Radiant heat has long been popular due to its comfort, quiet operation and cleanliness. The radiant element is directly coupled to the adjacent room, with no drafts or convection currents common with forced air systems. Duct noise and dust distribution is also reduced.
Concrete masonry has been successfully used as an integral part of these systems in both residential and commercial applications, as both radiant floors and walls. Hollow concrete masonry cores are aligned and interconnected to form air distribution channels. Heated or cooled air in the channels heats or cools the masonry, which then radiates to the interior space. These systems are generically termed “air core.”
Concrete masonry heats up and cools down slowly, improving comfort by moderating indoor temperature swings and helping shift utility loads to off-peak hours, when utility rates are generally lower. Further economy is provided by using the building’s structure as part of the heating and air-conditioning system. Depending on the application, culls or reclaimed concrete masonry units may be utilized as well.
GENERAL DESIGN GUIDANCE
To function as designed, the thermal mass of the concrete masonry should be as directly exposed to the interior air as possible. For floors, this means minimizing the use of insulating floor coverings such as carpeting and rugs. Concrete or tile flooring is a better choice in these applications.
Air core floors or walls can be used with either passive solar heat collection (see ref. 1) or conventional heating and cooling equipment. Air core elements can also be used to destratify, where warm air from the top floor is pulled down to a lower air core floor or wall for cooling.
It is important that the air core wall or floor be properly installed, and sealed where necessary, to minimize air leakage. When other building materials are integrated into an air core wall or floor, extreme care should be exercised to prevent leakage at joints. Use only high grade sealants and caulks.
For maximum efficiency, insulate the air core floor or exterior wall from the exterior, or utilize interior walls as air core walls. One possible exception to this rule of thumb is an air core floor on grade in a cooling-dominated climate, where the air core floor is being used as a heat sink.
Concrete masonry webs are tapered as a result of the manufacturing process. For maximum heat transfer, the concrete masonry units should be placed with the wide end of the taper upstream. This results in a series of small ridges which instills some turbulence to the air flow, increasing the heat transfer to some degree, although also causing a slight pressure drop. These effects should be evaluated for the each project, considering the core area and design air flow rate.
Where the concrete masonry wall is reinforced, the ungrouted cores can still be used for air flow.
During humid weather, the masonry also act as a desiccant, absorbing moisture from the air around it. When conditioned air is distributed through the masonry at night or as needed for cooling, it also removes stored moisture from the masonry.
When outdoor air is used for night cooling, thermal simulation should be performed to determine concrete masonry temperature versus air dew point temperature to prevent situations that could lead to condensation within the walls or floor.
AIR CORE WALLS
Properly insulated exterior walls, such as that shown in Figure 1, may be used for the air core system. Insulating to an R-value of 10 ft2. hr. oF/Btu (1.76 m2.oC/W) greater than that required by local code is recommended. In climates with greater than 6,500 heating degree days (HDD, base 65°F), consider locating the concrete masonry air core walls entirely inside the conditioned space, as interior walls are more thermally efficient in colder climates. The use of solid or solidly grouted concrete masonry exterior walls, insulated on the outside face to provide an R25 (4.4 m2.oC/W) envelope, increases the effectiveness of the mass and reduces the passive solar collection area required for efficient passive solar use.
Sheet metal ducts, where used, should be properly insulated. Some codes may require fuse-link fire dampers as required in active solar air systems. These provide a good safety feature, even if their use is not mandatory. Smoke detectors can also be interfaced with hybrid fans in order to render them inoperable in the event of fire or smoke.
Although air core walls are typically sealed, i.e., air in cores is not distributed within the building interior, some designers have successfully accomplished this by slightly modifying the system. One example is the OPTM (off-peak thermal mass) wall system shown in Figure 2. In this case, a space between the masonry and interior gypsum wallboard is used for conditioned air distribution. This system uses fully grouted concrete masonry with exterior insulation to provide maximum thermal storage capability and isolation from exterior temperatures. To facilitate the flow of conditioned air from the air duct at the base of the wall upward into the space between the gypsum board and the masonry wall, venturis are placed in the air duct at appropriate spacing. (Note that although this system was patented when first developed, the patent has expired.)
AIR CORE FLOORS
Air core floors tend to be less complex than air core walls, so can often be more economical. Air core floors are typically constructed on grade, and finished with a concrete topping. Typical air core floor elements are shown in Figure 3.
Perimeter insulation is a primary factor affecting the performance of air core floors. To prevent excessive heat loss around the perimeter, recommended insulation levels are:
In warmer climates (lower HDD), it may be cost effective to insulate the masonry from high summer soil temperatures, improving its potential for cooling. In colder climates, the high insulation R-value not only improves air core performance, but reduces the overall building heat load.
Below 4,500 HDD, insulation under the floor slab may consist of a simple skirt extending inwards from the foundation approximately 2 to 3 ft (610 to 914 mm).
Continuous under-the-slab insulation is warranted in climates above 4,500 HDD; a minimum of 1 in. (25 mm) of high density extruded board insulation is recommended. The insulation should always be placed on top of a vapor retarder. In climates above 6,000 HDD, 2 to 3 in. (51 to 76 mm) of continuous insulation is recommended; while above 8,000 HDD, it becomes cost effective to use up to 4 in. (102 mm) of insulation to isolate the slab system from the cold soil. Note that these recommendations may not be applicable to floors carrying significant structural loads.
Air leaks should also be minimized by using vapor retarders and thorough caulking and sealing of ducts and plenums.
AIR FLOW
In a single story (8 to 10 ft (2.4- 3.0 m) ceiling height) sunspace or direct gain area serving an air core system, a general rule of thumb for forced air flow is 2 to 5 cfm per square foot (0.61-1.5 m3/min./m2) of collector glazing (glazing area > 7% south glazing/floor area). For taller ceiling heights, the cfm should be increased.
The path length of heat exchanged in the system is used to determine air flow rates. For path lengths of 15 to 20 ft (4.6-6.1 m) through one core of a masonry wall, an air flow of 100 ft/min. (30.5 m/h) for solar heating and 150 ft/min. (45.7 m/h) for mechanically heated air has proven to be satisfactory. Runs in block cores in excess of 20 ft (6.1 m) should be avoided whenever possible. A pressure drop of 0.10 in. (2.5 mm) of water is an indication that the system is performing as desired, while a pressure drop of 0.25 in. (6.4 mm) of water indicates excessive flow restrictions or improper fan size.
In the case of systems expected to store both solar gains and wood burning energy released into room air, two fan speeds should be provided. The faster fan speed should create about 1.5 to 2 times the air flow of the solar fan speed. A proportional fan control based on temperature near the supply inlet could be used, creating a simple variable air volume system.
Flow balancing using small trim tabs at the return plenum side has been used successfully to align core air flow. Large alterations of the air core outlets may result in feedback to change the overall system flow rate, and should be avoided.
For cooling spaces with ventilation, a general guideline of 10 to 15 air changes per hour (ACH) has been recommended. The air flow generally required for this operation is determined using the relationship:
Cooling a 10 x 14 ft (3.0 x 4.3 m) air core room in this way would require about 200 cfm (5.7 m3/min.). Note that for wholehouse venting, the building air volume can be exchanged by a whole-house fan, while the air core operates in the cooling mode. Different fan speeds may be required for winter and summer operation. Care should be taken to seal all joints, ducts, dampers, housings, fan cages, plenums, slabs, vapor retarders, perimeter insulation and all other fittings.
CONTROLS
Figures 4 and 5 show fan control methods that have been used with the air core system for heating mode and for cooling using night ventilation, respectively.
Sensors must be placed during construction. It is very difficult to obtain consistent readouts if sensors are installed later by drilling and insertion. Always use a dependable brand of controller similar to those used for active solar systems. Differential controllers for heating should initially be set as follows:
The following temperatures should be continuously monitored for improved system performance: mass, supply air, building air near the thermostat, and outdoor air.
The sensor near the supply grille should be protected from sunlight which can cause faulty readings. A dust filter should be installed if possible; a standard sheet metal box, well insulated and sealed, is satisfactory for enclosing the filter and fan unit. In high solar fraction buildings, an automatic override is recommended. Operation of the solar system should preclude operation of auxiliary equipment, and vice versa.
THERMAL STORAGE CAPACITY
A simple and accurate method for properly sizing distributed thermal mass is diurnal heat capacity (dhc), which is helpful in determining the useful daily energy flux to and from a mass storage system (ref. 3). Using this method, any structure can be properly massed to achieve optimal temperature swings in the 10°F to 12°F (5-6.7 oC) range. Performance is best predicted using computer modelling.
A typical design value of 0.4 Btu/hr. ° F.ft2 (2.27 W/o C.m2) of surface area is used for the heat transfer coefficient of the inside surface areas of the concrete masonry units. Inside the cores, heat transfer is convectively dominated during charging, with some radiative coupling from shell to shell during discharge. The proper sizing and distribution of thermal mass in passive solar applications is discussed in reference 1.
REFERENCES
Passive Solar Design Strategies, TEK 06-05A, Concrete Masonry & Hardscapes Association, 2006.
Howard, B. D. The Air-Core System for Thermal Storage. Passive Solar Journal. Marcel Dekker and ASES, Vol. 3 No. 3, 1986.
Concrete masonry can make a significant contribution to meeting LEED Green Building certification. Leadership in Energy and Environmental Design (LEED) is a voluntary rating system developed by the United States Green Building Council (USGBC) to evaluate a building’s environmental impact and performance. LEED provides design guidelines and third-party certification for defining what constitutes a “green” building. LEED’s overall goals are to improve: occupant well-being, the environmental impacts and the economic return of new buildings. USGBC offers several green building certification programs, each tailored to a specific market or application.
LEED version 2.2 will remain in effect until LEED 2009 is completed, around September 2009. Once LEED 2009 is active, version 2.2 registered projects can continue to certification as version 2.2 projects. This TEK provides details on LEED for New Construction & Major Renovations 2009 (LEED NC 2009) (ref. 1).
The USGBC has incorporated several changes into LEED 2009. These major changes are:
Prerequisite and credit harmonization across rating systems (LEED Bookshelf). The intents and requirements for common prerequisites and credits among the various rating systems (i.e., New Construction, Schools, Retail, etc.) have been standardized to minimize differences or contradictions between rating systems. There are still unique prerequisites and credits in each rating system. In addition to harmonization, credit interpretation requests (CIR) that have set precedents or shown a need for clarification have been incorporated.
A set schedule for updating the rating systems is being established, similar to building code development schedules.
The quantity and distribution among the categories of points was changed to better reduce or improve a buildings environmental impact. These changes are most readily seen in the increase in the total number of points and in the allocation of points in each category. One of the major tools for weighting the categories was TRACI (Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts), established by the EPA.
The new Regional Priority category provides an opportunity for a project to achieve credit for addressing regional environmental concerns. The details on this category are still being finalized as the USGBC councils and chapters assemble and submit their requests to the USGBC.
LEED NC 2009 is comprised of seven categories each with its own prerequisites and credits. The prerequisites in all categories must be met to earn LEED certification. Using concrete masonry and concrete hardscape products can contribute to certification in the following LEED categories: Sustainable Sites, Energy & Atmosphere, Materials & Resources, Innovation and Design, and possibly Regional Priority*.
POINTS FOR CERTIFICATION
LEED NC 2009 provides a checklist of mandatory prerequisites as well as voluntary credits in seven basic categories: Sustainable Sites, Water Efficiency, Energy and Atmosphere, Materials and Resources, Indoor Environmental Quality, Innovation and Design Process, and Regional Priority Credits. Projects earn one or more credits by meeting or exceeding a checklist item’s technical requirements. Figure 1 illustrates the categories where concrete masonry products can contribute.
Points for voluntary credits add up to a final score which can earn the building one of four possible levels of certification. As shown in Figure 1, concrete masonry and hardscape products can make a significant contribution to LEED certification. A project must earn at least 40 points for LEED certification. Silver (50 pts), gold (60 pts), and platinum (80 pts) certification levels are also available. There are a total of 110 points available from the seven credit categories.
EARNING LEED POINTS
The following sections briefly describe how concrete masonry and hardscape products can contribute to earning points in each of the LEED credit categories.
Sustainable Site Prerequisite—Construction Activity Pollution Prevention
Reducing pollution due to construction activity is a prerequisite. Pollution includes soil erosion, waterway sedimentation and dust. Articulated concrete block (ACB) and open grid pavers can be used for soil erosion control and dust control. See References 3 and 4 for more information.
Sustainable Site Credit—Development Density
Developing an urban lot in lieu of an undeveloped “greenfield” area can earn a project one point towards certification. Concrete masonry and retaining walls enable designs that take advantage of small, irregularly shaped lots, where access and open area are often at a premium. Concrete masonry, because of its relatively small modular size does not require large equipment for delivery or placement, nor are large staging areas required for construction.
Sustainable Site Credit—Site Development
The overall goal of this credit is to preserve or restore natural habitats and to promote biodiversity. There are two parts to this credit, Protect or Restore Habitat and Maximize Open Space, each worth one LEED point.
On greenfield sites, segmental retaining walls and/or cantilever retaining walls can be used to limit cut and fill requirements. Reducing cut and fill operations preserves habitat and limits the disturbed area.
A second point can be earned by exceeding the local zoning requirement for open space by 25%. This 25% threshold can be met by reducing the site disturbance or by creating vegetated open space (the credit also provides requirements for projects where no local zoning requirement exists). Once again, retaining walls can limit site disturbance by reducing the amount of slope alteration.
For urban projects earning the Development Density credit, pedestrian hardscape areas, such as those provided by concrete pavers, can be counted as open space, although only 25% of the open space can be hardscape. Wetlands and ponds may count as open space if the side slope ratio does not exceed 1:4 and are vegetated. Articulated concrete block is well suited to wetland and pond construction.
Sustainable Site Credit—Stormwater Management
The intent of this credit is to limit disruption of natural hydrology by reducing stormwater runoff quantity and by improving the quality of stormwater runoff, through methods such as reducing impervious cover, increasing on-site infiltration, and managing stormwater runoff. One of the LEED-suggested strategies is to use permeable paving that promotes infiltration.
The requirement for controlling stormwater rate and quality is implemented either by limiting the post-development peak discharge rate or by implementing a plan that protects the receiving streams from excessive erosion.
An additional point may be earned by implementing Best Management Practices to capture and treat at least 90% of the average annual rainfall and removing 80% of the post development suspended solids.
Pavements are major contributors to stormwater runoff, and permeable pavements that directly pass water from the pavement surfaces to the underlying soil can help alleviate runoff. Benefits of permeable pavements include reduced stormwater runoff, direct recharge of underlying groundwater systems, partial treatment of pollutants in the runoff and increased usable space. Both permeable pavers and open-cell pavers (also known as turf stone or grid pavers) offer the option of replacing impermeable pavement with permeable pavement. These pavers can help earn one point each for reducing stormwater runoff and for treatment of stormwater. References 4, 5, and 6 provide more detailed design information for these pavements.
For the protection of receiving stream channels, ACB products can effectively reduce soil erosion and allow for a protected and vegetated area.
Sustainable Site Credit—Heat Island Effect, Non-Roof
Urban heat islands are localized areas of high temperature, caused by the retention of solar energy on constructed dark surfaces. The effect is elevated temperatures in urban areas and a greater energy demand for cooling.
LEED offers one credit point for non-roof heat island reduction on projects if 50% or more of the site hardscape (including roads, sidewalks, courtyards and parking lots) are either shaded or use paving materials with Solar Reflectance Index of at least 29. Typical values for SRI are 35 for new gray concrete and 19 for weathered (unclean) concrete.
The SRI requirement can be met either by using light-colored concrete pavers in lieu of asphalt; or by using open-cell pavers, which can support grass or other plant materials in the pavers’ open grid areas. The open cell pavers must be considered at least 50% pervious.
Energy & Atmosphere Prerequisite—Minimum Energy Performance
This prerequisite requires a 10% improvement (for new construction) or a 5% improvement (for major renovations) over the ASHRAE 90.1 2007, Appendix G, Building Performance Rating Method (ref. 7). There are two other options for meeting this prerequisite: using the ASHRAE Advanced Energy Design Guide appropriate to the building type; and using the Advanced Buildings Core Performance Guide by the New Buildings Institute (NBI) (refs. 8, 9). These publications are prescriptive design guides that have specific building size and usage criteria.
The intent of these credits is to improve energy efficiency beyond ASHRAE 90.1-2007. Energy savings attributable to thermal mass inherent in concrete masonry construction contribute to this goal when used in conjunction with passive solar heating and/or ventilation cooling. Because concrete masonry has high thermal mass and specific heat, it provides very effective thermal storage. Masonry walls remain warm or cool long after the heat or air-conditioning has shut off. This, in turn, can effectively: reduce heating and cooling loads; improve occupant comfort by moderating indoor temperature swings; and shift peak heating and cooling loads to off-peak hours.
Using ASHRAE/IESNA Standard 90.1 Appendix G, Performance Rating Method, entails using a comprehensive, whole-building analysis software simulation program, capable of projecting the building’s energy consumption and associated costs based on an hour-by-hour simulation of a full year of weather data. Examples of such programs include DOE-2 and BLAST. These programs can accurately model concrete masonry’s thermal mass and predict the associated energy savings. These energy simulations have been used to demonstrate in many cases that, with all other variables kept the same, a high mass concrete masonry building can be heated and cooled using less energy than a similar frame building. See References 7 and 10 for more information.
Energy & Atmosphere Credit—Optimize Energy Performance
As many as 19 points can be achieved under this credit by incrementally increasing the energy efficiency of the building by 12 44% for new construction using Appendix G of ASHRAE 90.1-2007. One point is awarded for each 2% improvement. The Advanced Energy Design Guide and the Advanced Buildings Core Performance Guide can be used to achieve this credit. However, using these guides limits the potential points earned to 3.
Note that for the purposes of this credit, savings attributable to the building thermal envelope are cumulative, and so are added to savings from high efficiency HVAC, heat recovery equipment, daylighting, etc. Thus, all incremental improvements contribute toward project certification.
Materials & Resources Credit—Building Reuse
The building reuse credit is intended to extend the life of the existing building stock, thereby conserving resources, reducing waste and reducing the environmental impacts of new construction. Credits are earned when developers maintain the majority of an existing building’s structure and envelope. The building envelope is the exterior skin and framing excluding window assemblies and non structural roofing.
This credit is often obtainable when renovating buildings with exterior concrete masonry walls, because concrete masonry is an exceptionally durable material with a life cycle measurably longer than many other building envelope products. Concrete masonry construction provides the opportunity to refurbish the building should the building use or function change, rather than tearing down and starting anew.
Three points are available under this credit. These points are awarded based on the percentage of building reused. One point is awarded for 55% reuse, 2 points for 75% reuse, and 3 points for 95% reuse. An additional point is available if at least 50% of the interior, non-structural items remain. The percentages are based on the area of the completed building (existing and additions).
This item encourages contractors to divert demolition and landclearing debris from landfills and incinerators.
The construction waste management credit is awarded based on recycling or salvaging at least 50% of construction waste, based on either weight or volume. Because concrete masonry is a relatively heavy construction material and can be recycled into aggregate for road bases or other concrete products, pipe bedding or construction fill, this credit is obtainable either when buildings with concrete masonry are demolished or, in new construction when saw-cut scraps and broken pieces of concrete masonry are crushed and reused. In addition, intact and unused concrete masonry units can be redirected to other projects or donated to charitable organizations such as Habitat for Humanity.
This credit is worth 1 point if 50% of the construction, demolition and land clearing waste is recycled or salvaged and 2 points for 75%.
Materials & Resources Credit—Materials Reuse
This checklist item encourages the reuse of salvaged materials on the project, such as crushed concrete masonry, and it awards one point if the value of all reused materials is at least 5% of the total value of materials on the project. Two points are awarded at the 10% threshold. Note that the same materials cannot be claimed for both the construction waste management credit, above, and the materials reuse credit on the same project.
Materials & Resources Credit—Recycled Content
The use of building products with recycled content can earn the project one or two points. To earn the point(s), the project must meet the threshold percentage (10% for 1 point; 20% for 2 points) based on the total of all building materials used in the project.
Concrete masonry can potentially incorporate recycled materials, with due consideration to ensure that the use of these materials does not adversely affect the quality of the masonry units or construction. Recycled materials can be used as a partial replacement for cement, or as aggregate.
A full discussion, including calculation methods for recycled content, is discussed in TEK 06-06B, Determining the Recycled Content of Concrete Masonry Products (ref. 11).
Materials & Resources Credit—Regional Materials
Using materials and products that are locally extracted and manufactured supports the use of indigenous resources and reduces environmental impacts of transportation. Concrete masonry materials are most commonly extracted and manufactured close to the jobsite, thus helping to fulfill this LEED credit.
The LEED requirement is to “specify that a minimum of 10% of building materials be extracted, processed & manufactured within a radius of 500 miles.” Concrete masonry usually qualifies, since block plants are often within 50 mi (80 km) of a job site. The percentage of materials is calculated on a cost basis. If only a fraction of a product or material is extracted/harvested/recovered and manufactured within the region, then only that percentage (by weight) contributes to the regional value.
Innovation and Design Process
The intent of this item is to provide design teams with an incentive to go beyond the LEED requirements and/or to award points for innovative strategies not specifically addressed in the LEED rating system. Examples that may qualify are: substantially exceeding the building energy performance criteria (Energy & Atmosphere Credit 1), or including characteristics not directly referenced by LEED, such as acoustic performance and life cycle analysis of materials used.
Potential strategies for achieving Innovation & Design points with concrete masonry and hardscape products include:
Show concrete masonry’s advantage in life cycle environmental impact over other building materials such as steel and aluminum due to its durability, low maintenance, and low embodied energy.
Address indoor air quality issues, by eliminating the need for paints with exposed concrete masonry walls, thereby reducing the potential for VOC (volatile organic compounds) emissions.
Improve indoor air quality using concrete masonry due to the reduced potential for mold growth ( concrete masonry is not a food source food for mold) and concrete masonry’s ability to be cleaned instead of being replaced in the event of a mold incident.
Show concrete masonry’s material usage efficiency by incorporating partial grouting and prestressed masonry design techniques.
Demonstrate concrete masonry’s intrinsic acoustical characteristics. See Reference 12 for further information.
Make a case for concrete masonry’s superior fire resistance and fire containment qualities. See Reference 13 for further information.
Regional Priority
These credits provide a means for addressing local environmental priorities. There are six credits, but no more than four can be earned per project. These credits will pertain only to certain geographic locals, and the projects must be located within the relevant region to be eligible. Details on these credits will be available on the USGBC web site, www.usgbc.com.
REFERENCES
LEED 2009 New Construction for Member Ballot. U.S. Green Building Council, 2008.
LEED-NC for New Construction v2.2 Reference Guide. First Edition, U. S. Green Building Council, October 2005.
Energy Standard for Buildings Except Low-Rise Residential Buildings, ANSI/ASHRAE/IESNA Standard 90.1-2007. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2007.
Advanced Energy Design Guides. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Available for free download from: http://www.ashrae.org/technology/page/938.
Advanced Buildings Core Performance Guide. New Buildings Institute, 2007.
Standard 90.1-2007 User’s Manual. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2008.
Determining the Recycled Content of Concrete Masonry Products, TEK 06-06B, Concrete Masonry & Hardscapes Association, 2009.
Sound Transmission Class Rating for Concrete Masonry Walls, TEK 13-01D Concrete Masonry & Hardscapes Association, 2012.
Fire Resistance Rating of Concrete Masonry Assemblies, TEK 07-01D Concrete Masonry & Hardscapes Association, 2018.
Earth-sheltering refers to using earth as part of a building’s thermal control system Earth-sheltered buildings can be either built into the earth or an existing hillside, or can be built above grade, and earth bermed around the exterior after construction. Earth-sheltered buildings can be built entirely underground, but are more often only partially earth-sheltered to allow adequate natural light into the interior. These buildings are most widely recognized for their energy efficiency, due to the insulating capacity of the earth and lower air infiltration through the earth-sheltered surfaces. In addition, earth sheltered buildings also offer superior protection from storms, insulation from outside noise, lower maintenance costs, and less impact on the surrounding landscape.
Concrete masonry is often the material of choice for earth-sheltered buildings. In addition to strength, durability, low maintenance and resistance to fire, soil gas, termites and other pests, the wide range of concrete masonry colors and textures provides an unlimited palette for the interior. Earth-sheltered buildings can also be constructed of conventional gray units, and easily finished with furring strips and drywall, since masonry is constructed square and plumb.
Earth-sheltered buildings are typically designed with passive solar features to further reduce mechanical heating and cooling requirements. In these designs, the concrete masonry absorbs solar energy, preventing interior overheating and providing heat after the sun sets. Types of passive solar systems and design considerations are covered in more detail in Passive Solar Design, TEK 06-05A (ref.1).
Many design and construction considerations for earth-sheltered buildings are the same as those for basements. For this reason, the reader is referred to Basement Manual: Design and Construction Using Concrete Masonry (ref. 2), which includes detailed information on structural design, water penetration resistance, crack control, insect protection, soil gas resistance as well as construction recommendations.
ENERGY EFFICIENCY
Earth-sheltered buildings save energy in several ways when compared to conventional structures. First, earth-sheltered buildings have a lower infiltration, or air leakage, rate. In homes, up to 20% of the total heating requirement can be due to infiltration. Almost half of that figure results from air leakage through walls other than windows or door openings. The earth covering effectively eliminates these losses.
Earth-sheltered construction also saves energy by reducing conduction heat losses through the walls and roof. The temperature difference between the building and the adjacent ground is typically much less than between an above grade structure and the outside air. In other words, the earth moderates the outdoor temperature swings, so that the earth-sheltered building is not subjected to as harsh an environment. In hot climates, the earth acts an a heat sink, helping keep the interior cooler.
CLIMATE AND SITE CONSIDERATIONS
Local climate can effect the practicality of earth-sheltering. Studies have shown that earth-sheltered houses are more cost effective in climates with larger daily temperature swings and low humidity, such as the northern Great Plains and the Rocky Mountains (ref. 3). In these locations, the earth temperature tends to be more stable than air temperatures, which allows the earth to act as a heat sink in hot weather and to insulate the building during cold weather.
Climate should also be considered when deciding on the type of earth-sheltered structure to build.
The site should be evaluated for water drainage. Choosing a site where the water will naturally drain away from the building is ideal. The finished grade should slope away from the building at least 6 in. in 10 ft (152 mm in 3.05 m) to carry surface water away. Where the topography is such that water flows towards the building, a shallow swale or trench can be constructed to intercept the water and divert it away from the structure.
In addition to the effect on water runoff, the site’s slope can significantly impact construction and design. Steeply sloping sites require much less excavation than flat or slightly sloping sites. South facing slopes work well in climates with a longer heating season, because the building can be easily designed with south-facing windows for direct solar gain (see also ref. 1). In climates with milder winters and hot summers, a north-facing slope may be preferable.
BUILDING MATERIALS
The choice of construction materials should consider the type of structure, depth below grade and soil type. Deeply buried buildings require stronger, more durable construction materials. The following is a brief list of recommendations for below-grade concrete masonry construction. Basement Manual: Design and Construction Using Concrete Masonry (ref. 2) contains more detailed recommendations as well as minimum requirements.
Concrete masonry units should be 8-in. (203-mm) or larger, depending on structural requirements. Use of unit shapes such as “A” or “H” facilitates unit placement around vertical reinforcing bars.
Type S mortar is generally recommended.
Joint reinforcement or horizontal reinforcing bars may be required to reduce potential shrinkage cracking and meet certain code requirements.
Grout, if used, must have a minimum compressive strength of 2,000 psi (13.8 MPa).
The concrete slab is typically a minimum of 2,500 psi (17.2 MPa) and 4 in. (102 mm) thick to allow the slab to span over weak soil areas without excessive cracking. Follow industry recommendations for subslab aggregate base and vapor barrier.
Backfill should preferably be free-draining material and should only be placed after the wall has gained sufficient strength and has been properly braced or supported.
TYPES OF EARTH-SHELTERED BUILDINGS
Earth-sheltered buildings can be constructed completely below grade or with part of the building above grade. An earth-sheltered building constructed completely below grade is referred to as an underground structure. More typically, though, the building is built partially or fully above grade, then earth is bermed up around one or more exterior walls. These bermed structures can in general accommodate more conventional building plans.
Underground Structures
Underground structures are most often designed using an atrium or courtyard design. This floorplan uses a subgrade central open area as the entry and focal point, and achieves an open feeling because it has four walls exposed to daylight. The structure is built completely below grade, typically on a flat site, and the interior spaces are arranged around a central outdoor courtyard. Windows and glass doors opening into the courtyard supply light, solar heat, natural ventilation, views and access to the ground level. Atrium/courtyard buildings are typically covered with less than 3 ft (0.91 m) of earth. Greater depths do not significantly improve the energy efficiency.
The atrium design provides minimal interruption to the natural landscape, good protection from winter winds and exterior noise, and a private outdoor space. Design considerations specific to atrium/courtyard structures are courtyard drainage and snow removal, as well as possibly limited passive solar gains, depending on the courtyard size and depth below grade.
Bermed Structures
Two general types of bermed structures are elevational and penetrational. Elevational floorplans have one whole building face exposed, while the other sides, and sometimes the roof, are covered with earth. The covered sides protect and insulate, while the exposed face, typically facing south, provides views, natural light and solar heat. This type of structure is typically set into the side of a hill, and tends to be the easiest type of earth sheltered building to construct, and therefore the most economical. Skylights and/or additional ventilation may need to be considered for the north-facing interior spaces.
Penetrational designs are built above grade, with earth bermed around and on top of it. The entire building is covered, except at windows and doors, where the earth is retained. This design allows natural light from all walls of the building, as well as cross-ventilation.
INSULATION PLACEMENT
Not all experts agree on the amount of insulation required nor the optimum placement around the structure, but two points are generally well agreed on:
(1) It is generally not cost-effective to insulate below the floor slab in an earth-sheltered building. Edge insulation is a good investment on walls that are not bermed.
(2) Insulation should be placed on the exterior side of the walls. Exterior insulation protects the waterproofing from abrasion damage and allows the thermal mass of the below-grade concrete masonry walls to contribute to the energy savings and indoor temperature moderation.
Because of the insulating effect of the soil, insulation is more effective for buildings located closer to the surface of the ground. Normally, for earth cover less than 5 ft (1.52 m) over the ceiling, the ceiling should be insulated. In most cases, it is less expensive to insulate the ceiling than to increase the roof capacity to carry the load of the additional earth.
Figure 1 shows four variations of insulation placement, with some general performance guidelines for underground buildings. Note that in some cases, insulation which is effective at reducing winter heat losses can actually be detrimental when cooling needs are considered. For this reason, it is important to match the insulation strategy to the heating and cooling needs of the building.
WATER PENETRATION RESISTANCE
All below-grade spaces are potentially vulnerable to water penetration from rainfall, melting snow, irrigation and natural groundwater, regardless of the construction materials used. For adequate protection, the following should be employed (see ref. 2 for a complete discussion):
Identify the water sources (precipitation, irrigation, groundwater and/or condensation) and address potential water entry points prior to construction.
Follow proper construction techniques and details
Provide drainage to direct surface and roof water away from the structure.
Install a subsurface drainage system to collect and direct water away from the foundation.
Apply damp-proofing or waterproofing systems to the masonry walls. A drainage board can also be used to drain water quickly and reduce backfill pressure.
OTHER CONSIDERATIONS
Earth-sheltered buildings require all of the considerations typically associated with basement design and construction, such as structural capacity, insect protection and soil gas protection. In addition, considerations such as adequate ventilation, egress and natural light may also be considerations for earth-sheltered structures.
Adequate ventilation must be carefully planned for earth sheltered buildings. Ventilation is the exchange of indoor air for outdoor air, and reduces indoor pollutants, odors and moisture. For buildings with low air leakage, such as earth-sheltered buildings, natural ventilation alone should not be relied upon. Instead, the building should utilize a mechanical ventilation system. ASHRAE recommends balanced air to-air systems with heat recovery for very air-tight homes (ref. 6).
The International Residential Code (IRC) (ref. 7) requires all habitable rooms to have a minimum glazing area of 8% of the room’s floor area, with minimum openable area of 4% of the floor area to provide natural ventilation. For earth-sheltered homes, if these requirements cannot be met using windows, doors, window wells and skylights, the IRC includes exceptions for homes with mechanical ventilation capable of providing 0.35 air changes per hour in the room or a whole-house system capable of supplying 15 ft3 /min. (7.08 L/s) of outdoor air per occupant. Supplementary artificial lighting may also be required.
In addition, bedrooms are required to have at least one openable emergency escape and rescue opening with a minimum net clear opening of 5.7 ft2 (0.530 m2). Window wells must have a horizontal area at least 9 ft2 (0.84 m2), with a minimum projection and width of 36 in. (914 mm). These emergency egress requirements may drive the building layout. Homes designed using an “elevational” floorplan, for example, tend to be long and narrow, so that the bedrooms and main living spaces are aligned along the above-grade wall.
Indoor humidity can increase during the summer, which can lead to problems such as condensation and mold if not addressed. Exterior insulation on the walls prevents the walls from cooling down to the earth temperature, but also reduces the heat sink effect for summer cooling. Mechanical dehumidification or air conditioning may be required to control indoor humidity levels.
REFERENCES
Passive Solar Design, TEK 06-05A, Concrete Masonry & Hardscapes Association, 2006.
Basement Manual: Design and Construction Using Concrete Masonry, CMU-MAN-002-01, Concrete Masonry & Hardscapes Association, 2001.
EERE Consumers Guide: Site-Specific Factors for Earth Sheltered Home Design. U. S. Department of Energy, 2005.
Earth-Sheltered Home Design. U. S. Department of Energy, http://www.eere.energy.gov/consumer/your_home/designing_rem deling/index.cfm/mytopic=10100.
Forowicz, T. Z. An Anlaysis of Different Insulation Strategies for Earth-Sheltered Buildings. ASHRAE Transactions, Vol. 100 Part 2, 1994.
2005 ASHRAE Handbook Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2005.
2003 International Residential Code for One and Two Family Dwellings. International Code Council, 2003.
Sustainable development has been defined as development that meets the needs of the present without compromising the ability of future generations to meet their own needs (ref.1). This is often expressed as a holistic approach to building design, with the goal of optimizing environmental, economic and social impacts, from site selection through building operation and maintenance. A sustainable building optimizes resource management and operational performance, while minimizing risks to human health and the environment. As such, providing a sustainable building project encompasses far-reaching design decisions, and recognizes the interrelationships between virtually all elements and phases of the project.
A range of products and programs has been developed to help designers achieve a more sustainable built environment. Whether in the form of design guidelines for particular building types, or rating systems that step the design team through a series of design considerations, all aim to provide practical guidance for achieving the almost overwhelming goal of sustainability.
Referenced and in some cases mandated by some branches of the Federal government, as well as many state and local governments, the United States Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED®) has become perhaps the most widely used of these programs in recent years. LEED is a voluntary rating system designed to provide guidance as well as national third-party certification for defining what constitutes a “green” building.
Concrete masonry building and hardscape products can make a significant contribution to meeting LEED certification. This contribution is augmented by the recycled content potential of the companion products necessary for a concrete masonry wall, such as grout, mortar and reinforcement products.
Concrete masonry building and hardscape materials can contribute to earning credits in several LEED categories, including Sustainable Sites, Energy and Atmosphere, Materials and Resources and Innovation in Design. More detail on LEED strategies incorporating concrete masonry and hardscape materials is available in TEK 06 09C, Concrete Masonry and Hardscape Products in LEED 2009 and PAV-TEC-016-16, Achieving LEED Credits with Segmental Concrete Pavement (refs. 2, 3).
LEED includes specific rating systems for various applications. The information in this TEK is applicable to LEED for new construction, school, retail, and core and shell development (refs. 4-7).
For these LEED programs, Materials and Resources Credit 4: Recycled Content allows up to two LEED certification points for using materials with recycled content. The inert nature of concrete masonry lends itself well to incorporating recycled materials as cement replacements, as aggregates and as other constituents in the concrete mix. This TEK provides guidance on determining the recycled content of concrete masonry products for the purpose of earning LEED credit under the new construction, school, retail, and core and shell development LEED programs.
The LEED for Homes (ref. 8) recycled content credit differs from these other programs. Concrete masonry walls are eligible for recycled content credit under the LEED for Homes Materials and Resources Credit 2: Environmentally Preferable Products, provided the masonry contains at least 25% recycled content (post-consumer plus one-half pre-consumer, as described in the following sections). Note, however, that the National Association of Home Builders with the International Code Council has developed their own green building standard that has similar requirements (ref. 9). See www.nahbgreen.org for more information.
USE OF RECYCLED MATERIALS IN CONCRETE MASONRY AND HARDSCAPE UNITS
When concrete masonry products incorporate recycled materials, due consideration must be given to ensure that the use of these materials does not adversely affect the quality or safety of the units or construction. Note that some recycled materials may only be regionally available. Designers should work closely with concrete masonry manufacturers to substantiate recycled content.
Unit Specifications
Whether produced using recycled or virgin materials, concrete masonry products are required to meet the applicable ASTM unit specification (see Table 1). These standards contain minimum requirements that assure properties necessary for quality performance. For example, many concrete masonry units are required to conform to ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units (ref. 11). ASTM C90 requirements include material requirements for aggregates, cementitious materials, and other constituent materials, physical requirements, finish and appearance requirements, and permissible variations in dimensions.
Aggregates, including recycled aggregates, for concrete masonry units are required to meet ASTM C33, Standard Specification for Concrete Aggregates, or C331, Standard Specification for Lightweight Aggregates for Concrete Masonry Units (refs. 19, 20), except that grading requirements do not have to be met. Aggregate characteristics governed include limits on deleterious substances and aggregate soundness.
Cements are required to meet ASTM C150 and supplemental cementitious materials such as fly ash must meet ASTM C618 (refs. 27, 28). In addition to cementitious materials and aggregates, the ASTM unit specifications also allow for the inclusion of “Other Constituents,” such as pigments, integral water repellents and finely ground silica. For a material to qualify for inclusion in a concrete masonry product under this provision, the material:
must have been previously established as suitable for use in the product, and
must either conform to applicable ASTM standards or be shown, via test or experience, not to be detrimental to the durability of the units or other masonry materials.
Fire Resistance Ratings
For construction requiring a fire resistance rating, the use of recycled aggregates may impact the method used to determine the hourly rating, because concrete masonry fire resistance ratings vary with the aggregate type(s) used to manufacture the units. Concrete masonry fire ratings can be determined by: model building code prescriptive tables (ref. 21), a standard calculation method as provided in Section 721 of the International Building Code (IBC) (ref. 21) and the ACI/TMS 216 (ref. 22); testing in accordance with ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials (ref. 23); commercial listing services; and deemed-to comply assemblies included in some building codes. These tools also include ways to increase a wall system’s fire resistance rating through careful placement of additional materials.
Currently, the standard calculation procedure applies to the following aggregate types: expanded slag, pumice, expanded clay, expanded shale, expanded slate, limestone, cinders, aircooled slag, calcareous gravel, and siliceous gravel. When units are made with a combination of these aggregates, the fire rating is determined by interpolation (see ref. 23 for more detail). When aggregate types other than those listed above are used, the fire resistance rating is determined using a method other than the standard calculation procedure.
TEK 07-01D, Fire Resistance Rating of Concrete Masonry Assemblies (ref. 24) contains a detailed discussion of concrete masonry fire ratings. Additional considerations for recycled aggregates which are not listed in the standard calculation procedure are their stability, safety and load-carrying ability when subjected to fire.
By increasing the demand for products that incorporate recycled materials, the Recycled Content credits are intended to reduce the environmental and societal impacts associated with extracting and processing virgin materials.
LEED awards 1 point to projects that demonstrate that the total amount of a project’s recycled content exceeds 10% based on both weight and the total building product costs. An additional point is awarded if the recycled content reaches 20%. Also, if the recycled content reaches 30%, a third point can be earned as an Innovation & Design credit.
LEED refers to the International Organization for Standardization (ISO) for the definition of what constitutes recycled content, and for the basis of determining the percentage – i.e., weight (ref. 25). Recycled materials are those materials diverted from the solid waste stream, either during the manufacturing process (pre-consumer) or after their intended use (post-consumer). The recycled content for LEED credit is determined as the sum of all post-consumer recycled content plus one-half of the pre-consumer recycled content.
To claim this credit, the LEED NC Reference Guide suggests establishing a project goal for recycled content materials, and dentifying product suppliers who can achieve this goal. The following sections describe how concrete masonry and hardscape products can contribute to recycled content goals.
Pre-Consumer Recycled Content
Pre-consumer (post-industrial) content as defined by the LEED v2.2 reference manual is “material diverted from the waste stream during the manufacturing process. Excluded is reutilization of materials such as rework, regrind or scrap generated in a process and capable of being reclaimed within the same process that generated it (Source ISO 14021). Examples in the pre-consumer category include planer shavings, plytrim, sawdust, chips, bagasse, sunflower seed hulls, walnut shells, culls, trimmed materials, print overruns, over-issue publications, and obsolete inventories.” (refs. 4, 25) It is important for the producer to work with the material suppliers to determine which materials can be considered recycled and which cannot. It is important for the producer to have documentation from the material supplier stating that a material is considered recycled for the purposes of contributing to LEED certification.
Post-Consumer Recycled Content
Post-consumer recycled content is consumer waste that can no longer be used for its intended purpose. The official LEED definition of a post-consumer material is “material generated by households or by commercial, industrial and institutional facilities in their role as end users of the product which can no longer be used for its intended purpose. This includes returns of materials from the distribution chain (ref. 26). Examples of materials in this category include construction and demolition debris, materials collected through curbside and drop off recycling programs, broken pallets (if from a pallet refurbishing company, not a pallet-making company), discarded products (e.g. furniture, cabinetry and decking) and urban maintenance waste (leaves, grass clippings, tree trimmings, etc.) (refs. 4, 25).
As with pre-consumer materials, a producer should work with the material supplier to document that the materials being used are specifically documented as post-consumer recycled material for the purposes of contributing to LEED certification.
DETERMINING RECYCLED CONTENT
The LEED recycled content credit(s) is based on the recycled content percentages, based on the total value of all permanently installed materials on the project. Note that mechanical, electrical and plumbing components are excluded from this total, as are specialty items such as elevators. In determining the percentages of recycled content, the contribution from concrete masonry and hardscape products is added to the contribution from other building components.
The following sections describe the procedure for determining the recycled content of a particular product, then combining all such data to determine the overall recycled content percentage for the project. The percentages are based on both weight and cost, as described below.
For a Product
The producer is responsible for reporting the percentages of reconsumer and post-consumer recycled content for each product sold. If the producer supplies other products in addition to block such as reinforcement, mortar, etc., the producer should also document the recycled percentages in each of these products and report them to the contractor who purchased them.
The percentages are based on weight, as follows:
As an aid to the producer, CMHA has developed a simple spreadsheet to calculate these percentages (see Figure 1). Figure 1 illustrates the process of determining the weights of all constituent materials; determining the total weight; then determining the percent by weight of each recycled material. The total pre-consumer and post consumer percentages are simply the sum of the individual material percentages in each category.
Note that Figure 1 includes an alternate calculation, applicable to concrete products only. This alternate calculation is described below.
For a Product: Alternate Calculation per LEED for New Construction and Major Renovations
LEED for New Construction and Major Renovations, Version 2.2 and the LEED Reference Guide for Green Building Design and Construction, 2009 Edition (ref. 5, 26) provide an alternate method to calculate and report the recycled content for concrete products that use supplementary cementitious materials (SCMs), such as fly ash or ground blast furnace slag cement. This alternate method allows the recycled content calculation to be based on only the cementitious materials, rather than on all materials in the concrete mix. This alternate method helps offset the fact that the recycled content calculation is based on weight, and SCMs are typically very low in weight. For concrete mixes with SCMs as the only recycled content, this alternate method will result in a higher recycled content value than the conventional approach. For concrete mixes that incorporate both SCMs and other recycled materials, the manufacturer may want to evaluate the percent recycled content using both methods to determine which method yields the best result.
The basic calculation is the same as that described in the previous section, except:
when determining the percent post-consumer and percent pre consumer recycled content, divide by the total weight of the cementitious materials only, and
when determining the recycled content value, multiply the percent recycled content by the total value of the cementitious materials only.
Use of the alternative calculation method requires that the value of the cementitious materials be used in place of the total value of the product when the LEED project team determines the value of the recycled content. The producer would likely benefit from describing this value as a percentage of the value of the whole product and not as a monetary figure. When requested, the producer should report this value to the direct customer and not to a third party.
For the Project as a Whole
Based on information from the product suppliers, the design team determines the recycled content value for the project as a whole as follows:
For each product, the percent recycled content is determined as the percent post-consumer (reported by the supplier) plus one-half of the percent pre-consumer. For the example in Figure 1, the percent recycled content for the concrete masonry units is 17.9% + 1/2(37.1%) = 36.5%
For each product, the recycled content value is determined as the percent recycled content multiplied by the total product cost for the project. For the hypothetical project referenced in Figure 1, if the total cost of the concrete masonry units is $90,000, the recycled content value of the concrete masonry units is 0.365($90,000) = $32,805. It is important to note that the cost used in this calculation is the amount paid to the producer or the contractor for the product. It is not the cost of the individual materials that constitute the concrete masonry or hardscape product. The product cost should be supplied by the contractor. It is the contractor’s responsibility to separate their labor charges from the material charges.
For the project as a whole, the recycled content percentage is determined as the sum of the recycled content values of each product, divided by the total cost of all of these products. If this total recycled content percentage is 10% or higher, the project earns one LEED point; if it is 20% or higher the project earns two LEED points. Projects with recycled content percentages of 30% or more may be eligible for an additional Innovation in Design point.
CONCRETE MASONRY UNITS RETURNED FROM A JOB SITE
Unused concrete masonry units returned to the manufacturer from a job site are considered under Materials and Resources Credit 2: Construction Waste Management. Under Credit 2, the building project with unused materials can earn LEED point(s) for returning those materials, and hence diverting them from a landfill. If subsequently used on another project, the recycled content of the units as manufactured is reported to the contractor or design team, as for unused concrete masonry products.
REFERENCES
Standard Terminology for Sustainability Relative to the Performance of Buildings, ASTM E2114-06a. ASTM International, Inc., 2006.
Concrete Masonry and Hardscape Products in LEED 2009, TEK 06-09C, Concrete Masonry & Hardscapes Association, 2009.
LEED for New Construction and Major Renovations, Version 2.2, 3rd ed. U. S. Green Building Council, 2005.
LEED for Schools for New Construction and Major Renovations, Version 2007. U. S. Green Building Council, 2007.
LEED for Retail: New Construction and Major Renovations, Version 3. U. S. Green Building Council, 2008.
LEED Green Building Rating System for Core and Shell Development, Version 2.0. U. S. Green Building Council, 2006.
LEED for Homes Rating System. U. S. Green Building Council, 2008.
NAHB Model Green Home Building Guidelines. National Association of Home Builders, 2006.
Standard Specification for Concrete Brick, ASTM C55-06e1. ASTM International, 2006.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-06b. ASTM International, Inc., 2006.
Standard Specification for Nonloadbearing Concrete Masonry Units, ASTM C129-06. ASTM International, 2006.
Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units, ASTM C744-05. ASTM International, 2005.
Standard Specification for Concrete Facing Brick, ASTM C1634-06. ASTM International, 2006.
Standard Specification for Solid Concrete Interlocking Paving Units, ASTM C936-08. ASTM International, 2008.
Standard Specification for Concrete Grid Paving Units, ASTM C1319-01(2006). ASTM International, 2006.
Standard Specification for Dry-Cast Segmental Retaining Wall Units, ASTM C1372-04e2. ASTM International, 2002.
Standard Specification for Concrete Masonry Units for Construction of Catch Basins and Manholes, ASTM C139-05. ASTM International, 2005.
Standard Specification for Concrete Aggregates, ASTM C33-07. ASTM International, Inc., 2007.
Standard Specification for Lightweight Aggregates for Concrete Masonry Units, C331-05. ASTM International, Inc., 2005.
International Building Code, International Code Council. 2006 and 2009 editions.
Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1-07/TMS 216-07. American Concrete Institute and The Masonry Society, 2007.
Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM E119-08a. ASTM International, Inc., 2008.
Fire Resistance Rating of Concrete Masonry Assemblies, TEK 07-01D, Concrete Masonry & Hardscapes Association, 2018.
Environmental Labels and Declarations – Self-Declared Environmental Claims (Type II Environmental Labeling), ISO 14021-1999. International Organization for Standardization, 1999.
LEED Reference Guide for Green Building Design and Construction, 2009 Edition. U.S. Green Building Council, 2009.
Standard Specification for Portland Cement, ASTM C150-07. ASTM International, 2007.
Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. C618-08a. ASTM International, 2008.
Passive solar design involves utilizing a building’s basic elements walls, windows and floors—to produce a comfortable environment with less reliance on mechanical heating and cooling. Passive solar systems can provide space heating, natural ventilation, cooling load avoidance, daylighting and water heating. The U. S. Department of Energy estimates that 30 to 50% energy cost reductions are economically realistic in new office design with an optimum mix of energy conservation and passive solar design strategies (ref. 1). In addition, most passive solar design strategies integrate well with active solar applications such as photovoltaics.
Concrete masonry plays a vital role in effective passive solar design, by providing thermal mass to absorb and slowly release solar heat. Without sufficient thermal mass, passive solar buildings can overheat and be uncomfortable.
It is most economical to evaluate passive solar strategies early in the design process. The rules of thumb included in this TEK are intended as a starting point for determining preliminary size and location for concrete masonry and glazing. As the design progresses, a more detailed analysis should be performed, preferably using software designed to accommodate passive solar interactions. Some appropriate software programs are briefly discussed near the end of this TEK.
PASSIVE SOLAR IN BUILDING CODES AND LEED
Renewable energy sources, such as passive solar, are typically not explicitly included in energy code criteria. Often, a passive solar building will comply with energy code requirements when the passive solar elements are neglected. Where this is not the case, for example where the prescriptive limit on glazing area is exceeded, most building codes allow compliance using an analysis based on whole building performance. For residential buildings three stories or less in height, Chapter 4 of the International Energy Conservation Code (IECC, ref. 2) describes the criteria for an annual energy analysis to demonstrate compliance.
For commercial and high-rise residential buildings, IECC compliance falls under section 806, Total Building Performance, or via Chapter 7 which references Energy Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE/IESNA 90.1-2001 (ref. 3). Section 11 of Standard 90.1, Energy Cost Budget Method, is used for passive solar compliance. Note that a 2004 edition of ASHRAE 90.1 is also available, but the energy cost budget provisions are essentially the same as those in the 2001 edition. As for IECC Chapter 4, these sections define criteria for the annual whole-building analysis required to demonstrate compliance.
In the LEED program (ref. 4), passive solar systems are included under Energy and Atmosphere Credit 1, Optimize Energy Performance. This credit again references the energy cost budget method defined in Section 11 of ASHRAE/IESNA Standard 90.1 for demonstrating the building energy performance. Note that E&A Credit 2, Renewable Energy, applies only to renewable energy systems that generate power, such as photovoltaics, biomass and wind turbines.
ELEMENTS OF A PASSIVE SOLAR DESIGN
The components of a passive solar design are familiar parts of any building. In a passive solar building, however, these elements are carefully chosen, sized and located to work together to provide comfort. The function of each of these components is briefly described here. Some general guidance regarding sizing and location is included in the section Passive Solar Design Rules of Thumb.
Thermal Mass
Thermal mass in passive solar design provides three functions: it quickly absorbs solar heat to help avoid overheating when the sun shines; it stores the solar heat; and it slowly releases the heat to provide warmth after the sun sets. Concrete masonry walls and concrete paver floors are very efficient thermal storage mediums, and are commonly used in passive solar buildings to provide these functions. Masonry location and thickness are important to passive solar design, as are the conduction, specific heat and density. As these three properties increase, the heat storage effectiveness generally increases accordingly.
However, very high conductances should be avoided since this can shorten the time lag for heat delivery.
One important performance measure of passive solar buildings is the ability to maintain comfortable indoor temperatures. The amplitude of the indoor temperature swing is determined by the amount of effective thermal mass in the building. As the amount of thermal mass increases, the daily indoor temperature swing typically decreases.
Glazing
Glazing allows solar heat and light into the building. Choice of particular glazing products, sizes and locations will vary with the desired heat gain, cooling load avoidance and daylighting needs. These may vary within the building according to how the interior spaces are used. For example, because of glare, areas such as lobbies and atria may be more appropriate on a south-facing wall with a large amount of direct sunlight, than, for example, an office space.
Shading
Appropriate shading helps prevent solar heat gain during the summer. Shading may include permanent overhangs or porch roofs, moveable awnings, shutters, vegetation to shade east and west-facing windows, and/or limiting east/west glass.
Ventilation
Venting can rid the building of heat when the thermal mass is saturated. It can also provide outdoor air to cool the building when the outside air is cooler than the building’s thermostat setting, such as by precooling the building at night. Ventilation can be accomplished using natural ventilation or using an exhaust fan tied to a thermostatic control.
Types of Passive Solar Designs
Passive solar designs can generally be classified as one of three types, depending on where the solar heat is collected relative to where it is used: direct gain, indirect gain or isolated gain. The basic components are illustrated in Figure 1.
Direct Gain Systems
In a direct gain space, solar energy penetrates directly into the space where it is stored and used. Direct gain systems are the simplest to install since only windows and mass are required. Figure 1a shows the proper use of thermal mass in the walls and on the floor. Heat is collected and distributed by transmission through the windows, absorption at the mass surface, and convection and radiation within the room. Using sufficient thermal mass improves performance and comfort.
Indirect Gain
With indirect gain, a thermal storage material is used between the glazing and the space to be heated to collect, store and distribute solar radiation. An example is the trombe wall (see Figure 1b). A trombe wall uses a south-facing masonry wall faced with glazing placed 3/4 to 2 in. (19-51 mm) from the masonry. Heat from sunlight passing through the glass is absorbed by the masonry and slowly transferred through the wall to the interior space. Shading and/or ventilation are used to prevent unwanted heat gains during warmer periods. Vents at the top and bottom of a trombe wall are sometimes included to set up a convective current for passive cooling.
Isolated Gain
Isolated gain systems, such as sunspaces, collect solar energy in an area that can be closed off from the rest of the building. In addition to thermal mass floors, sunspaces typically use concrete masonry walls for thermal storage and as a heat transfer “valve” between the sunspace and the living or working space. Sunspace heat can be moved through vents with backdraft dampers to prevent improper flow. A fan, doors and/or windows can also be used to circulate warm air to the living space.
Because of the potential for overheating, care must be used when designing with sloped (i.e., overhead) glazings. Vertical glazings and pop-up skylights can be used with only a small decrease in performance. Vertical glazing is less expensive than sloped glazing, and overheating is more easily prevented.
PASSIVE SOLAR DESIGN RULES OF THUMB
Rules of thumb are useful early in the design process, as a first step in laying out the building and sizing the systems and materials. The rules of thumb listed below are most appropriate for buildings with skin-dominated heating loads, such as residential and small commercial buildings. Note that many of these design considerations involve compromises. For example, allowing maximum solar heat gain while minimizing summer heat gain. The designer should consider the specifics of the site and climate when evaluating appropriate passive solar design strategies. Software is particularly effective for evaluating these interactions for a particular building. General suggestions for successful passive solar performance include:
Building orientation. Ideally, the building south wall should face within 15 degrees of true south. With this orientation, the building receives maximum winter and minimum summer heat gains. Between 15 and 30 degrees east or west of true south, performance tends to be reduced about 15 percent from the optimum.
Buffer the north side of the building. Place rooms with low heating, lighting and use requirements, such as utility rooms, storage rooms, and garages on the north side of the building to buffer the other spaces. This can reduce the normally higher heat loss through north walls while not interfering with solar access.
Match the solar heating system to the room use. Consider occupancy patterns when choosing a system: what are the heating, daylighting and privacy requirements of the room? For example, a bedroom requires privacy and needs heat after sunset, so a thermal storage wall might be the logical choice. A living room, on the other hand, needs daytime and early evening heat and has higher lighting requirements; therefore, a direct gain system or sunspace may be more appropriate.
Include adequate thermal mass. For buildings with southfacing glass area more than 7% of the floor area, additional thermal mass must be included to absorb heat and maintain comfort. Thermal mass, such as concrete masonry walls and concrete paver floors, should be relatively thin (2 to 4-in. (51 to 102-mm) thick) to allow heat absorption and release within a 24-hour cycle; and should be spread over a large area to help prevent localized hot or cold spots.
Eight-in. (203-mm) fully grouted concrete masonry should be used if the wall is used as a north-south division wall separating two direct gain rooms. Such a wall can store heat on both sides, optimizing the mass storage. Do not fill the cores of these walls with sand, soil or insulation.
For trombe walls, the concrete masonry wall is typically 8 to 16 in. (203 to 406 mm) thick, depending on the desired time lag for heat distribution indoors.
Minimum recommended ratios of thermal mass area to additional glass area (i.e., south-facing glass area > 7%) are (ref. 5):
1: 5.5 for floor in direct sunlight,
1:4 for floor not in direct sunlight, and
1:8.3 for wall and ceilings.
5. Distribute the thermal mass throughout the room. In direct gain systems, the primary collection mass is placed in direct sunlight. In addition to this mass, comfort is improved if mass is distributed evenly around the room because localized hot or cold spots are less likely to develop. Performance is relatively the same whether the mass is located on the east, west or north walls, or in the floor. The mass should be distributed over an interior surface area approximately equal to six times the solar glass area.
6. Avoid “insulating” thermal mass. Rugs or carpets in the solar collection space will significantly impact thermal mass performance. In general, an exposed strip of massive floor about 8 ft (2.4 m) wide provides a good floor collection area.
7. Select an appropriate thermal mass color. Masonry walls can be any color in direct gain systems, although the mass should be somewhat darker (0.5 < a < 0.8) than the low-mass materials. (Absorptivity, a, ranges from 0 to 1, indicating the percentage of incident solar energy that is absorbed. a = 1 indicates 100% absorption) The absorptivity of natural or colored concrete masonry falls in this range without paints or special treatments. Mass walls that are too dark (0.8 < a < 1.0), can result in high surface temperatures where surfaces are exposed directly to the sun, and less absorption elsewhere on the wall. Masonry floors, on the other hand, should be dark (0.7 < a < 1.0) to increase the absorption of the concrete pavers or masonry units and to counter the tendency of the heat to be released too rapidly to the atmosphere. A matt floor finish will maximize absorption and reduce glare. Thermal storage walls should be dark (a > 0.8) or coated with a selective surface material, such as a metallic film specifically designed to maximize absorption and minimize heat loss due to radiation back towards the glass. Materials without significant thermal mass, such as frame walls, should be lighter in color.
Choose appropriate glass. Windows are typically rated by their solar heat gain coefficient (SHGC) and conductance (U-factor). Regardless of climate, the U-factor should be as low as possible (0.35 or less), to minimize conductive heat loss through the windows. For south-facing glass in most climates, choose windows with a SHGC as high as possible (0.6 or higher) to allow maximum heat gain during the winter, and rely on overhangs or other shading to limit summer sun. In cooling climates, such as south Florida, choose windows with a SHGC as low as possible.
Use appropriate window shading. Windows facing within about 30o of true south can be shaded with properly sized overhangs. Figure 2 shows guidelines for sizing the overhangs to allow sunlight entry from about mid September through mid-March (software is also available for sizing overhangs). Off-south wall orientations reduce overhang effectiveness.
East- and west-facing glass can be a significant source of heat gain and glare year-round, because of low morning and evening sun angles. Of the two, west-facing glass is more of a concern, because afternoons are typically hotter than mornings. There are several strategies to address these issues: limit the area of east- and west-facing glass; use wide overhangs (such as porch roofs; smaller overhangs will not be effective); use evergreens for shading; or use glass that blocks solar heat (although this may require different glass types for different walls of the building).
Landscaping. Consider solar access and prevailing wind patterns when choosing trees and shrubs. Issues to consider include: shading of south-, east- and west-facing glass; channeling summer breezes; summer shading of the roof and paved areas; and blocking prevailing winter winds. Because leafless deciduous trees can block as much as 30% of winter solar energy, trees should not be placed where they block the south-facing windows in locations where significant winter solar heating is expected. However, in climates where summer heat is a significant problem, trees on the south-facing side may be appropriate.
SOFTWARE TOOLS
Software to evaluate passive solar buildings should include an annual whole-building analysis, and be able to correctly model solar gains and thermal mass. Programs such as DOE2 and BLAST are very comprehensive and well-documented. However, these programs require a high level of user expertise and can be cumbersome to use.
Energy-10 is a conceptual design tool focused on making whole building trade-offs during early design phases for residential and small commercial buildings (less than 10,000 ft2 (930 m2 ) floor area). The program performs an hourly annual whole-building energy analysis, including dynamic thermal and daylighting calculations. Outputs include a summary table, detailed tabular results and 20 graphical outputs.
EnergyPlus builds on the capabilities of BLAST and DOE2, with some additional simulation capabilities such as time steps of less than an hour. Input and output are via ASCII text files, although the program was developed to facilitate thirdparty user-friendly interfaces (see http://www.eere.energy.gov/ buildings/energyplus/ for available interfaces).
These programs are described on the Department of Energy website, http://www.eere.energy.gov/buildings/tools_directory. The website contains a list of over 300 building analysis programs, searchable by name, subject or platform. The site also includes information on program capabilities, input, output, user expertise required, how to obtain the software, as well as strengths and weaknesses.
REFERENCES
Passive Solar Design. U. S. Department of Energy, www.eere.energy.gov/buildings/info/design/integratedbuilding/passive.html.
2003 International Energy Conservation Code. International Code Council, 2003.
Energy Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE/IESNA 90.1-2001/2004. American Society of Heating, Refrigerating and AirConditioning Engineers, Inc., 2001/2004.
LEED for New Construction and Major Renovations, LEED-NC version 2.2. U. S. Green Building Council, 2005.
Green Building Guidelines: Meeting the Demand for LowEnergy, Resource-Efficient Homes. Sustainable Buildings Industry Council, 2004.
ASHRAE Handbook, Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2005.
COMcheckTM (ref. 1) is software developed by the U.S. Department of Energy specifically for demonstrating compliance with nationally recognized energy codes. Versions are available for download (for various software platforms) as well as for online use. Using the tradeoff compliance method allowed by energy codes, such as COMcheck software, may provide more design flexibility when compared to prescriptive table requirements. For example, parameters such as fenestration area can be increased above the prescriptive limitations, and the additional energy demand offset by adjusting fenestration characteristics and/or increasing roof or wall insulation levels. In addition, once the basic building description has been entered into the program and saved, design changes and/or the building location can be quickly modified, and compliance immediately redetermined. COMcheck has another advantage in that various national and state energy codes and energy standards are included within the program, making it easy for designers who work in several states to be able to use the same compliance tool for many different project locations.
After the building data is entered, COMcheck indicates the percentage by which the proposed building envelope passes or fails the chosen energy code requirements. The program can be downloaded free of charge from: http://www.energycodes.gov/ comcheck. It is advisable to also review the known problems in COMcheck, which are documented on this same site.
This TEK provides a basic overview of the program as well as some guidance on concrete masonry building envelope compliance.
APPLICABILITY
COMcheck enables the user to choose the code and year for compliance. This is a critical first step, as energy code requirements can be significantly different from one edition of the code to the next. If unknown, the local building department can provide this information. Currently, the following codes are included:
the International Energy Conservation Code (ref. 2), IECC (2000, 2001, 2003, 2004, 2006 and 2009 editions), ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings (ref. 3) (2001 2004, 2007 and 2010 editions), and
state energy codes for New York, North Carolina, Oregon and Vermont, as well as for Puerto Rico. COMcheck is applicable to all buildings other than lowrise residential, i.e., most commercial, industrial, hotels and educational buildings as well as residential buildings over three stories in height. For low-rise residential buildings, the program REScheckTM (ref. 4) can be downloaded from http://www.energycodes.gov/comcheck.
BUILDING ENVELOPE COMPLIANCE
After choosing the appropriate code, the Project screen is used to enter the building location (which determines climate) and the building use category, such as school, office or restaurant (which determines the internal heat loads, such as lighting loads). The building’s gross floor area is also entered on the Project screen. Note that for multi story buildings, the total area of all floors is entered, while for single story buildings, the gross floor area is generally equal to roof area. The user can also add descriptive text about the building, client and location.
After the basic information has been entered, the user chooses the Envelope tab to display the envelope compliance screen (see Figure 1). Building envelope data input for COMcheck is straightforward. The user describes the building envelope, component by component, either from a series of drop-down menus or from user-entered data.
Individual envelope elements (roof, skylight, exterior wall, etc.) are chosen from the row above the data table. Then, the table is populated with a description of each element: the element size, R values or U-factors to describe the steady-state resistance to heat transfer, and solar heat gain coefficient (SHGC) for windows.
When the building envelope has been completely described, the software combines this input with the weather data embedded in the program to perform a location-specific analysis.
The output is a pass/fail rating (in the lower left-hand corner of the screen), along with an indication of how close the proposed building is to meeting the specified code requirements. In Figure 1, the proposed building exceeds the minimum code requirements by 1%. This percentage can help the designer understand the building envelope’s sensitivity to various design changes and help optimize building components.
If the program returns Fails, one or more envelope parameters can be quickly modified and compliance immediately redetermined. Thus, the user can choose from various ways to improve the envelope performance, whether it be in the roof insulation, high-performance glazing or wall performance, based on the economics of the products involved and on other project goals or restrictions.
ABOVE-GRADE CONCRETE MASONRY WALLS
Figure 1 shows the above-grade concrete masonry wall options within COMcheck that can be used to demonstrate compliance with the 2009 IECC. Note that the specific walls listed may vary somewhat with the code chosen for compliance.
COMcheck contains a database of precalculated thermal properties or various systems. As a result, once the user chooses a concrete block wall construction, the program applies an associated R-value and, for masonry walls, a heat storage capacity describing the wall’s thermal mass. Figure 1 shows that COMcheck includes various single wythe concrete masonry walls, with or without insulation in the ungrouted cells, as follows:
Concrete Block, Solid Grouted applies to fully grouted masonry walls. The R-value of any insulation installed between furring should be entered under the Cavity Insulation R-Value column, while the R value of continuous insulation should be entered under the Continuous Insulation R-Value column.
Concrete Block, Partially Grouted, Cells Empty applies to masonry with at least 50% of the masonry cells free of grout or cells that are grouted no more than 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally, and with no insulation in the ungrouted cells. Similar to solid masonry, the R-value of any insulation installed between furring should be entered under the Cavity Insulation R-Value column, while the R-value of continuous insulation should be entered under the Continuous Insulation R Value column.
Concrete Block, Partially Grouted, Cells Insulated applies to masonry with at least 50% of the masonry cells free of grout or cells that are grouted no more than 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally, and with insulation in the ungrouted cells. Masonry core insulation is typically molded polystyrene inserts, expanded perlite or vermiculite granular fills or foams (see Insulating Concrete Masonry Walls, TEK 06-11A (ref. 5), for more information on insulating concrete masonry walls). Although the R-value of this cell insulation is already accounted for in the program and need not be entered by the user, note that the U-factor included in COMcheck for cell-insulated concrete masonry is conservative. Often, the actual wall U-factor will be lower (i.e., R value will be higher) than that reflected in the program. In these cases, the user can enter their own wall performance data (see below). Additional insulation installed on the interior or exterior side of the masonry, such as EIFS or insulation between furring, should be entered separately in the Continuous Insulation R-Value or Cavity Insulation R-Value column, respectively.
Concrete Block, Unreinforced, Cells Empty applies to masonry without reinforcement and without insulation in the ungrouted cells. Although by definition these walls do not include reinforcement, up to 50% of the masonry cells are permitted to be grout-filled.
Concrete Block, Unreinforced, Cells Insulated applies to masonry without reinforcement and with insulation in the cells.
In some cases, the concrete masonry wall being used for the project is significantly different from those listed in the program (see refs. 6 and 7 for R-values of concrete masonry walls). For example, a variety of special unit shapes have been developed to increase energy efficiency. These units often have reduced web areas to reduce heat loss due to thermal bridging through the webs. Even conventional concrete masonry units may have significantly better thermal performance than that assumed in COMcheck, because the thermal values in COMcheck for concrete masonry are conservative for many walls. R-values in the COMcheck database for concrete masonry with insulate cells are based on loose fill insulation, which has a relatively low R-value per inch of thickness. In addition, partially grouted walls are assumed to be grouted at 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally.
Buildings with masonry walls utilizing better-performing cell insulation systems, special unit shapes and/or less grout can demonstrate compliance by using the Other option from the exterior wall pull-down menu.
Note that when the Other and Mass options are chosen, the screen displays a new column for heat capacity (which allows COMcheck to distinguish masonry wall requirements from frame wall requirements when determining compliance). When custom data is entered using this option, the user enters both the overall U-factor of the wall (including all insulation and finish materials) as well as the wall heat capacity (see TEK 06-16A, Heat Capacity (HC) Values for Concrete Masonry Walls, ref. 8).
When the building’s exterior is constructed of more than one type of construction (one story is masonry and another is frame, for example), each construction type should be entered into the program as a separate wall. When all above grade exterior walls are the same construction, they can be entered into the program as a single wall, unless the optional wall orientation option is selected.
Choosing Orientation from the Options tab on the main program menu bar allows the user to enter solar orientation (north, east, south or west) for each exterior wall, and displays this information in an additional column on the Envelope screen. When Orientation is not selected, the building envelope assemblies are assumed to be equally distributed. Therefore, compliance results may be slightly different. Selecting Orientation may be an advantage when fenestration for the proposed building has been located to maximize energy efficiency (see Passive Solar Design, TEK 06-05A (ref. 9), for more detailed information).
Multi-Wythe Masonry Walls
Only single wythe masonry walls are explicitly included in the COMcheck drop-down menus. When using multi-wythe walls, such as a masonry cavity wall, the user has two options. The first is to select the backup masonry wythe from COMcheck’s drop-down menu, and enter the R-value of the cavity insulation under Continuous Insulation R-Value, which effectively ignores the masonry veneer. The second option is to determine the overall wall U-factor and heat capacity of the cavity wall (using TEK 06-01C, R-values of Multi-Wythe Concrete Masonry Walls (ref. 7), and TEK 06-16A or other data) and enter this data under the Other wall option described above.
Concrete Masonry Basement Walls
The masonry wall types in COMcheck for basement walls are the same as those listed for above grade walls, and data entry is similar. In addition to gross wall area and continuous or cavity insulation R values, for basement walls the user also enters the basement wall height and the depth below grade (i.e., average grade level to the depth of the basement floor). This information allows the program to account for basement walls that are partially above grade.
Additional Mandatory Requirements
In addition to passing COMcheck’s envelope criteria, there is a list of mandatory requirements that must be met. To access the mandatory requirements, choose View then Mandatory Requirements from the main program menu. After choosing the applicable code, the program displays a list of items that must be accomplished.
For the building envelope, these mandatory requirements include:
installation of insulation: ensuring that insulation is installed without large gaps, without being compressed, and that blown-in insulation is installed to the specified density, to help ensure that the rated R value is achieved,
fenestration and doors: certified to meet air leakage requirements, and
air leakage: requires sealing, caulking, gasketing and/or weather- tripping at joints and penetrations to minimize energy losses and the associated moisture migration due to air leakage through the envelope.
LIGHTING AND MECHANICAL COMPLIANCE
In COMcheck, the mechanical, lighting and envelope compliance are independent of each other (i.e., improved HVAC performance cannot be used to help offset lighting or envelope requirements, for example). The lighting input has a similar format to the envelope, with various lighting fixture types, and drop-down menus for the ballast, number of lamps, wattage per fixture, etc. Similar to the envelope compliance, any combination of lighting components can be used, as long as the total meets the lighting budget for the proposed building. The mandatory lighting requirements primarily cover lighting controls and exterior lighting requirements.
Mechanical compliance for COMcheck is different from the envelope and lighting. The mechanical section generates a list of mandatory requirements based on the list of mechanical components input by the user. So, rather than producing a pass or fail message, the program generates a checklist of requirements that must be met.
International Energy Conservation Code. International Code Council, 2009.
Energy Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE/IESNA Standard 90.1-2010. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. and the Illuminating Engineering Society of North America, 2010.
Single wythe concrete masonry walls are often constructed of hollow units with cores filled with insulation and/or grout. This construction method allows insulation and reinforcement to be used to increase thermal and structural performance, respectively, without increasing the wall thickness.
U-factors and R-values are used to estimate heat flow under steady state conditions (neglecting the effects of thermal mass). These steady-state values can be used in conjunction with factors such as thermal mass, climate, and building orientation to estimate a building envelope’s thermal performance, typically using software.
This TEK lists thermal resistance (R) and thermal transmittance (U) values of single wythe walls. Cavity wall R-values are listed in TEK 06-01C, R-Values of Multi-Wythe Concrete Masonry Walls (ref. 1).
The R-values/U-factors listed in this TEK were determined by calculation using the code-recognized series-parallel (also called isothermal planes) calculation method (refs. 2, 3, 4). The method accounts for the thermal bridging (energy loss) that occurs through the webs of concrete masonry units. The method is fully described in TEK 06-01C. Alternate code approved means of determining R values of concrete masonry walls include two-dimensional calculations and testing (ref. 2).
CONCRETE MASONRY ENERGY PERFORMANCE
Although this TEK presents a compendium of concrete masonry assembly R-values and U-factors, it is important to note that R values/U-factors alone do not fully describe the thermal performance of a concrete masonry assembly.
Concrete masonry’s thermal performance depends on both its steady state thermal characteristics (described by R-value or U-factor) as well as its thermal mass (heat capacity) characteristics. The steady state and mass performance are influenced by the size, type, and configuration of masonry unit, type and location of insulation, finish materials, density of masonry, climate, and building orientation and exposure conditions.
Thermal mass describes the ability of materials to store energy. Because of its comparatively high density and specific heat, masonry provides very effective thermal storage. Masonry walls retain their temperature long after the heat or air-conditioning has shut off. This, in turn, effectively reduces heating and cooling loads, moderates indoor temperature swings, and shifts heating and cooling loads to off-peak hours.
Due to the significant benefits of concrete masonry’s inherent thermal mass, concrete masonry buildings can provide similar energy performance to more heavily insulated light frame buildings.
These thermal mass effects have been incorporated into energy code requirements as well as sophisticated computer models. Due to the thermal mass, energy codes and standards such as the International Energy Conservation Code (IECC) (ref. 5) and Energy Efficient Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE Standard 90.1 (ref. 2), require less insulation in concrete masonry assemblies than equivalent light-frame systems. Although applicable to all climates, the greater benefits of thermal mass tend to be found in warmer climates (lower-numbered Climate Zones).
Although the thermal mass and inherent R-value/U-factor of concrete masonry may be enough to meet energy code requirements (particularly in warmer climates), concrete masonry assemblies may require additional insulation, particularly when designed under more contemporary building code requirements or to achieve above-code thermal performance. For such conditions, there are many options available for insulating concrete masonry construction.
Although in general higher R-values reduce energy flow through a building element, R-values have a diminishing impact on the overall building envelope energy use. In other words, it’s important not to automatically equate higher R-value with improved energy efficiency. As an example, consider a two story elementary school in Bowling Green, Kentucky. If this school is built using single wythe concrete masonry walls with cell insulation only and a resulting wall R-value of 7 hr.ft2.oF/Btu (1.23 m2.K/W), an estimate of the building envelope energy use for this structure is approximately 27,800 Btu/ft2 (87.7 kW.h/m2), as shown in Figure 1. If we increase the R-value of the wall to R14 by adding additional insulation while holding the other envelope variables constant, the building envelope energy use drops by only 2.5%, which is not in proportion to doubling the wall R-value. Figure 1 illustrates this trend: as wall R-value increases, it has less and less impact on the building envelope thermal performance.
In this example, a wall R-value larger than about R12 no longer has a significant impact on the envelope energy use. At this point, it makes more sense to invest in energy efficiency measures other than wall insulation.
When required, concrete masonry can provide assemblies with R values that exceed code minimums. For overall project economy, however, the industry recommends balancing needs and performance expectations with reasonable insulation levels.
ENERGY CODE COMPLIANCE
Compliance with prescriptive energy code requirements can be demonstrated by:
the concrete masonry wall by itself or the concrete masonry wall plus a prescribed R-value of added insulation, or
the overall U-factor of the wall.
The IECC prescriptive R-value table calls for “continuous insulation” on concrete masonry and other mass walls. This refers to insulation uninterrupted by furring or by the webs of concrete masonry units. Examples of continuous insulation include rigid insulation adhered to the interior of the wall with furring and drywall applied over the insulation, continuous insulation in the cavity of a masonry cavity wall, and exterior insulation and finish systems. These and other insulation options for concrete masonry assemblies are discussed in TEK 06 11A, Insulating Concrete Masonry Walls (ref. 6).
If the concrete masonry assembly will not include continuous insulation, there are several other options to comply with the IECC requirements—concrete masonry assemblies are not required to have continuous insulation in order to meet the IECC, regardless of climate zone.
Other compliance methods include: prescriptive U-factor tables, and computer programs which may require U-factors and heat capacity (a property used to indicate the amount of thermal mass) to be input for concrete masonry walls. See TEK 06-04B, Energy Code Compliance Using COMcheck, (ref. 7) for more detailed information. Another compliance method, the energy cost budget method, incorporates sophisticated modeling to estimate a building’s annual energy cost.
A more complete discussion of concrete masonry IECC compliance can be found in TEK 06-12E (for the 2012 IECC) (ref. 8).
CONCRETE MASONRY UNIT CONFIGURATIONS
Revisions in 2011 to ASTM C90¸ Standard Specification for Loadbearing Concrete Masonry Units (ref. 9) have significantly reduced the minimum amount of web material required for CMU. Values in this TEK are based on concrete masonry units with three webs, with each web being the full height of the unit, and having a minimum thickness as provided in historical versions of ASTM C90 (see Table 1).
The changes in C90, however, allow a much wider range of web configurations, with corresponding changes in R-values and U-factors (because the webs of a CMU act as thermal bridges, reducing the CMU web area increases the R-value of the corresponding concrete masonry assembly). More discussion on the impact of web configuration and thermal performance can be found in CMU-TEC 001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (ref. 10).
The Thermal Catalog of Concrete Masonry Assemblies (ref.11) lists R-values and U-factors of traditional units, as included here, as well as wall assemblies with smaller web areas, as now allowed by ASTM C90. The additional wall assemblies are based on:
CMU having two full-height 3/4 in. (19 mm) thick webs, and
a ‘hybrid’ system of CMU, intended to maximize thermal efficiency. The hybrid system uses the two-web units described above for areas requiring a grouted cell, and a one-web unit where grout confinement is not required.
Although the R-values/U-factors in Table 2 are based on typical 8-in. (203-mm) high concrete masonry units, 4-in. (102 mm) high units (commonly called “half-high” units) are also widely available, and other heights may be available in some markets. Because the wall R values vary so little with different unit heights, the values in Table 2 can be applied to units with heights other than 8 in. (203 mm).
U-FACTOR AND R-VALUE TABLES – TRADITIONAL THREE-WEB UNITS
Table 2 lists calculated U-factors and R-values of various thicknesses of concrete masonry walls, for concrete densities of 85 to 135 lb/ft3 (1,362 to 2,163 kg/m3), with various core fills. Table 3 shows the approximate percentage of grouted and ungrouted wall area for different vertical and horizontal grout spacings, which can be used to determine R-values of partially grouted walls (see following section).
In addition to the core insulations listed across the top of Table 2, polystyrene inserts are available which fit in the cores of concrete masonry units. Inserts are available in many shapes and sizes to provide a range of insulating values and accommodate various construction conditions. Specially designed concrete masonry units may incorporate reduced height webs to accommodate inserts. Such webs also reduce thermal bridging through masonry, since the reduced web area provides a smaller cross-sectional area for energy flow. To further reduce thermal bridging, some manufacturers have developed units with two webs rather than three. In addition, some inserts have building code approval to be left in the grouted cores, thus improving the thermal performance of fully or partially grouted masonry walls.
The values for insulated and grouted cores in Table 2 are based on the assumption that all masonry cores are insulated or grouted, respectively. In other words, for ungrouted walls and fully grouted, the values in Table 2 can be used directly. For partially grouted walls, refer to the following section.
R-values of various interior and exterior insulation and finish systems are listed in Table 4. (Note that the use of batt insulation is not recommended, due to its susceptibility moisture.) These R-values can be added to the wall R-values in Table 2. After adding the R-values, the wall U-factor can be found by inverting the total R-value (i.e., U = 1 /R) (see also the following example). Note that tables of precalculated R-values and U-factors, including the various insulation and finish systems, are available in Thermal Catalog of Concrete Masonry Assemblies.
Thermal properties used to compile the tables are listed in Table 5.
R-VALUES AND U-FACTORS OF PARTIALLY GROUTED CONCRETE MASONRY
For partially grouted walls, the values in Table 2 must be modified to account for the grouted cores, using an area weighted average approach. The first step is to determine how much of the wall area is grouted (see Table 3). The U-factor of the wall is calculated from the area-weighted average of the U-factors of the grouted area and ungrouted areas as follows:
For example, consider an 8 in. (203 mm) wall composed of hollow 105 lb/ft3 (1682 kg/m3) concrete masonry, and grouted at 48 in. (1,219 mm) o.c. both vertically and horizontally. The ungrouted cores contain polyurethane foamed-in-place insulation, and the wall is finished on the interior with gypsum wallboard.
REFERENCES
R-Values of Multi-Wythe Concrete Masonry Walls, TEK 06-01C, Concrete Masonry & Hardscapes Association, 2013.
Energy Standard for Buildings Except Low-Rise Residential Buildings, ANSI/ASHRAE/IESNA 90.1-2010. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2010.
ASHRAE Handbook, Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2009.
Guide to Thermal Properties of Concrete and Masonry Systems. ACI 122R-02. American Concrete Institute, 2002.
International Energy Conservation Code. International Code Council, 2006, 2009, 2012.
Multi-wythe concrete masonry construction lends itself to placing insulation between two wythes of masonry when the wythes are separated to form a cavity. Placing insulation between two wythes of masonry offers maximum protection for the insulation while allowing a vast amount of the thermal mass to be exposed to the conditioned interior to help moderate temperatures. Masonry cavity walls can easily meet or exceed energy code requirements, because the cavity installation allows a continuous layer of insulation to envelop the masonry. When properly sealed, this continuous insulation layer can also increase energy efficiency by mitigating air infiltration/exfiltration.
Cavity wall construction provides hard, durable surfaces on both sides of the assembly, efficiently utilizing the inherent impact resistance and low maintenance needs of concrete masonry. While these needs are most commonly associated with multifamily dwellings, hospitals, schools and detention centers, the benefits of resistance to damage from hail, shopping and loading carts, gurneys, motorized chairs, and even sports make cavity construction ideal for any application.
This TEK lists thermal resistance (R) values of multi-wythe walls. Single wythe R-values are listed in TEK 06-02C, R-Values and U Factors of Single Wythe Concrete Masonry Walls (ref. 1).
The R-values listed in this TEK were determined by calculation using the code-recognized series-parallel (also called isothermal planes) calculation method (refs. 2, 3, 4). The method accounts for the thermal bridging (energy loss) that occurs through the webs of concrete masonry units. The method is fully described on page 4 of this TEK. Alternate code-approved means of determining R-values of concrete masonry walls include two dimensional calculations and testing (ref. 2).
CAVITY WALLS
The term cavity insulation, which in some codes refers to the insulation between studs in lightweight framing systems, should not be confused with the long established term “masonry cavity wall.” Cavity walls are comprised of at least two wythes of masonry separated by a continuous airspace (cavity).
Under current building code requirements a 1 in. (25-mm) clear airspace between the insulation and the outer wythe is required (2 in. (51 mm) is preferred) to help ensure free water drainage (ref. 5).
Cavity walls are typically designed and detailed using actual out-to out dimensions. Thus, a 14-in. (356-mm) cavity wall with a nominal 4 in. (102-mm) exterior wythe and 8-in. (203-mm) backup wythe has an actual cavity width of 23/4 in. (68 mm), allowing for 11 /2 in. (38 mm) of rigid board insulation.
Typical cavity walls are constructed with a 4, 6, 8, 10 or 12 in. (102, 152, 203, 254 or 305 mm) concrete masonry backup wythe, a 2 to 41 /2 in. (51 to 114 mm) wide cavity, and a 4-in. (102-mm) masonry veneer. By reference to Specification for Masonry Structures (ref. 6), the International Building Code (ref. 7) allows cavity widths up to 41/2 in. (114 mm), beyond which a detailed wall tie analysis must be performed. More detailed information on cavity walls can be found in References 8 through 11.
Changing the interior finish materials of a multi-wythe assembly does not typically change the overall assembly R-value significantly, unless the finish material itself is insulative. For cavity assemblies with interior-side finish materials installed on furring, such as wood paneling, the R-values for 1/2 in. (13 mm) gypsum wallboard on furring in Table 4 can be used as a very close approximation.
CONCRETE MASONRY ENERGY PERFORMANCE
Although this TEK presents concrete masonry assembly R-values, it is important to note that R-values or U-factors alone do not fully describe the thermal performance of a concrete masonry assembly.
Concrete masonry’s thermal performance depends on both its steady state thermal characteristics (described by R-value or U-factor) as well as its thermal mass (heat capacity) characteristics. The steady state and mass performance are influenced by the size, type, and configuration of masonry unit, type and location of insulation, finish materials, density of masonry, climate, and building orientation and exposure conditions.
Thermal mass describes the ability of materials to store energy. Because of its comparatively high density and specific heat, masonry provides very effective thermal storage. Masonry walls retain their temperature long after the heat or air-conditioning has shut off. This , in turn, effectively reduces heating and cooling loads, moderates indoor temperature swings, and shifts heating and cooling loads to off-peak hours.
Due to the significant benefits of concrete masonry’s inherent thermal mass, concrete masonry buildings can provide similar energy performance to more heavily insulated light frame buildings.
These thermal mass effects have been incorporated into energy code requirements as well as sophisticated computer models. Due to the thermal mass, energy codes and standards such as the International Energy Conservation Code (IECC) (ref. 12) and Energy Efficient Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE Standard 90.1 (ref. 2), require less insulation in concrete masonry assemblies than equivalent light-frame systems. Although applicable to all climates, the greater benefits of thermal mass tend to be found in warmer climates (lower-numbered Climate Zones).
Although the thermal mass and inherent R-value/U-factor of concrete masonry may be enough to meet energy code requirements (particularly in warmer climates), concrete masonry assemblies may require additional insulation, particularly when designed under more contemporary building code requirements or to achieve above-code thermal performance. For such conditions, there are many options available for insulating concrete masonry construction.
Although in general higher R-values reduce energy flow through a building element, R-values have a diminishing impact on the overall building envelope energy use. In other words, it’s important not to automatically equate higher R-value with improved energy efficiency. As an example, consider a two-story elementary school in Bowling Green, Kentucky. If this school is built using single wythe concrete masonry walls with cell insulation only and a resulting wall R-value of 7 hr. ft2.oF/Btu (1.23 m2.K/W), an estimate of the building envelope energy use for this structure is approximately 27,800 Btu/ft2 (87.7 kW. h/m2), as shown in Figure 1. If we increase the R-value of the wall to R14 by adding additional insulation while holding the other envelope variables constant, the building envelope energy use drops by only 2.5%, which is not in proportion to doubling the wall R-value. Figure 1 illustrates this trend: as wall R-value increases, it has less and less impact on the building envelope thermal performance.
In this example, a wall R-value larger than about R12 no longer has a significant impact on the envelope energy use. At this point, it makes more sense to invest in energy efficiency measures other than wall insulation. The effect of adding insulation to a multi-wythe wall is virtually the same.
When required, concrete masonry can provide assemblies with R values that exceed code minimums. For overall project economy, however, the industry recommends balancing needs and performance expectations with reasonable insulation levels.
ENERGY CODE COMPLIANCE
Compliance with prescriptive energy code requirements can be demonstrated by:
the concrete masonry wall by itself or the concrete masonry wall plus a prescribed R-value of added insulation, or
the overall U-factor of the wall.
The IECC prescriptive R-value table calls for “continuous insulation” on concrete masonry and other mass walls. This refers to insulation uninterrupted by furring or by the webs of concrete masonry units. Examples of continuous insulation include rigid insulation adhered to the interior of the wall with furring and drywall applied over the insulation, continuous insulation in the cavity of a masonry cavity wall, and exterior insulation and finish systems. These and other insulation options for concrete masonry assemblies are discussed in TEK 06 11A, Insulating Concrete Masonry Walls (ref. 13).
If the concrete masonry assembly will not include continuous insulation, there are several other options to comply with the IECC requirements—concrete masonry assemblies are not required to have continuous insulation in order to meet the IECC, regardless of climate zone.
Other compliance methods include prescriptive U-factor tables and computer programs which may require U-factors and heat capacity (a property used to indicate the amount of thermal mass) to be input for concrete masonry walls. See TEK 06-04B, Energy Code Compliance Using COMcheck, (ref. 14) for more detailed information. Another compliance method, the energy cost budget method, incorporates sophisticated modeling to estimate a building’s annual energy cost. A more complete discussion of concrete masonry IECC compliance can be found in TEK 06-12E (for the 2012 IECC) (ref. 15).
CONCRETE MASONRY UNIT CONFIGURATIONS
Revisions in 2011 to ASTM C90¸ Standard Specification for Loadbearing Concrete Masonry Units (ref. 16) have significantly reduced the minimum amount of web material required for CMU. Values in this TEK are based on concrete masonry units with three webs, with each web being the full height of the unit, and having a minimum thickness as provided in historical versions of ASTM C90 (see Table 1).
The changes in C90, however, allow a much wider range of web configurations, with corresponding changes in R-values and U-factors (because the webs of a CMU act as thermal bridges, reducing the CMU web area increases the R-value of the corresponding concrete masonry assembly). More discussion on the impact of web configuration and thermal performance can be found in CMU-TEC 001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (Ref. 17).
The Thermal Catalog of Concrete Masonry Assemblies (ref.18) lists R-values and U-factors based on traditional units, as included here, as well as units with smaller web areas, as now allowed by ASTM C90. The additional wall assemblies are based on:
CMU having two full-height 3 /4 in. (19 mm) thick webs, and
a ‘hybrid’ system of CMU, intended to maximize thermal efficiency. The hybrid system uses the two-web units described above for areas requiring a grouted cell, and a one-web unit where grout confinement is not required.
R-VALUE TABLES-TRADITIONAL THREE-WEB UNITS
Table 2 presents R-values of uninsulated concrete masonry cavity walls with 4, 6, 8, 10 and 12 in. (102, 152, 203, 254 and 305 mm) backup wythes and a 4 in. (102 mm) hollow unit concrete masonry veneer. These R-values should be added to the applicable R-values in Tables 3 and 4 to account for cavity insulation and/or interior furring with insulation, respectively. Table 5 contains the thermal data used to develop the tables.
To convert the R-value to U-factor (as may be needed for code compliance), simply invert the R-value, i.e.: U = 1/R. Note that U factors of various wall components cannot be directly added together. To determine the overall cavity wall U-factor, first add the component R-values together, then determine overall U-factor by inverting the total R-value.
As an example, to determine the R-value of a concrete masonry cavity wall with 8 in. (152 mm) 105 pcf (1,682 kg/m3) backup insulated with 2 in. (51 mm) of extruded polystyrene insulation in the cavity, first determine the R-value of the uninsulated wall from Table 2 (4.22 ft2.hr.oF/Btu, 0.74 m2.K/W), then add the cavity insulation R value from Table 3 (10 ft2.hr.oF/Btu, 1.8 m2.K/W), to obtain the total R-value of 14.2 ft2.hr.oF/Btu (2.5 m2.K/W). The corresponding U factor for this wall is:
Note that tables of precalculated R-values and U-factors, including the various insulation and finish systems, are available in Thermal Catalog of Concrete Masonry Assemblies.
The values in Table 2 are based on an ungrouted backup wythe. However, the addition of grout to a hollow concrete masonry backup wythe does not significantly affect the overall R-value of an insulated cavity wall. For example, the R-value of a cavity wall with 8 in. (203 mm) ungrouted 105 pcf (1,682 kg/m3) backup and insulated cavity decreases only about 5% when the backup wythe is solidly grouted. With a partially-grouted backup, the difference in R-value is smaller than 5%.
Calculations are performed using the series-parallel (also called isothermal planes) calculation method (refs. 2, 3, 4). The method accounts for the thermal bridging that occurs through the webs of concrete masonry units. The method is briefly described below, and its use is demonstrated in Appendix C of Thermal Catalog of Concrete Masonry Assemblies.
REFERENCES
R-Values and U-Factors of Single Wythe Concrete Masonry Walls, TEK 06-02C, Concrete Masonry & Hardscapes Association, 2013.
Energy Standard for Buildings Except Low-Rise Residential Buildings, ANSI/ASHRAE/IESNA 90.1-2010. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2010.
ASHRAE Handbook, Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2009.
Guide to Thermal Properties of Concrete and Masonry Systems. ACI 122R-02. American Concrete Institute, 2002.
Building Code Requirements for Masonry Structures, TMS 402/ACI 530/ASCE 5. Reported by the Masonry Standards Joint Committee, 2005, 2008, 2011.
Specification for Masonry Structures, TMS 602/ACI 530.1/ASCE 6. Reported by the Masonry Standards Joint Committee, 2005, 2008, 2011.
International Building Code. International Code Council, 2006, 2009, 2012.
One aspect of concrete masonry that has kept it at the forefront of building materials is its ability to incorporate and reflect a broad spectrum of existing architectural styles, as well as providing the designer with the ability to develop and present unique aesthetic affects and techniques. When skillfully designed, simple materials can provide unparalleled aesthetic enhancement. Inventive patterns, color choices (unit and mortar), unit sizes, and surface finishes (split face and standard) can be used in various concrete masonry bond patterns to evoke a sense of strength, modernity, tradition, or even whimsy.
Within the confines of meeting applicable building codes and specifications, concrete masonry’s modular sizes and range of colors, textures and patterns provide ample opportunity to demonstrate a design technique or overcome design challenges. In addition to the architectural finish, concrete masonry can provide the wall’s structure, fire resistance, acoustic insulation, and energy envelope.
This TEK addresses the proper application of architectural enhancements in concrete masonry wall systems. Where appropriate, related TEK and other documents are referenced to provide further information and detail.
Communication With Clients
Common dilemmas faced by designers are a client’s changing expectations and responses to the project’s changing appearance over time and under varying conditions. As discussed below, there are some basic requirements relative to aesthetics, but these are far from comprehensive. It is important to realize that code requirements primarily govern structural performance, not aesthetics. For example, code required construction tolerances are designed to ensure that masonry units are placed such that the completed wall can act structurally as an integrated unit.
These requirements assume an understanding of the techniques unique to the nature of masonry. The design and construction team should establish and consistently support ground rules affecting aesthetic interpretations of a project. It is also important for the client to realize the aesthetic standard that the project is based on, and that unusually high aesthetic standards can be more costly. In addition, certain high-profile areas, such as a building entrance, may require a custom level of quality, commensurate with an additional cost for the defined area. Several state and local masonry associations have developed guidelines for defining aesthetic requirements, and these can be a good resource for clarifying a project’s aesthetic standards.
Sample panels are a good means to communicate the minimum contract-based aesthetic standard to all parties. The sample panel is typically constructed prior to the project, and in some cases a portion of the work can serve as the sample panel. The sample panel remains in place or at least available until the finished work has been accepted, since it serves as a comparison for the finished work. The sample panel should contain the full acceptable range of unit and mortar color, as well as the minimum expected level of workmanship. Cleaning procedures, as well as application of any coatings or sealants, should also be demonstrated on the sample panel. See TEK 08-04A, Cleaning Concrete Masonry, (ref. 1) for more information on cleaning.
CONSIDERATIONS FOR CHOOSING CONCRETE MASONRY UNITS
Architectural Concrete Masonry Units
One of the most significant architectural benefits of designing with concrete masonry is its versatility—the finished appearance of a concrete masonry wall can be varied with the unit size and shape, color of units and mortar, bond pattern, and surface finish of the units. The term “architectural concrete masonry units” typically is used to describe units displaying any one of several surface finishes that affect the color or texture of the unit, allowing the structural wall and finished surface to be installed in a single step. CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, (ref. 2) provides an overview of some of the more common architectural units, although local manufacturers should be consulted for final unit selection.
Architectural concrete masonry units are used for interior and exterior walls, partitions, terrace walls and other enclosures. Some units are available with the same treatment or pattern on both faces, to serve as both exterior and interior wall finish material, increasing both the economic and aesthetic advantages. Architectural units comply with the same performance-based quality standards as conventional concrete masonry, such as Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90 (ref. 3). See Aesthetics in ASTM C90 (page 4) for more information.
Concrete Masonry Unit Color
Being produced from natural aggregates, concrete masonry has natural color variations from unit to unit. When a more monotone appearance is desired, there are various techniques that may be specified to increase the color uniformity in concrete masonry. Perhaps the best method is to specify the use of mineral pigments in the concrete mix, which are available in a wide range of colors. Pigments provide an integral color throughout the unit and minimize variations in color and texture found naturally in aggregate and sand deposits. Using several colors of integrally-colored concrete masonry units in the same wall is an effective technique for producing other visual impacts, such as two-tone banding or complementary color palates (see Figure 1).
Other methods are also used to improve color uniformity. One method is to specify the use of a post-applied stain, paint or coating on the units. With a paint or coating, the resulting film minimizes the texture of the masonry surface as well as the visual impact of the mortar joints. Paints and coatings for concrete masonry should be compatible with the masonry, and should in general allow for water vapor transmission. TEK 19-01, Water Repellents for Concrete Masonry Walls, (ref. 4) contains information on the applicability of different types of paints and coatings for concrete masonry walls.
A more laborious method to improve color uniformity is to arrange with the masonry contractor for a pre-sorting of on-site supplied block during certain stages of construction.
Interaction With Sunlight
Because it is produced from natural materials, concrete masonry walls often interact with changing sunlight in much the same way that natural stone does, appearing to change color as the light hits the wall at different angles. Figure 2 shows how even a conventional gray concrete masonry wall can interact with sunlight to present a range of color. This same attribute can be used to advantage with electric lighting, as well as on interior walls.
Fluted concrete masonry units provide a rich texture and tend to enhance the sound attenuating properties of concrete masonry.
The vertical flutes also provide an interesting interplay of light and shadow, which can be much more dramatic than smoothfaced units.
MORTAR JOINTS
While mortar generally comprises less than ten percent of a typical concrete masonry wall surface area, it can have a significant impact on the overall aesthetics of the completed structure. Mortar joint finishing, profiles and color can all impact the overall wall aesthetics. See also Concrete Masonry Handbook for Architects, Engineers, Builders (ref. 5) for information on mortar joints.
Mortar Joint Tooling
Tooling refers to finishing the mortar joints with a profiled tool that shapes and compacts the surface of the joint and provides a sharper, cleaner appearance for the wall. The surface shape of the tool determines the joint’s profile (discussed in more detail in the following section). Tooling mortar joints also helps seal the outer surface of the joint to the adjacent masonry unit, improving the joint’s weather resistance. For this reason, tooled joints that compact the mortar and do not create ledges to hold water are recommended for construction that will be exposed to weather.
Mortar joints should be tooled when the mortar is thumbprint hard (a clear thumbprint can be pressed into the mortar without leaving cement paste on the thumb). Tooling the joints before they reach this stage results in lighter colored joints, because more cement paste is brought to the surface of the joints. Joints tooled too early can also subsequently shrink away slightly from the adjacent concrete masonry unit. Tooling at the proper time allows this initial shrinkage to occur, then restores contact between the mortar and the unit producing a more weatherresistant joint. Conversely, later tooling can produce a darker joint. A consistent time of tooling will minimize variations in the final mortar color.
For the cleanest result, horizontal mortar joints should be tooled before vertical joints. For white and light-colored mortar, Plexiglas jointers can be used to avoid staining the joints during tooling. After all joints are tooled, any mortar burrs on the wall should be trimmed off with a trowel or other tool (a tool such as a plastic loop is easier to use on a split face wall than a trowel, for example). As a final step the joints are dressed using a brush, a piece of burlap, or similar material.
Mortar Joint Profiles
Traditional mortar joint profiles are illustrated in Figure 3. For walls not exposed to weather, the joint profile selection can be based on aesthetics and economics (as some joint profiles are more labor intensive to produce). For exterior exposures, however, the mortar joint profile can impact the wall’s weather resistance, as discussed above.
Unless otherwise specified, mortar joints should be tooled to a concave profile when the mortar is thumbprint hard (refs. 6, 7). For walls exposed to weather, concave joints (Figure 3a) improve water penetration resistance by directing water away from the wall surface. In addition, because of the shape of the tool, the mortar is compacted against the concrete masonry unit to seal the joint. V-shaped joints (Figure 3b) result in sharper shadow lines than concave joints.
Grapevine and weather joints (Figures 3c, 3d) provide a water shedding profile, but do not result in the same surface compaction as concave or V-shaped joints. Both are used in interior walls to provide strong horizontal lines.
Beaded joints (Figure 3e) are formed by tooling the extruded mortar into a protruding bead shape. Care must be taken to obtain a straight line with the bead. Although technically a tooled joint, the beaded tooler does not produce the same mortar surface compaction as a concave or V-shaped tool. In addition, the protruding bead can allow water, ice or snow to collect. Therefore, beaded joints are not recommended for weather-exposed construction.
Flush joints (Figure 3f) are typically specified when a wall will be plastered. Excess mortar is simply struck off the face of the wall with the trowel, then dressed with a brush or other tool.
Squeezed or extruded joints (Figure 3g) are made using excess mortar that is squeezed out as units are laid. They may be specified for interior walls.
Struck joints (Figure 3h) provide a strong horizontal line, similar to weather joints, however because the shape provides a ledge for rain, ice or snow, they are not recommended for walls that will be exposed to weather. Raked joints (Figure 3i) provide a dramatic contrast between the units and mortar joints. They are formed using a joint raker, which removes the mortar to a maximum depth of 1/2 in. (13 mm). With raked joints, small imperfections on unit edges can be more noticeable, because the mortar is not compacted against the unit (the compaction tends to fill in small surface irregularities along the unit edge). The resulting joint is not weather-resistant, and may not leave enough mortar cover over horizontal joint reinforcement (joint reinforcement is required to have 5/8 in. (16 mm) mortar cover in walls exposed to weather or earth (refs. 6, 7)). A better option for exterior surfaces is to specify an integrally colored mortar to provide the visual contrast.
Mortar Joint Color
Choosing a specific mortar color allows additional creativity by specifying integral color to either provide a visual contrast or to match the unit color, as shown in Figure 4. Note that using a mortar color that matches the surrounding units minimizes the effects of minor mortar staining; i.e., with a contrasting mortar color, greater care should be used to remove mortar droppings and splatters from the masonry units.
Because foreign material in mortar sand can affect the mortar quality, as well as appearance, ASTM C144, Standard Specification for Aggregate for Masonry Mortar (ref. 8), limits deleterious substances in aggregates for masonry mortars. Sand can also affect mortar color: sands from different natural sources may have different hues. Therefore, all of the sand for a particular project should come from the same source. Silica sand, which is more expensive than typical masonry sand, is often specified for white mortar. Consistent batching and mixing procedures also help produce uniform mortar color from batch to batch. See TEK 03-08A, Concrete Masonry Construction (ref. 9), for further information.
Using a consistent amount of mix water is important to maintain color uniformity for all mortars and especially when using integrally colored mortar. Changing the amount of water can significantly change the resulting mortar color intensity. For this reason there are special methods and equipment, such as shading materials and equipment from direct sunlight, the use of cooled water, and the use of damp, loose sand piles to reduce excessive retempering. Mortar that is too stiff or older than 2 1/2 hours after initial mixing is to be discarded.
EXPECTATIONS FOR UNITS AND CONSTRUCTION
Aesthetics in ASTM C90
ASTM C90 provides minimum requirements for concrete masonry units that assure properties necessary for quality performance. The specification includes requirements for materials, as well as dimensional and physical requirements such as minimum compressive strength, maximum water absorption, maximum dimensional tolerances, and maximum linear drying shrinkage. It also includes finish and appearance criteria for concrete masonry units.
It should be noted that the requirements in ASTM C90 are intended to address the performance of the masonry units when installed, not the aesthetics of the units nor of the constructed masonry. The time for product inspection is before placement. As such, the finish and appearance criteria, for example, prohibits defects that would impair the strength or permanence of the construction, but permit minor cracks or chips incidental to usual manufacturing, shipping and handling methods.
Qualities that are not included in C90 include color, surface texture, surface features such as scores or flutes, density choice, water repellency, fire resistance rating, thermal properties and acoustic properties. If required, these properties must be addressed in project contract documents. ASTM C90 does, however, include acceptance criteria for unit color and surface texture: namely, that the finished unit surfaces that will be exposed in the final structure conform to an approved sample of at least four units. The sample should represent the range of color and texture permitted on the job. As a practical matter, color and texture should be expected to vary somewhat due to the nature of the material.
The ASTM C90 specification is described in more detail in CMU-TEC 001-23, (ref. 2).
Considerations for Integrally Colored Smooth-Faced Units
Integrally-colored concrete masonry units are available in a wide variety of colors and shades. The mineral oxide pigments are evenly dispersed throughout the concrete mix, producing a low-maintenance enhancement that lasts the life of the structure.
During unit manufacture, the integrally-colored concrete mix is placed into a steel mold, which is stripped off while the concrete is still plastic. This stripping of the mold draws moisture and coloring pigment to the unit surface, which impacts the surface appearance. On split-faced or ground-faced units, this surface is either ground away or not exposed (in the case of split-faced units). Because the formed surface is the final exposed surface on smooth-faced units, however, these units will have a wider color variation than is seen with split-faced or ground-faced units. Understanding this color variation will help avoid possible disappointment that the finished wall does not have the color uniformity of a painted or stained wall.
Construction Tolerances
The International Building Code and Specification for Masonry Structures (refs. 6, 7) contain site tolerances for masonry construction which allow for deviations in the construction. The permissible tolerances are intended to ensure that misalignment of units or structural elements does not impede the structural performance of the wall. Although the tolerances are not intended for the purpose of producing an aesthetically pleasing wall, these tolerances are generally adequate for most aesthetic applications as well. If tighter tolerances are desired, they must be specified in the project documents.
As an example, unless otherwise specified, the actual location of a masonry element is required to be within a certain tolerance of where the element is shown on the construction drawings: + 1/2 in. in 20 ft, + 3/4 in. max (+ 13 mm in 6.2 m, + 19 mm max). More precise placement dimensions can be specified, typically at a higher cost.
Tolerances apply to: plumb, alignment, levelness and dimensions of constructed masonry elements, location of elements, levelness of bed joints, mortar joint thickness, and width of collar joints, grout spaces and cavities. A full discussion of code-required masonry construction tolerances is presented in TEK 03-08A, Concrete Masonry Construction (ref 9).
MODULAR COORDINATION
Concrete masonry structures can be constructed using virtually any layout dimension. However, for maximum construction efficiency, economy, and aesthetic benefit, concrete masonry elements should be designed and constructed with modular coordination in mind. Modular coordination is the practice of laying out and dimensioning structures to standard lengths and heights to accommodate modularly-sized building materials.
Standard concrete masonry modules are typically 8 in. (203 mm) vertically and horizontally, but may also include 4-in. (102 mm) modules for some applications. These modules provide the best overall design flexibility and coordination with other building products such as windows and doors. Designing a concrete masonry building to a 4- or 8-in. (102- or 203-mm) module will minimize the number of units that need to be cut, providing a more harmonious looking masonry structure. TEK 05-12, Modular Layout of Concrete Masonry (ref. 10) provides details of modular wall layouts and openings.
CONTROL JOINTS
Control joints, a type of movement joint, are one method used to relieve horizontal tensile stresses due to shrinkage of concrete products and materials. They are essentially vertical planes of weakness built into the wall to reduce restraint and permit longitudinal movement due to anticipated shrinkage. When control joints are required, concrete masonry requires only vertical control joints. When materials with different movement properties are used in the same wythe (such as clay masonry and concrete masonry), this movement difference needs to be accommodated, and may require horizontal movement joints as well (see the Banding section, below). Recommendations for band in a split-faced wall (see Figure 5); with different unit sizes, such as the use of a 4-in. (102-mm) high band in a wall of 8-in. (203-mm) units; or with a combination of these techniques. Combining masonry units of different size, color and finish provides a virtually limitless palette.
The use of concrete masonry bands in clay brick veneer has also become very popular. The architectural effect is very pleasing; however, proper detailing must be provided to accommodate the different movement properties of the two materials to prevent racking. The detail shown in Figure 6 has demonstrated good performance in many areas of the United States and is the preferred detail, as it is economical and maintenance free. Horizontal joint reinforcement is placed in the mortar joints above and below the band, as well as in the band itself if it is more than two courses high. In addition, lateral support (wall ties) are provided within 12 in. (305 mm) of the top and bottom of the band and the band itself must contain at least one row of ties. Some designers prefer placing joint reinforcement in every bed joint of the concrete masonry band. In this case, a tie which accommodates both the tie and reinforcement in the same joint (such as seismic clips) should be used. Another, but less recommended, option is to use horizontal slip planes between clay masonry and the concrete masonry band (see TEK 05-02A, Clay and Concrete Masonry Banding Details, Reference 12).
The maximum spacing of expansion joints in the clay masonry wall should be reduced to no more than 20 ft (6.1 m) when concrete masonry banding is used. When the clay masonry expansion joint spacing exceeds 20 ft (6.1 m), an additional control joint should be placed near mid-panel in the concrete masonry band, although the joint reinforcement should not be cut in this location. At locations control joint spacing, locations and construction details can be found in CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction (ref. 11).
Aesthetically, control joints typically appear as continuous vertical lines in the field of the masonry walls, and perhaps at other areas of stress concentration, such as adjacent to openings, at changes in wall height, etc. Several strategies can be used to make control joints less noticeable. Perhaps the simplest approach is to align the control joint with another architectural feature, such as a pilaster or recess in the wall. In this case, the vertical shadow line provided by the architectural feature provides an inconspicuous control joint location.
BANDING
Concrete masonry banding is successfully used in many architectural applications. Banding can be accomplished with different colors of block; with different textures, for example a smooth-faced of expansion joints in the clay masonry, joints should be continued through the concrete masonry band and the joint reinforcement cut at these locations. TEK 05-02A provides a fuller discussion and additional details for combining these two materials, including details for incorporating clay masonry bands into concrete masonry walls.
LIGHTING DESIGN CONSIDERATIONS FOR CONCRETE MASONRY WALLS
Masonry has historically been associated with diffuse illumination located on or recessed into ceilings, as step (walkway) fixtures located below the waist, or generally placed at a distance from the masonry wall assembly. Diffuse lighting does not concentrate a focused beam but rather spreads the light to provide soft illumination. Although this is sometimes accomplished using an array of many individual light sources at a distance, it is more typically accomplished with fixtures and devices made for this purpose. When wall-mounted light sources are necessary, there are specialized fixtures adapted for masonry that internally refract, reflect, deflect, partially block, diffuse, and/or shade light from directly impinging on the wall surface. Often, the fixture includes additional light diffusers facing away from the wall surface to assist in softly lighting the adjacent area. No noticeable shadows are cast onto the wall, because the shadow is intentionally located away from the wall surface, thus masonry aesthetics are enhanced with a lower lighting intensity and more graceful illumination. These concepts are illustrated in Figure 7.
Non-diffuse light shining onto a concrete masonry wall from a surface mounted light fixture or sconce can sometimes cast unwanted long shadows, giving the erroneous visual appearance of unacceptably poor materials or workmanship (see Figure 7). With non-diffuse light, glossy surface treatments and coatings could also inadvertently magnify this problem. Well-designed diffuse light can eliminate such concerns.
Certain concrete masonry units, such as ground face (also called honed or burnished), can be highly reflective. Figure 8 shows a residential project using a custom-fabricated white ground face block. The designer used a complementary balance of several lighting fixtures with what might have otherwise been a challenging masonry reflective finish. The harmonious use of interior lighting combined with exterior overhead (recessed trim) and step lighting is an effective way of solving this challenge.
Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry & Hardscapes Association, 2023.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-09. ASTM International, 2009.
Water Repellents for Concrete Masonry Walls, TEK 19-01. Concrete Masonry & Hardscapes Association, 2006.
J. A. Farney, Melander, J. M., and Panarese, W. C., Concrete Masonry Handbook for Architects, Engineers, Builders, Sixth Edition, Engineering Bulletin 008. Portland Cement Association, 2008.
International Building Code, International Code Council, 2009.
Specification for Masonry Structures, TMS 602/ACI 530.1/ASCE 6. Reported by the Masonry Standards Joint Committee, 2008.
Standard Specification for Aggregate for Masonry Mortar, ASTM C144-04. ASTM International, 2004.
Concrete masonry offers numerous functional advantages, such as structural load bearing, life and property protection, durability and low maintenance. Half-high concrete masonry units offer the additional advantages of a veneer-like appearance in economical single wythe construction. As for all concrete masonry units, integrally colored half high brick-like units provide enduring strength and lasting resistance to fire and wind while maintaining a virtually maintenance-free façade. These attributes are appealing for both new construction and renovations in historic districts.
Many designers are turning to half-high masonry because of its economy. As an alternative to a traditional cavity wall, these walls offer the same finished appearance, exterior durability and low maintenance coupled with a shorter construction time because of the single wythe loadbearing design. This TEK describes the use of half high units for single wythe masonry construction. For veneer applications, see Refs. 1 and 2.
HALF-HIGH UNITS
Half-high concrete masonry units are produced to the same quality standards as other concrete masonry units. ASTM C 90 (ref. 3) governs physical requirements such as minimum compressive strength, minimum face shell and web thicknesses, finish and appearance, and dimensional tolerances.
Like other concrete masonry units, half-highs are produced in a variety of sizes, unit configurations, colors and surface textures. In addition, special shapes, such as corners and bond beam units are also available.
WALL PERFORMANCE
Structural design considerations for half-high construction are virtually the same as those for conventional concrete masonry units. One aspect that may be different for half-high units is the unit strength. Typical nonarchitectural concrete masonry units have a minimum unit strength of 1,900 psi (13.10 MPa), corresponding to a specified compressive strength of masonry, f’m, of 1,500 psi (10.34 MPa). Half high and other architectural units, however, are typically manufactured to a higher unit strength. Designers should check with producers about the strength of locally available units, with the intent of taking advantage of these higher strengths in their designs when available.
Section properties for half-high units are essentially the same as for full-height units, and the same design aids can be used for both (see Ref. 4). In addition, because the core sizes are also typically the same as for full-height units of the same thickness, considerations for maximum reinforcing bar size as a percentage of the cell area are the same as well. See Ref. 5 for more detailed information.
Because there are more horizontal mortar joints in a wall constructed using half-high units, there is slightly less concrete web area in the wall overall. Although this theoretically reduces the wall weight, in practice the wall weights of walls constructed using half-high units are within 1 psf (0.05 kPa) of those for full height units (see Ref. 6).
To facilitate the construction of bond beams, half-high bond beam units are typically available with depressed webs to accommodate horizontal reinforcement. Grouting two half-high units provides an 8 in. (203-mm) deep bond beam, as shown in Figures 1 through 3. Note that the bottom unit of the bond beam should have depressed webs to accommodate the horizontal reinforcement, but the top unit need not have depressed webs.
Performance criteria for fire resistance, energy efficiency and acoustics of half-high units can be considered to be the same as for similar full height units. See Refs. 7 through 11 for further information. In addition, detailing window openings, door openings, etc., is the same as for single wythe masonry walls constructed using full-height units.
CONSTRUCTION
Construction with half-highs is very similar to that for conventional units. Some differences include: an increased number of courses laid per wall height, greater amount of mortar needed, as well as the difference in bond beam construction noted above. Crack control considerations are the same as for full height units.
As an alternative to supporting trusses by means of a pocket in the masonry wall or by joist hangers, Figure 4 shows a unique application where half-high units have been corbelled out to provide bearing for a wood truss floor. This also provides continuous noncombustible bearing thickness without the need to stagger the joists. See Ref. 12 for additional floor and roof connection details.
As for any single wythe construction, particular care should be taken to prevent water from entering the building interior. Dry walls are attained when both the design and construction address water movement into, through and out of the wall. Considerations include potential sources of water, unit and mortar characteristics, crack control, workmanship, mortar joint tooling, flashing and weeps, sealants, and water repellents. For single wythe masonry, an integral water repellent in both the units and mortar, as well as a compatible post-applied surface water repellent are recommended. See Refs. 13 -18 for more information.
Figure 1 shows a proprietary flashing system that collects and directs water to the exterior of the wall and out weep holes, without compromising the bond at mortar joints in the face shells (see Ref. 15 for recommended flashing locations). There are a number of generic and proprietary flashing, drainage, weep, mortar dropping control, and rain screen systems available. Single wythe flashing details using conventional flashing are included in Ref. 14.
Solid grouted single wythe walls tend to be less susceptible than ungrouted or partially grouted walls to moisture penetration, since voids and cavities where moisture can collect are absent. As a result, solid grouted walls do not require flashing and weeps, although they do require other moisture control provisions, such as sealants and water repellents. For partially grouted walls, flashing should be placed in ungrouted cells.
