Concrete Masonry and Hardscapes AssociationConcrete Masonry and Hardscapes AssociationThe CMHA R-Value/U-Factor Calculator is a spreadsheet-based calculator tool that determines steady state thermal properties of concrete masonry assemblies. Calculations can be performed for a wide variety of unit configurations, grouting schedules, and insulation types. Both single-wythe and multi-wythe calculations can be performed. The results can be used for determining compliance with energy codes as well as inputs for COMcheck and other energy modeling software.
Heat capacity is a material property used to assess a wall’s thermal mass, and it is often used as a criteria in energy codes and standards. Thermal mass is defined as: the absorption and storage of significant amounts of heat in a building or in walls of a building (ref. 1). Wall thermal mass, such as that present in concrete masonry construction, tends to decrease both heating and cooling loads in a given building, thus saving energy. The amount of savings realized by incorporating thermal mass into a building’s design is a function of several variables. These include local climate, wall heat capacity, fenestration (window) area, fenestration orientation, fenestration solar gain, building occupancy load and other internal gains such as lights and office equipment. The most manageable approach to account for energy savings due to thermal mass is to relate the savings to the wall heat capacity and local climate.
Heat capacity (HC) is defined as the amount of heat necessary to raise the temperature of a given mass one degree (refs. 2, 3), and is calculated as the product of a wall’s mass per unit area by its specific heat.
A building with massive walls, such as concrete masonry, often uses less energy for heating and cooling than does one with lightweight frame walls, wood or steel studs for example. Because of this, the International Energy Conservation Code and ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings (refs. 4, 3), prescribe lower R-value requirements for concrete masonry walls than those for frame walls and metal buildings, in many cases. (This lower required R-value corresponds to a higher required U-factor.) In order to qualify for this lower minimum R-value, ASHRAE Standard 90.1 requires the wall to achieve a minimum HC value. The International Energy Conservation Code defines “mass walls” in terms of wall weight, rather than heat capacity (see reference 5 for detailed information).
This TEK is intended for use by engineers and designers as a guide to determining the heat capacity (HC) of concrete masonry walls.
ASHRAE Standard 90.1 defines a mass wall as one with a heat capacity exceeding:
Walls meeting either of these criteria are considered mass walls, and are eligible to comply to the Standard using the lower mass wall requirements in the prescriptive compliance tables.
In addition to these prescriptive compliance tables, heat capacity is also used in the ENVSTD compliance software, that forms a part of ASHRAE Standard 90.1, when defining a mass wall assembly. See TEKs 06-12E, Concrete Masonry in the 2012 Edition of the IECC, and 06-04B, Energy Code Compliance Using COMcheck (refs. 5, 6) for further information on energy code compliance options.
Concrete masonry heat capacity values are also required for more rigorous energy analyses, such as those necessary to demonstrate compliance with the Total Building Performance option in the International Energy Conservation Code, demonstrate compliance with the Energy Cost Budget Method in ASHRAE Standard 90.1, or demonstrate energy savings to qualify for LEED® program points.
Wall heat capacity is defined as: the sum of the products of the mass of each individual material in the wall per unit area times its individual specific heat (ref. 3). As indicated previously, HC is equal to the mass, or wall weight, multiplied by the specific heat. Therefore, for example, a single wythe concrete masonry wall weighing 34 lb/ft 2 (166 kg/m2) has a HC of 7.14 Btu/ft2 · ºF (1.26 kJ/m2.K), as follows:
This simple calculation is based on a rule-of-thumb that the specific heat of most concretes is very close to 0.21 Btu/lb · ºF (880 J/kg.
K). The actual value depends on the aggregate type used to manufacture the concrete masonry units. Concrete masonry units produced using sand-gravel aggregates tend to have specific heat values of approximately 0.22 Btu/lb · °F (922 J/kg. K), while most other units have a specific heat of 0.20 to 0.21 Btu/lb · ºF (840 to 880 J/kg. K) (ref. 2).
Tables 1 through 5 contain calculated heat capacity values for 4-in. to 12-in. (102- to 305-mm) thick concrete masonry walls. The values are based on: minimum face shell and web thickness requirements of Standard Specification for Loadbearing Concrete Masonry Units, ASTM C-90 (ref. 7); 125 pcf (2,003 kg/m3) mortar; and 140 pcf (2,243 kg/m3) grout. For walls not included in these tables, the heat capacity can be calculated by multiplying the wall weight in lb/ft2 (kg/m2) by the specific heat of 0.21 Btu/lb· ºF (880 J/kg. K). Note that concrete masonry wall weights are tabulated in CMU-TEC-002-23, Weights and Section Properties of Concrete Masonry Assemblies (ref. 8).
For multiwythe walls or walls including a finish such as gypsum wallboard or plaster, the heat capacity of the second masonry wythe and/or finish is simply added to that of the first masonry wythe, as follows:
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.
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 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.
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.
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.
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.
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:
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*.
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.
The following sections briefly describe how concrete masonry and hardscape products can contribute to earning points in each of the LEED credit categories.
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.
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.
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.
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.
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.
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.
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.
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%.
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.
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).
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.
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:
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.
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.
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:
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.
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.
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).
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.
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.
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:
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.
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).
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.
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.
Compliance with prescriptive energy code requirements can be demonstrated by:
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).
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:
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:
U = 1/R = 1/14.2 = 0.070 Btu/ hr.oF/Btu (0.4 W/ m2.K)
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.
