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Concrete Masonry in the 2012 Edition of the IECC

  • November 2018

INTRODUCTION

Although masonry is an ancient material, today’s concrete masonry can be a significant benefit to modern sustainable buildings. In addition to its energy efficiency, concrete masonry is a locally produced natural material that is durable and long lived, minimizing the need for repair or replacement. Concrete masonry can incorporate recycled materials, and can itself be reused or recycled at the end of its life. Various architectural finishes are available that can eliminate the need for paint or other coatings which can impair indoor air quality or impede moisture control.

The International Energy Conservation Code (IECC) (ref. 1) serves as a written model for states, counties, cities or other jurisdictions to develop local codes for energy efficient building design. Concrete masonry construction can help meet these energy requirements, while also providing superior structural capacity, durability, and resistance to fire, sound transmission, insects and mold.

This TEK describes concrete masonry wall compliance for commercial buildings in accordance with the 2012 edition of the IECC.

CONCRETE MASONRY ENERGY PERFORMANCE

The thermal performance of a masonry wall depends on its steady state thermal characteristics (described by R-value and U-factor) as well as its thermal mass (described by heat capacity).

The steady-state and thermal mass performance are influenced by the size and type of masonry unit, cross-web configuration, type and location of insulation, finish materials, and density of masonry. Concrete masonry units (CMU) made with lower density concrete have higher R-values (i.e., lower U-factors) than units made with higher density concrete.

Thermal mass is the ability of materials such as concrete masonry to store heat—they heat up and cool down slowly, which can help mitigate heating and cooling loads. Due to the significant benefits of concrete masonry’s inherent thermal mass, concrete masonry buildings can often provide similar thermal performance to more heavily insulated frame buildings. The benefits of thermal mass have been incorporated into energy code requirements and computer models. The IECC and ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings (ref. 2), permit concrete masonry walls to have less insulation than frame wall systems and metal buildings to meet the energy requirements

COMPLIANCE OPTIONS

For commercial buildings, there are three alternatives for demonstrating building energy compliance:

  1. Prescriptive compliance, which requires each building envelope element to independently meet the R-value or U-factor requirements listed in IECC Table C402.1.2 or Table C402.2(1). The approach is simple to implement, but offers no design flexibility.
  2. A building envelope trade-off procedure, which demonstrates that the envelope as a whole meets the energy requirements. This approach allows more design flexibility, since elements that do not meet prescriptive IECC requirements can be offset by other elements with higher performance. Although not specifically referenced in the code, COMcheck software (ref. 3) developed by the U. S. Department of Energy is a commonly accepted compliance tool for demonstrating total envelope performance.
  3. A total building performance analysis, which simulates a full year of building operation. The analysis treats the entire building as a system, and accounts for virtually all aspects of building energy use, including the building envelope, mechanical systems, service water heating, and electric power and lighting. Detailed energy simulation software, such as EnergyPlus or eQuest (refs. 4, 5), is typically used when employing this option. This compliance path offers the maximum design flexibility, but requires a fairly rigorous and detailed analysis. For a project which will be LEED certified, a total building analysis is required, and the prescriptive criteria need not be met. See TEK 06-09C, Concrete Masonry and Hardscape Products in LEED 2009, (ref. 6) for more detailed information.

Note that a project need only comply under one of these compliance options, not all three. The following sections briefly describe prescriptive and trade-off compliance. See the design example at the end of the TEK for how these options may be implemented.

IECC PRESCRIPTIVE COMPLIANCE

Of the three compliance methods, the prescriptive method is easiest to apply. Requirements for building envelope components are listed in table format, by climate zone. Figure 1 illustrates the IECC climate zones.

Under the prescriptive option, requirements for individual elements are independent of each other. Hence, although using the prescriptive tables is very straightforward, it can also be very limiting in terms of design flexibility.

The IECC prescriptive building envelope requirements for commercial buildings (IECC section 402) list minimum energy performance criteria for roofs, above and below grade walls, slab-on-grade floors, and fenestration. The wall, roof and floor requirements are stated in terms of maximum assembly U-factor or minimum insulation R-value. The user may choose to use whichever table is more applicable to the project’s assemblies.

In the IECC prescriptive tables, concrete masonry walls fall under the Mass Wall category, which is defined as walls weighing at least 35 psf (171 kg/m2), or 25 psf (122 kg/m2) if the material weight is not more than 120 lb/ft3 (1,900 kg/m3).

Prescriptive Compliance via Overall Wall U-Factor

The 2012 IECC prescriptive U-factor requirements for above and below grade walls are shown in Table 1. U-factor is numerically the inverse of R-value. So, a wall with an overall R-value of 10 h. ft2.oF/Btu (1.76 m2.oK/W) has a U-factor of 0.10 Btu/h. ft2.oF (0.568 W/m2.oK) and vice-versa.

Using the U-factor criteria, rather than the insulation R-values discussed below, allows walls without continuous insulation (such as concrete masonry with insulated cores) to comply prescriptively. The U-factor compliance table may also be a good option for concrete masonry walls with proprietary inserts, or other walls that have better thermal performance than that assumed in the code.

CMHA TEK 06-02B, R-Values and U-Factors of Single Wythe Concrete Masonry Walls, and TEK 06-01C, R-Values of Multi-Wythe Concrete Masonry Walls, as well as the Thermal Catalog of Concrete Masonry Assemblies (refs. 7, 8, 9) list R-values, and in some cases U-factors, for a wide variety of concrete masonry walls.

Prescriptive Compliance via Insulation R-Value

Table 2 shows minimum insulation R-value requirements for above grade walls. These R-values apply to the insulation only, regardless of the underlying wall’s R-value.

In the table “ci” means continuous insulation. There is a widespread misconception that all walls must have continuous insulation in order to meet the IECC. In fact, continuous insulation is required only to comply with this particular table – other compliance options are available, including compliance via the prescriptive U-factor table discussed above, and COMcheck, discussed below.

Note that in Climate Zones 1 and 2, the IECC includes an exception for single wythe concrete masonry walls with insulated cores. Where the table requires R5.7 continuous insulation, the IECC allows concrete masonry walls with ungrouted cells filled with insulation such as vermiculite, perlite or foamed-in-place (with a thermal conductivity less than or equal to 0.44 Btu-in./h. ft2.oF, or R-value per inch > 2.27 (63.4 W.mm/m2.oK)) to comply, as long as the amount of grouting does not exceed 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally. The exception allows the majority of single wythe ungrouted and partially grouted concrete masonry walls containing insulation in the ungrouted cells to comply with the IECC, regardless of that wall’s R-value.

TRADE-OFF COMPLIANCE USING COMCHECK

The trade-off option allows the user to demonstrate compliance based on the building envelope as a whole, rather than on the prescriptive component-by-component basis. These trade-offs are most often implemented using easy-to-use software, such as COMcheck (ref. 3).

There are two main benefits to using trade-off software for compliance rather than prescriptive tables. First, the user gains design flexibility because parameters such as increased glazing area can be offset by increasing roof or wall insulation. Second, once the basic building data is entered into the program and saved, design changes or building location can be quickly modified, and compliance immediately redetermined.

More detailed information on using COMcheck for concrete masonry buildings can be found in TEK 06-04B, Energy Code Compliance Using COMcheck (ref. 10).

DESIGN EXAMPLE

Consider a “big-box” retail building in St. Louis, MO (Climate Zone 4). For aesthetics and durability, the designer opts for single wythe core insulated 12-in. (305-mm) concrete masonry walls with an overall wall R-value of 6.3 Btu/h. ft2.oF (1.11m2.oK/W).

The first step is to check if the walls comply using one of the prescriptive compliance options, since they are the easiest to implement. Table 1 shows an above grade mass wall requirement of U0.104 h.ft2.oF/Btu (0.59 W/ m2.oK), which corresponds to a wall R-value of 9.6 Btu/h.ft2.oF (1.69 m2.oK/W). The R6.3 wall does not meet this requirement, nor can Table 2 be used because core insulation does not qualify as continuous insulation.

Using COMcheck, however the building can easily comply, by using the prescriptive minimum level of roof insulation (R25). Note that at the time of publication, because COMcheck did not yet include compliance via the 2012 IECC, compliance was demonstrated using ASHRAE 90.1-2010, which the IECC accepts as an alternate. See TEK 06-04B for more information on compliance using COMcheck.

BUILDING ENVELOPE AIR LEAKAGE REQUIREMENTS

The 2012 IECC includes substantially new air leakage criteria, requiring a continuous air barrier throughout the building envelope. The code also includes several “deemed-to-comply” materials and assemblies which are considered to comply with the code, including fully grouted concrete masonry, various surface coatings, and certain board insulations when installed with taped or sealed joints. The criteria, as well as a full discussion of how the criteria apply to concrete masonry assemblies can be found in TEK 06-14A, Control of Air Leakage in Concrete Masonry Walls, (ref. 11) Figures 2 and 3 in particular.

Heat Capacity (HC) Values for Concrete Masonry Walls

  • August 2018

INTRODUCTION

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.

HEAT CAPACITY VALUES IN CODES AND STANDARDS

ASHRAE Standard 90.1 defines a mass wall as one with a heat capacity exceeding:

  • 7 Btu/ft2 °F (45.2 kJ/m2.K), or
  • 5 Btu/ft2 °F (32.2 kJ/m2.K) provided that the wall has a material unit weight not greater than 120 lb/ft3 (1,922 kg/m3).
    This criteria clarifies that most lightweight concrete masonry
    walls are defined as mass walls for the purposes of the
    Standard.

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.

CALCULATING HEAT CAPACITY

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).

Single Wythe Walls

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).

Multiwythe Walls and Finishes

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:

REFERENCES

  1. Guide to Thermal Properties of Concrete and Masonry Systems, ACI 122R-02. American Concrete Institute, 2002.
  2. 2005 ASHRAE Handbook, Fundamentals, Chapter 23. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., 2005.
  3. Energy Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE 90.1. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2004 & 2007.
  4. International Energy Conservation Code. International Code Council, 2003 & 2006.
  5. Concrete Masonry in the 2012 Edition of the IECC, TEK 06-12E, Concrete Masonry & Hardscapes Association, 2012.
  6. Energy Code Compliance Using COMcheck, TEK 06-04B, Concrete Masonry & Hardscapes Association, 2012.
  7. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C-90-06b. ASTM International, Inc., 2006.
  8. Weights and Section Properties of Concrete Masonry Assemblies, CMU-TEC-002-23, Concrete Masonry & Hardscapes Association, 2023.

Insulating Conrete Masonry Walls

  • August 2018

INTRODUCTION

The variety of concrete masonry wall constructions provides for
a number of insulating strategies, including: interior insulation,
insulated cavities, insulation inserts, foamed-in-place insulation,
granular fills in block core spaces, and exterior insulation
systems. Each masonry wall design has different advantages
and limitations with regard to each of these insulation
strategies. The choice of insulation will depend on the desired
thermal properties, climate conditions, ease of construction,
cost, and other design criteria. Note that insulation position
within the wall can impact dew point location, and hence affect
the condensation potential. See TEK 06-17B, Condensation
Control in Concrete Masonry Walls (ref. 1) for more detailed
information. Similarly, some insulations can act as an air barrier
when installed continuously and with sealed joints. See TEK
06-14B, Control of Infiltration in Concrete Masonry Walls, (ref.
2) for further information.

MASONRY THERMAL PERFORMANCE

The thermal performance of a masonry wall depends on its steady state thermal characteristics (described by R-value or U-factor) as well as the thermal mass (heat capacity) characteristics of the wall. The steady state and mass performance are influenced by the size and type of masonry unit, type and location of insulation, finish materials, and density of masonry. Lower density concrete masonry mix designs result in higher R-values (i.e., lower U-factors) than higher density concretes. Thermal mass describes the ability of materials to store heat. Because of its comparatively high density and specific heat, masonry provides very effective thermal storage. Masonry walls remain warm or cool long after the heat or airconditioning 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 performance to more heavily insulated frame buildings.

The benefits of thermal mass have been incorporated into energy code requirements as well as sophisticated computer models. 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/IESNA Standard 90.1 (ref. 6), permit concrete masonry walls to have less insulation than frame wall systems to meet the energy requirements.

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 walls often require additional insulation. When they do, there are many options available for insulating concrete masonry construction. When required, concrete masonry can provide walls with R-values that exceed code minimums (see refs. 3, 4). For overall project economy, however, the industry suggests a parametric analysis to determine reasonable insulation levels for the building envelope elements.

The effectiveness of thermal mass varies with factors such as climate, building design and insulation position. The effects of insulation position are discussed in the following sections. Note, however, that depending on the specific code compliance method chosen, insulation position may not be reflected in a particular code or standard.

There are several methods available to comply with the energy requirements of the IECC. One of the options, the IECC prescriptive R-values (IECC Table 502.2(1)) 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 include rigid insulation adhered to the interior of the wall with furring and drywall applied over the insulation, continuous insulation in a masonry cavity wall, and exterior insulation and finish systems. If the concrete masonry wall will not include continuous insulation, there are several other options to comply with the IECC requirements—concrete masonry walls are not required to have continuous insulation in order to meet the IECC. See TEK 06-12E, Concrete Masonry in the 2012 Edition of the IECC and TEK 06-04B, Energy Code Compliance Using COMcheck (refs. 7, 8).

INTERIOR INSULATION

Interior insulation refers to insulation applied to the interior side of the concrete masonry, as shown in Figure 1. The insulation may be rigid board (extruded or expanded polystyrene or polyisocyanurate), closed-cell spray polyurethane foam, cellular glass, fibrous batt, or fibrous blown-in insulation (note, however, that fibrous insulation is susceptible to moisture). The interior wall surface is usually finished with gypsum wallboard or paneling.

Interior insulation allows for exposed masonry on the exterior, but isolates the masonry from the building’s interior and so may reduce the effects of thermal mass.

With rigid board insulation, an adhesive is used to temporarily hold the insulation in place while mechanical fasteners and a protective finish are applied. Furring may be used and held away from the face of the masonry with spacers. The space created by the spacers provides moisture protection, as well as a convenient and economical location for additional insulation, wiring or pipes.

As an alternative, wood or metal furring can be installed with insulation placed between the furring. The furring size is determined by the type of insulation and R-value required. Because the furring penetrates the insulation, the furring properties must be considered in analyzing the wall’s thermal performance. Steel penetrations through insulation significantly affect the thermal resistance by conducting heat from one side of the insulation to the other. Although not as conductive as metal, the thermal resistance of wood and the cross sectional area of the wood furring penetration should be taken into account when determining overall R-values. See TEK 06-13B, Thermal Bridges in Wall Construction (ref. 9) for more information.

Closed cell spray polyurethane foam is typically installed between interior furring. The foam is applied as a liquid and expands in-place. Proper training helps ensure a quality installation. The foam is resistant to both air and water vapor transmission.

When using interior insulation, concrete masonry can accommodate both vertical and horizontal reinforcement with partial or full grouting without interrupting the insulation layer. The durability, weather resistance, and impact resistance of the exterior of a wall remain unchanged with the addition of interior insulation. Impact resistance on the interior surface is determined by the interior finish.

INTEGRAL INSULATION

Figure 2 illustrates some typical integral insulations in single-wythe masonry walls. Integral insulation refers to insulation placed between two layers of thermal mass. Examples include insulation placed in concrete masonry cores and continuous insulation in a masonry cavity wall (note that an insulated masonry cavity wall can also be considered as exterior insulation if the thermal mass effect of the veneer is disregarded).

With integral insulation, some of the thermal mass (masonry) is directly in contact with the indoor air, which provides excellent thermal mass benefits, while allowing exposed masonry on both the exterior and interior.

Multi-wythe cavity walls contain insulation between two wythes of masonry. The continuous cavity insulation minimizes thermal bridging. The cavity width can be varied to achieve a wide range of R-values. Cavity insulation can be rigid board, closed cell spray polyurethane foam, or loose fill. To further increase the thermal performance, the cores of the backup wythe may be insulated.

When rigid board insulation is used in the cavity, the inner masonry wythe is typically completed first. The insulation is precut or scored by the manufacturer to facilitate placement between the wall ties. The board insulation may be attached with an adhesive or mechanical fasteners. Tight joints between the insulation boards maximize the thermal performance and reduce air leakage. In some cases, the joints between boards are set into an expandable bead of sealant, or caulked or taped to act as an air barrier.

Integral insulations placed in masonry cores are typically molded polystyrene inserts, foams, or expanded perlite or vermiculite granular fills. As for the furring used for interior insulation, the thermal resistance of the concrete masonry webs and any grouted cores should be accounted for when determining the thermal performance of the wall (see TEK 06-02C, ref. 3, for tabulated R-values of walls with core insulation). When using core insulation, the insulation should occupy all ungrouted core spaces (although some rigid inserts are configured to accommodate reinforcing steel and grout in the same cell).

Foamed-in-place insulation is installed in masonry cores after the wall is completed. The installer either fills the cores from the top of the wall or pumps the foam through small holes drilled into the masonry. Foams may be sensitive to temperature, mixing conditions, or other factors. Therefore, manufacturers’ instructions should be carefully followed to avoid excessive shrinkage due to improper mixing or placing of the foam.

Polystyrene inserts may be placed in the cores of conventional masonry units or used in specially designed units. Inserts are available in many shapes and sizes to provide a range of R-values and accommodate various construction conditions. In pre-insulated masonry, the inserts are installed by the manufacturer. Inserts are also available which are installed at the construction site.

Specially designed concrete masonry units may incorporate reduced height webs to accommodate inserts in the cores. Such webs also reduce thermal bridging through masonry, since the reduced web area provides a smaller cross-sectional area for heat flow through a wall. To further reduce thermal bridging, some manufacturers have developed concrete masonry units with two cross webs rather than three.

Vertical and horizontal reinforcement grouted into the concrete masonry cores may be required for structural performance. Cores to be grouted are isolated from cores to be insulated by placing mortar on the webs to confine the grout. Granular or foam insulation is placed in the ungrouted cores within the wall. Thermal resistance is then determined based on the average R-value of the wall area (see TEK 06-02C, ref. 3, for an explanation and example calculation). Some rigid inserts are configured to accommodate reinforcing steel and grout, to provide both thermal protection and structural performance. When inserts are used in grouted construction, the coderequired minimum grout space dimensions must be met (see TEK 03-02A, ref. 10).

Granular fills are placed in masonry cores as the wall is laid up. Usually, the fills are poured directly from bags into the cores. A small amount of settlement usually occurs, but has a relatively small effect on overall performance. Granular fills tend to flow out of any holes in the wall system. Therefore, weep holes should have noncorrosive screens on the interior or wicks to contain the fill while allowing water drainage. Bee holes or other gaps in the mortar joints should be filled. In addition, drilled-in anchors placed after the insulation require special installation procedures to prevent loss of the granular fill.

EXTERIOR INSULATION

Exterior insulated masonry walls are walls that have insulation on the exterior side of the thermal mass. In these walls, continuous exterior insulation envelopes the masonry, minimizing the effect of thermal bridges. This places the thermal mass inside the insulation layer. Exterior insulation keeps masonry directly in contact with the interior conditioned air, providing the most thermal mass benefit of the three insulation strategies.

Exterior insulation also reduces heat loss and moisture movement due to air leakage when joints between the insulation boards are sealed. Exterior insulation negates the aesthetic advantage of exposed masonry. In addition, the insulation requires a protective finish to maintain the durability, integrity, and effectiveness of the insulation.

For exterior stucco installation, a reinforcing mesh is applied to reinforce the finish coating, improving the crack and impact resistance. Fiberglass mesh, corrosion-resistant woven wire mesh or metal lath is used for this purpose. After the mesh is installed, mechanical fasteners are placed through the insulation, to anchor securely into the concrete masonry. Mechanical fasteners can be either metal or nylon, although nylon limits the heat loss through the fasteners.

After the insulation and reinforcing mesh are mechanically fastened to the masonry, a finish coating is troweled onto the surface. This surface gives the wall its final color and texture, as well as providing weather and impact resistance.

BELOW GRADE APPLICATIONS

Below grade masonry walls typically use single-wythe wall construction, which can accommodate interior, integral, or exterior insulation.

Exterior or integral insulation is effective in moderating interior temperatures and in shifting peak energy loads. The typical furring used for interior insulation provides a place to run electric and plumbing lines, as well as being convenient for installing drywall or other interior finishes.

When using exterior or integral insulation strategies, architectural concrete masonry units provide a finished surface on the interior. Using smooth molded units at the wall base facilitates screeding the slab. After casting the slab, a molding strip, also serving as an electric raceway, can be placed against the smooth first course. The remainder of the wall may be constructed of smooth, split-face, split ribbed, ground faced, scored or other architectural concrete masonry units.

Insulation on the exterior of below grade portions of the wall is temporarily held in place by adhesives until the backfill is placed. That portion of the rigid board which extends above grade should be mechanically attached and protected.

REFERENCES

  1. Condensation Control in Concrete Masonry Walls, TEK 06-17B, Concrete Masonry & Hardscapes Association, 2011.
  2. Control of Infiltration in Concrete Masonry Walls, TEK 06-14A, Concrete Masonry & Hardscapes Association, 2011.
  3. R-Values and U-Factors of Single Wythe Concrete Masonry Walls, TEK 06-02C, Concrete Masonry & Hardscapes Association, 2013.
  4. R-Values of Multi-Wythe Concrete Masonry Walls, TEK 06-01C, Concrete Masonry & Hardscapes Association, 2013.
  5. International Energy Conservation Code. International Code Council, 2003, 2006 and 2009.
  6. Energy Efficient Standard for Buildings Except LowRise Residential Buildings, ASHRAE/IESNA Standard 90.1. American Society of Heating, Refrigerating and Air Conditioning Engineers and Illuminating Engineers Society, 2001, 2004 and 2007.
  7. International Energy Conservation Code and Concrete Masonry, TEK 6-12C. Concrete Masonry & Hardscapes Association, 2007.
  8. Energy Code Compliance Using COMcheck TEK 06-04B, Concrete Masonry & Hardscapes Association, 2012.
  9. Thermal Bridges in Wall Construction, TEK 06-13B, Concrete Masonry & Hardscapes Association, 2010.
  10. Grouting Concrete Masonry Walls, TEK 03-02A, Concrete Masonry & Hardscapes Association, 2005.

Concrete Masonry Radiant Heating/Cooling Systems

  • August 2018

INTRODUCTION

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

  1. Passive Solar Design Strategies, TEK 06-05A, Concrete Masonry & Hardscapes Association, 2006.
  2. Howard, B. D. The Air-Core System for Thermal Storage. Passive Solar Journal. Marcel Dekker and ASES, Vol. 3 No. 3, 1986.
  3. Balcomb, J.D., 1982, ‘ Heat Storage Effectiveness of Concrete Masonry Units.’’

Concrete Masonry & Hardscape Products in LEED® 2009

  • August 2018

INTRODUCTION

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).

Materials & Resources Credit—Construction Waste Management

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

  1. LEED 2009 New Construction for Member Ballot. U.S. Green Building Council, 2008.
  2. LEED-NC for New Construction v2.2 Reference Guide. First Edition, U. S. Green Building Council, October 2005.
  3. Articulated Concrete Block for Erosion Control, ACB-TEC-001-14 Concrete Masonry & Hardscapes Association, 2014.
  4. Construction of Permeable Interlocking Concrete Pavement Systems, PAV-TEC-018-22, Concrete Masonry & Hardscapes Association, 2022.
  5. Concrete Grid Pavements, PAV-TEC-008-21, Concrete Masonry & Hardscapes Association, 2021.
  6. Achieving LEED Credits with Segmental Concrete Pavements, PAV-TEC-016-016, Concrete Masonry & Hardscapes Association, 2016.
  7. 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.
  8. 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.
  9. Advanced Buildings Core Performance Guide. New Buildings Institute, 2007.
  10. Standard 90.1-2007 User’s Manual. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2008.
  11. Determining the Recycled Content of Concrete Masonry Products, TEK 06-06B, Concrete Masonry & Hardscapes Association, 2009.
  12. Sound Transmission Class Rating for Concrete Masonry Walls, TEK 13-01D Concrete Masonry & Hardscapes Association, 2012.
  13. Fire Resistance Rating of Concrete Masonry Assemblies, TEK 07-01D Concrete Masonry & Hardscapes Association, 2018.

Earth-Sheltered Buildings

  • August 2018

INTRODUCTION

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

  1. Passive Solar Design, TEK 06-05A, Concrete Masonry & Hardscapes Association, 2006.
  2. Basement Manual: Design and Construction Using Concrete Masonry, CMU-MAN-002-01, Concrete Masonry & Hardscapes Association, 2001.
  3. EERE Consumers Guide: Site-Specific Factors for Earth Sheltered Home Design. U. S. Department of Energy, 2005.
  4. Earth-Sheltered Home Design. U. S. Department of Energy, http://www.eere.energy.gov/consumer/your_home/designing_rem deling/index.cfm/mytopic=10100.
  5. Forowicz, T. Z. An Anlaysis of Different Insulation Strategies for Earth-Sheltered Buildings. ASHRAE Transactions, Vol. 100 Part 2, 1994.
  6. 2005 ASHRAE Handbook Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2005.
  7. 2003 International Residential Code for One and Two Family Dwellings. International Code Council, 2003.

Passive Solar Design

  • August 2018

INTRODUCTION

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

  1. 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.
  2. 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).
  3. 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

  1. Passive Solar Design. U. S. Department of Energy, www.eere.energy.gov/buildings/info/design/integratedbuilding/passive.html.
  2. 2003 International Energy Conservation Code. International Code Council, 2003.
  3. 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.
  4. LEED for New Construction and Major Renovations, LEED-NC version 2.2. U. S. Green Building Council, 2005.
  5. Green Building Guidelines: Meeting the Demand for LowEnergy, Resource-Efficient Homes. Sustainable Buildings Industry Council, 2004.
  6. ASHRAE Handbook, Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2005.

R-Values of Multi-Wythe Concrete Masonry Walls

  • August 2018

INTRODUCTION

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:

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.

REFERENCES

  1. R-Values and U-Factors of Single Wythe Concrete Masonry Walls, TEK 06-02C, Concrete Masonry & Hardscapes Association, 2013.
  2. 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.
  3. ASHRAE Handbook, Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2009.
  4. Guide to Thermal Properties of Concrete and Masonry Systems. ACI 122R-02. American Concrete Institute, 2002.
  5. Building Code Requirements for Masonry Structures, TMS 402/ACI 530/ASCE 5. Reported by the Masonry Standards Joint Committee, 2005, 2008, 2011.
  6. Specification for Masonry Structures, TMS 602/ACI 530.1/ASCE 6. Reported by the Masonry Standards Joint Committee, 2005, 2008, 2011.
  7. International Building Code. International Code Council, 2006, 2009, 2012.
  8. Concrete Masonry Veneers, TEK 03-06C, Concrete Masonry & Hardscapes Association, 2012.
  9. Concrete Masonry Veneer Details, TEK 05-01B, Concrete Masonry & Hardscapes Association, 2003.
  10. Design of Concrete Masonry Noncomposite (Cavity) Walls, TEK 16-04A, Concrete Masonry & Hardscapes Association, 2004.
  11. Flashing Details for Concrete Masonry Walls, TEK 19-05A,
    Concrete Masonry & Hardscapes Association, 2008.
  12. International Energy Conservation Code. International Code Council, 2006, 2009, 2012.
  13. Insulating Concrete Masonry Walls, TEK 06-11A, Concrete
    Masonry & Hardscapes Association, 2010.
  14. Energy Code Compliance Using COMcheck, TEK 06-04B,
    Concrete Masonry & Hardscapes Association, 2012.
  15. Concrete Masonry in the 2012 Edition of the IECC, TEK 06-12E, Concrete Masonry & Hardscapes Association, 2012.
  16. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-11. ASTM International, 2011.
  17. Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry &
    Hardscapes Association, 2023.
  18. Thermal Catalog of Concrete Masonry Assemblies, Second Edition, CMU-MAN-004-12, Concrete Masonry & Hardscapes Association, 2012.