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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.

Energy Code Compliance Using Comcheck™

  • August 2018

INTRODUCTION

COMcheckTM (ref. 1) is software developed by the U.S. Department of Energy specifically for demonstrating compliance with nationally recognized energy codes. Versions are available for download (for various software platforms) as well as for online use. Using the tradeoff compliance method allowed by energy codes, such as COMcheck software, may provide more design flexibility when compared to prescriptive table requirements. For example, parameters such as fenestration area can be increased above the prescriptive limitations, and the additional energy demand offset by adjusting fenestration characteristics and/or increasing roof or wall insulation levels. In addition, once the basic building description has been entered into the program and saved, design changes and/or the building location can be quickly modified, and compliance immediately redetermined. COMcheck has another advantage in that various national and state energy codes and energy standards are included within the program, making it easy for designers who work in several states to be able to use the same compliance tool for many different project locations.

After the building data is entered, COMcheck indicates the percentage by which the proposed building envelope passes or fails the chosen energy code requirements. The program can be downloaded free of charge from: http://www.energycodes.gov/
comcheck. It is advisable to also review the known problems in COMcheck, which are documented on this same site.

This TEK provides a basic overview of the program as well as some guidance on concrete masonry building envelope compliance.

APPLICABILITY

COMcheck enables the user to choose the code and year for compliance. This is a critical first step, as energy code requirements can be significantly different from one edition of the code to the next. If unknown, the local building department can provide this information. Currently, the following codes are included:

  • the International Energy Conservation Code (ref. 2), IECC (2000, 2001, 2003, 2004, 2006 and 2009 editions), ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings (ref. 3) (2001 2004, 2007 and 2010 editions), and
  • state energy codes for New York, North Carolina, Oregon and Vermont, as well as for Puerto Rico. COMcheck is applicable to all buildings other than lowrise residential, i.e., most commercial, industrial, hotels and educational buildings as well as residential buildings over three stories in height. For low-rise residential buildings, the program REScheckTM (ref. 4) can be downloaded from http://www.energycodes.gov/comcheck.

BUILDING ENVELOPE COMPLIANCE

After choosing the appropriate code, the Project screen is used to enter the building location (which determines climate) and the building use category, such as school, office or restaurant (which determines the internal heat loads, such as lighting loads). The building’s gross floor area is also entered on the Project screen. Note that for multi story buildings, the total area of all floors is entered, while for single story buildings, the gross floor area is generally equal to roof area. The user can also add descriptive text about the building, client and location.

After the basic information has been entered, the user chooses the Envelope tab to display the envelope compliance screen (see Figure 1). Building envelope data input for COMcheck is straightforward. The user describes the building envelope, component by component, either from a series of drop-down menus or from user-entered data.

Individual envelope elements (roof, skylight, exterior wall, etc.) are chosen from the row above the data table. Then, the table is populated with a description of each element: the element size, R values or U-factors to describe the steady-state resistance to heat transfer, and solar heat gain coefficient (SHGC) for windows.

When the building envelope has been completely described, the software combines this input with the weather data embedded in the program to perform a location-specific analysis.

The output is a pass/fail rating (in the lower left-hand corner of the screen), along with an indication of how close the proposed building is to meeting the specified code requirements. In Figure 1, the proposed building exceeds the minimum code requirements by 1%. This percentage can help the designer understand the building envelope’s sensitivity to various design changes and help optimize building components.

If the program returns Fails, one or more envelope parameters can be quickly modified and compliance immediately redetermined. Thus, the user can choose from various ways to improve the envelope performance, whether it be in the roof insulation, high-performance glazing or wall performance, based on the economics of the products involved and on other project goals or restrictions.

ABOVE-GRADE CONCRETE MASONRY WALLS

Figure 1 shows the above-grade concrete masonry wall options within COMcheck that can be used to demonstrate compliance with the 2009 IECC. Note that the specific walls listed may vary somewhat with the code chosen for compliance.

COMcheck contains a database of precalculated thermal properties or various systems. As a result, once the user chooses a concrete block wall construction, the program applies an associated R-value and, for masonry walls, a heat storage capacity describing the wall’s thermal mass. Figure 1 shows that COMcheck includes various single wythe concrete masonry walls, with or without insulation in the ungrouted cells, as follows:

Concrete Block, Solid Grouted applies to fully grouted masonry walls. The R-value of any insulation installed between furring should be entered under the Cavity Insulation R-Value column, while the R value of continuous insulation should be entered under the Continuous Insulation R-Value column.

  • Concrete Block, Partially Grouted, Cells Empty applies to masonry with at least 50% of the masonry cells free of grout or cells that are grouted no more than 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally, and with no insulation in the ungrouted cells. Similar to solid masonry, the R-value of any insulation installed between furring should be entered under the Cavity Insulation R-Value column, while the R-value of continuous insulation should be entered under the Continuous Insulation R Value column.
  • Concrete Block, Partially Grouted, Cells Insulated applies to masonry with at least 50% of the masonry cells free of grout or cells that are grouted no more than 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally, and with insulation in the ungrouted cells. Masonry core insulation is typically molded polystyrene inserts, expanded perlite or vermiculite granular fills or foams (see Insulating Concrete Masonry Walls, TEK 06-11A (ref. 5), for more information on insulating concrete masonry walls). Although the R-value of this cell insulation is already accounted for in the program and need not be entered by the user, note that the U-factor included in COMcheck for cell-insulated concrete masonry is conservative. Often, the actual wall U-factor will be lower (i.e., R value will be higher) than that reflected in the program. In these cases, the user can enter their own wall performance data (see below). Additional insulation installed on the interior or exterior side of the masonry, such as EIFS or insulation between furring, should be entered separately in the Continuous Insulation R-Value or Cavity Insulation R-Value column, respectively.
  • Concrete Block, Unreinforced, Cells Empty applies to masonry without reinforcement and without insulation in the ungrouted cells. Although by definition these walls do not include reinforcement, up to 50% of the masonry cells are permitted to be grout-filled.
  • Concrete Block, Unreinforced, Cells Insulated applies to masonry without reinforcement and with insulation in the cells.

In some cases, the concrete masonry wall being used for the project is significantly different from those listed in the program (see refs. 6 and 7 for R-values of concrete masonry walls). For example, a variety of special unit shapes have been developed to increase energy efficiency. These units often have reduced web areas to reduce heat loss due to thermal bridging through the webs. Even conventional concrete masonry units may have significantly better thermal performance than that assumed in COMcheck, because the thermal values in COMcheck for concrete masonry are conservative for many walls. R-values in the COMcheck database for concrete masonry with insulate cells are based on loose fill insulation, which has a relatively low R-value per inch of thickness. In addition, partially grouted walls are assumed to be grouted at 32 in. (813 mm) o.c. vertically and 48 in. (1,219 mm) o.c. horizontally.

Buildings with masonry walls utilizing better-performing cell insulation systems, special unit shapes and/or less grout can demonstrate compliance by using the Other option from the exterior wall pull-down menu.

Note that when the Other and Mass options are chosen, the screen displays a new column for heat capacity (which allows COMcheck to distinguish masonry wall requirements from frame wall requirements when determining compliance). When custom data is entered using this option, the user enters both the overall U-factor of the wall (including all insulation and finish materials) as well as the wall heat capacity (see TEK 06-16A, Heat Capacity (HC) Values for Concrete Masonry Walls, ref. 8).

When the building’s exterior is constructed of more than one type of construction (one story is masonry and another is frame, for example), each construction type should be entered into the program as a separate wall. When all above grade exterior walls are the same construction, they can be entered into the program as a single wall, unless the optional wall orientation option is selected.

Choosing Orientation from the Options tab on the main program menu bar allows the user to enter solar orientation (north, east, south or west) for each exterior wall, and displays this information in an additional column on the Envelope screen. When Orientation is not selected, the building envelope assemblies are assumed to be equally distributed. Therefore, compliance results may be slightly different. Selecting Orientation may be an advantage when fenestration for the proposed building has been located to maximize energy efficiency (see Passive Solar Design, TEK 06-05A (ref. 9), for more detailed information).

Multi-Wythe Masonry Walls

Only single wythe masonry walls are explicitly included in the COMcheck drop-down menus. When using multi-wythe walls, such as a masonry cavity wall, the user has two options. The first is to select the backup masonry wythe from COMcheck’s drop-down menu, and enter the R-value of the cavity insulation under Continuous Insulation R-Value, which effectively ignores the masonry veneer. The second option is to determine the overall wall U-factor and heat capacity of the cavity wall (using TEK 06-01C, R-values of Multi-Wythe Concrete Masonry Walls (ref. 7), and TEK 06-16A or other data) and enter this data under the Other wall option described above.

Concrete Masonry Basement Walls

The masonry wall types in COMcheck for basement walls are the same as those listed for above grade walls, and data entry is similar. In addition to gross wall area and continuous or cavity insulation R values, for basement walls the user also enters the basement wall height and the depth below grade (i.e., average grade level to the depth of the basement floor). This information allows the program to account for basement walls that are partially above grade.

Additional Mandatory Requirements

In addition to passing COMcheck’s envelope criteria, there is a list of mandatory requirements that must be met. To access the mandatory requirements, choose View then Mandatory Requirements from the main program menu. After choosing the applicable code, the program displays a list of items that must be accomplished.

For the building envelope, these mandatory requirements include:

  • installation of insulation: ensuring that insulation is installed without large gaps, without being compressed, and that blown-in insulation is installed to the specified density, to help ensure that the rated R value is achieved,
  • fenestration and doors: certified to meet air leakage requirements, and
  • air leakage: requires sealing, caulking, gasketing and/or weather- tripping at joints and penetrations to minimize energy losses and the associated moisture migration due to air leakage through the envelope.

LIGHTING AND MECHANICAL COMPLIANCE

In COMcheck, the mechanical, lighting and envelope compliance are independent of each other (i.e., improved HVAC performance cannot be used to help offset lighting or envelope requirements, for example). The lighting input has a similar format to the envelope, with various lighting fixture types, and drop-down menus for the ballast, number of lamps, wattage per fixture, etc. Similar to the envelope compliance, any combination of lighting components can be used, as long as the total meets the lighting budget for the proposed building. The mandatory lighting requirements primarily cover lighting controls and exterior lighting requirements.

Mechanical compliance for COMcheck is different from the envelope and lighting. The mechanical section generates a list of mandatory requirements based on the list of mechanical components input by the user. So, rather than producing a pass or fail message, the program generates a checklist of requirements that must be met.

REFERENCES

  1. COMcheckTM, version 3.9.0 (build version 3.9.0.3). United States Department of Energy, http://www.energycodes.gov/comcheck, 2011.
  2. International Energy Conservation Code. International Code Council, 2009.
  3. Energy Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE/IESNA Standard 90.1-2010. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. and the Illuminating Engineering Society of North America, 2010.
  4. REScheckTM, version 4.4.2. United States Department of Energy, http://www.energycodes.gov/comcheck, 2011.
  5. Insulating Concrete Masonry Walls, TEK 06-11A, Concrete Masonry & Hardscapes Association, 2010.
  6. R-Values for Single Wythe Concrete Masonry Walls, TEK 06-02C, Concrete Masonry & Hardscapes Association, 2013.
  7. R-Values of Multi-Wythe Concrete Masonry Walls, TEK 06-01B, Concrete Masonry & Hardscapes Association, 2013.
  8. Heat Capacity (HC) Values for Concrete Masonry Walls, TEK 06 16A, Concrete Masonry & Hardscapes Association, 2008.
  9. Passive Solar Design, TEK 06-05A, Concrete Masonry & Hardscapes Association, 2006.

R-Values and U-Factors of Single Wythe Concrete Masonry Walls

  • August 2018

INTRODUCTION

Single wythe concrete masonry walls are often constructed of hollow units with cores filled with insulation and/or grout. This construction method allows insulation and reinforcement to be used to increase thermal and structural performance, respectively, without increasing the wall thickness.

U-factors and R-values are used to estimate heat flow under steady state conditions (neglecting the effects of thermal mass). These steady-state values can be used in conjunction with factors such as thermal mass, climate, and building orientation to estimate a building envelope’s thermal performance, typically using software.

This TEK lists thermal resistance (R) and thermal transmittance (U) values of single wythe walls. Cavity wall R-values are listed in TEK 06-01C, R-Values of Multi-Wythe Concrete Masonry Walls (ref. 1).

The R-values/U-factors listed in this TEK were determined by calculation using the code-recognized series-parallel (also called isothermal planes) calculation method (refs. 2, 3, 4). The method accounts for the thermal bridging (energy loss) that occurs through the webs of concrete masonry units. The method is fully described in TEK 06-01C. Alternate code approved means of determining R values of concrete masonry walls include two-dimensional calculations and testing (ref. 2).

CONCRETE MASONRY ENERGY PERFORMANCE

Although this TEK presents a compendium of concrete masonry assembly R-values and U-factors, it is important to note that R values/U-factors alone do not fully describe the thermal performance of a concrete masonry assembly.

Concrete masonry’s thermal performance depends on both its steady state thermal characteristics (described by R-value or U-factor) as well as its thermal mass (heat capacity) characteristics. The steady state and mass performance are influenced by the size, type, and configuration of masonry unit, type and location of insulation, finish materials, density of masonry, climate, and building orientation and exposure conditions.

Thermal mass describes the ability of materials to store energy. Because of its comparatively high density and specific heat, masonry provides very effective thermal storage. Masonry walls retain their temperature long after the heat or air-conditioning has shut off. This, in turn, effectively reduces heating and cooling loads, moderates indoor temperature swings, and shifts heating and cooling loads to off-peak hours.

Due to the significant benefits of concrete masonry’s inherent thermal mass, concrete masonry buildings can provide similar energy performance to more heavily insulated light frame buildings.

These thermal mass effects have been incorporated into energy code requirements as well as sophisticated computer models. Due to the thermal mass, energy codes and standards such as the International Energy Conservation Code (IECC) (ref. 5) and Energy Efficient Standard for Buildings Except Low-Rise Residential Buildings, ASHRAE Standard 90.1 (ref. 2), require less insulation in concrete masonry assemblies than equivalent light-frame systems. Although applicable to all climates, the greater benefits of thermal mass tend to be found in warmer climates (lower-numbered Climate Zones).

Although the thermal mass and inherent R-value/U-factor of concrete masonry may be enough to meet energy code requirements (particularly in warmer climates), concrete masonry assemblies may require additional insulation, particularly when designed under more contemporary building code requirements or to achieve above-code thermal performance. For such conditions, there are many options available for insulating concrete masonry construction.

Although in general higher R-values reduce energy flow through a building element, R-values have a diminishing impact on the overall building envelope energy use. In other words, it’s important not to automatically equate higher R-value with improved energy efficiency. As an example, consider a two story elementary school in Bowling Green, Kentucky. If this school is built using single wythe concrete masonry walls with cell insulation only and a resulting wall R-value of 7 hr.ft2.oF/Btu (1.23 m2.K/W), an estimate of the building envelope energy use for this structure is approximately 27,800 Btu/ft2 (87.7 kW.h/m2), as shown in Figure 1. If we increase the R-value of the wall to R14 by adding additional insulation while holding the other envelope variables constant, the building envelope energy use drops by only 2.5%, which is not in proportion to doubling the wall R-value. Figure 1 illustrates this trend: as wall R-value increases, it has less and less impact on the building envelope thermal performance.

In this example, a wall R-value larger than about R12 no longer has a significant impact on the envelope energy use. At this point, it makes more sense to invest in energy efficiency measures other than wall insulation.

When required, concrete masonry can provide assemblies with R values that exceed code minimums. For overall project economy, however, the industry recommends balancing needs and performance expectations with reasonable insulation levels.

ENERGY CODE COMPLIANCE

Compliance with prescriptive energy code requirements can be demonstrated by:

  • the concrete masonry wall by itself or the concrete masonry wall plus a prescribed R-value of added insulation, or
  • the overall U-factor of the wall.

The IECC prescriptive R-value table calls for “continuous insulation” on concrete masonry and other mass walls. This refers to insulation uninterrupted by furring or by the webs of concrete masonry units. Examples of continuous insulation include rigid insulation adhered to the interior of the wall with furring and drywall applied over the insulation, continuous insulation in the cavity of a masonry cavity wall, and exterior insulation and finish systems. These and other insulation options for concrete masonry assemblies are discussed in TEK 06 11A, Insulating Concrete Masonry Walls (ref. 6).

If the concrete masonry assembly will not include continuous insulation, there are several other options to comply with the IECC requirements—concrete masonry assemblies are not required to have continuous insulation in order to meet the IECC, regardless of climate zone.

Other compliance methods include: prescriptive U-factor tables, and computer programs which may require U-factors and heat capacity (a property used to indicate the amount of thermal mass) to be input for concrete masonry walls. See TEK 06-04B, Energy Code Compliance Using COMcheck, (ref. 7) for more detailed information. Another compliance method, the energy cost budget method, incorporates sophisticated modeling to estimate a building’s annual energy cost.

A more complete discussion of concrete masonry IECC compliance can be found in TEK 06-12E (for the 2012 IECC) (ref. 8).

CONCRETE MASONRY UNIT CONFIGURATIONS

Revisions in 2011 to ASTM C90¸ Standard Specification for Loadbearing Concrete Masonry Units (ref. 9) have significantly reduced the minimum amount of web material required for CMU. Values in this TEK are based on concrete masonry units with three webs, with each web being the full height of the unit, and having a minimum thickness as provided in historical versions of ASTM C90 (see Table 1).

The changes in C90, however, allow a much wider range of web configurations, with corresponding changes in R-values and U-factors (because the webs of a CMU act as thermal bridges, reducing the CMU web area increases the R-value of the corresponding concrete masonry assembly). More discussion on the impact of web configuration and thermal performance can be found in CMU-TEC 001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (ref. 10).

The Thermal Catalog of Concrete Masonry Assemblies (ref.11) lists R-values and U-factors of traditional units, as included here, as well as wall assemblies with smaller web areas, as now allowed by ASTM C90. The additional wall assemblies are based on:

  • CMU having two full-height 3/4 in. (19 mm) thick webs, and
  • a ‘hybrid’ system of CMU, intended to maximize thermal efficiency. The hybrid system uses the two-web units described above for areas requiring a grouted cell, and a one-web unit where grout confinement is not required.

Although the R-values/U-factors in Table 2 are based on typical 8-in. (203-mm) high concrete masonry units, 4-in. (102 mm) high units (commonly called “half-high” units) are also widely available, and other heights may be available in some markets. Because the wall R values vary so little with different unit heights, the values in Table 2 can be applied to units with heights other than 8 in. (203 mm).

U-FACTOR AND R-VALUE TABLES – TRADITIONAL THREE-WEB UNITS

Table 2 lists calculated U-factors and R-values of various thicknesses of concrete masonry walls, for concrete densities of 85 to 135 lb/ft3 (1,362 to 2,163 kg/m3), with various core fills. Table 3 shows the approximate percentage of grouted and ungrouted wall area for different vertical and horizontal grout spacings, which can be used to determine R-values of partially grouted walls (see following section).

In addition to the core insulations listed across the top of Table 2, polystyrene inserts are available which fit in the cores of concrete masonry units. Inserts are available in many shapes and sizes to provide a range of insulating values and accommodate various construction conditions. Specially designed concrete masonry units may incorporate reduced height webs to accommodate inserts. Such webs also reduce thermal bridging through masonry, since the reduced web area provides a smaller cross-sectional area for energy flow. To further reduce thermal bridging, some manufacturers have developed units with two webs rather than three. In addition, some inserts have building code approval to be left in the grouted cores, thus improving the thermal performance of fully or partially grouted masonry walls.

The values for insulated and grouted cores in Table 2 are based on the assumption that all masonry cores are insulated or grouted, respectively. In other words, for ungrouted walls and fully grouted, the values in Table 2 can be used directly. For partially grouted walls, refer to the following section.

R-values of various interior and exterior insulation and finish systems are listed in Table 4. (Note that the use of batt insulation is not recommended, due to its susceptibility moisture.) These R-values can be added to the wall R-values in Table 2. After adding the R-values, the wall U-factor can be found by inverting the total R-value (i.e., U = 1 /R) (see also the following example). Note that tables of precalculated R-values and U-factors, including the various insulation and finish systems, are available in Thermal Catalog of Concrete Masonry Assemblies.

Thermal properties used to compile the tables are listed in Table 5.

R-VALUES AND U-FACTORS OF PARTIALLY GROUTED CONCRETE MASONRY

For partially grouted walls, the values in Table 2 must be modified to account for the grouted cores, using an area weighted average approach. The first step is to determine how much of the wall area is grouted (see Table 3). The U-factor of the wall is calculated from the area-weighted average of the U-factors of the grouted area and ungrouted areas as follows:

For example, consider an 8 in. (203 mm) wall composed of hollow 105 lb/ft3 (1682 kg/m3) concrete masonry, and grouted at 48 in. (1,219 mm) o.c. both vertically and horizontally. The ungrouted cores contain polyurethane foamed-in-place insulation, and the wall is finished on the interior with gypsum wallboard.

REFERENCES

  1. R-Values of Multi-Wythe Concrete Masonry Walls, TEK 06-01C, 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. International Energy Conservation Code. International Code Council, 2006, 2009, 2012.
  6. Insulating Concrete Masonry Walls, TEK 06-11A, Concrete
    Masonry & Hardscapes Association, 2010.
  7. Energy Code Compliance Using COMcheck, TEK 06-04B, Concrete Masonry & Hardscapes Association, 2012.
  8. Concrete Masonry in the 2012 Edition of the IECC, TEK 06-12E, Concrete Masonry & Hardscapes Association, 2012.
  9. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-11. ASTM International, 2011.
  10. Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry & Hardscapes Association, 2023.
  11. Thermal Catalog of Concrete Masonry Assemblies, Second Edition, CMU-MAN-004-12, Concrete Masonry & Hardscapes Association, 2012.