program is required to be used, such as EnergyPlus, DOE-2 or BLAST. These programs analyze the energy impacts of the entire building, including factors such as interior components, lighting, HVAC, and occupancy patterns. The result is an estimate of annual energy use and/or cost for the building as a whole. The comprehensiveness of this analysis lends itself to parametric studies to evaluate the impacts of varying individual building elements, or the effectiveness of a particular strategy.
For LEED projects, a whole-building analysis must be used for Energy and Atmosphere (E&A) Prerequisite 2, Minimum Energy Performance, and for E&A Credit 1, Optimize Energy Performance. The whole-building analysis is performed in accordance with ASHRAE/IESNA
Standard 90.1 Appendix G, Performance Rating Method. Using Appendix G, LEED credits can be earned by demonstrating that the proposed building exceeds the Standard 90.1 minimums. E&A Credit 1 provides LEED credits incrementally from 1 to 19 credits for buildings that exceed the minimum requirements of ASHRAE/ESNA Standard 90.1-07 by 12 to 48%. This percentage reflects the performance of the entire building; improving the thermal performance of any one individual building element by 48% will do little to improve the overall building performance.
Although, in general, higher R-values reduce heat 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 core insulation only and a wall R-value of 7 hr.ft2.oF/Btu, a simplistic estimate of the building envelope energy use is 27,800 Btu/ft2. If we replace that wall with an R14 wall, the building envelope energy use drops by 2.5%, not in proportion to doubling the wall R-value. Figure 1 illustrates the trend: as wall R-value increases, it has less and less impact on the building envelope 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 may make more sense to invest in energy efficiency measures other than wall insulation.
When required, concrete masonry construction can provide walls with high R-values. For overall project economy, however, the industry recommends a parametric analysis similar to that shown in Figure 1 to determine reasonable insulation levels for the building envelope elements.
Figure 1: Diminishing Returns of Added Wall Insulation
Notes: Analysis is based on a two-story school in Bowling Green, Kentucky. Other building types and climates will have a similarly-shaped curve, although the individual numbers vary on a case-by-case basis. The y-axis values approximate the total heating and cooling energy associated with an average square foot of surface of building envelope. This analysis was performed using ENVSTD version 5.0.
Energy Tools for the Building Envelope: Energy Code Compliance If the project goal is to meet, rather than exceed, the energy code requirements, most codes allow three different methods to be used to show compliance: prescriptive, trade-off or system performance, and whole building energy analysis. The project need only comply under one of these methods, not all three.
Of the three compliance methods, the prescriptive method is the easiest to apply and perhaps the most well recognized. Requirements for building envelope components are listed in table format, with requirements listed separately for each component and climate zone, as shown in Table 1. Table 1 shows that in Chicago (Climate Zone 5), a flat roofed building must have R20 continuous insulation and masonry (Mass) walls must have R 7.6 continuous insulation to comply prescriptively. There is a misconception that these elements 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, as discussed below.
|Table 1: Excerpt from 2006 IECC Table 502.2 (1) Showing Prescriptive Wall and Roof R-Value Requirements|
NR = not required, i.e., and uninsulated concrete masonry wall complies in these climate zones.
Footnote c = Concrete masonry single wythe exception applies: ASTM C90 concrete
masonry, grouted up to 32 in. (813 mm) o.c. vertically and 48 in. (1219 mm) o.c.
horizontally, with ungrouted cores filled with insulation, is deemed to comply.
Footnote e = Insulation is not required for mass walls in Climate Zone 3A located below the “warm-
humid” line, and in Zone 3B.
Using these prescriptive tables, the requirements for individual elements are independent of each other. If in Climate Zone 5 the wall has R10 insulation and the roof has R18, the building cannot comply prescriptively based on R-value. Hence, although using the prescriptive tables is very straightforward, it is also very limiting in terms of design flexibility.
Although concrete masonry with insulated cores cannot comply under the prescriptive requirement for continuous insulation applied to the wall (because the webs of the masonry units interrupt the insulation) according to the IECC these walls can comply prescriptively based on the overall U-factor of the wall, per ASHRAE/IESNA Standard 90.1 (U-factor is the inverse of R-value, i.e. U = 1/R and R = 1/U). In this case, the U-factor of the wall must meet the prescriptive U-factor requirement instead of the insulation meeting the R-value shown in Table 1. For example, the equivalent mass wall U-factor requirement for Chicago is U0.123 (which corresponds to an R-value of 8.1). As long as the wall as a whole (not the insulation alone) meets the U0.123/R8.1 requirement, the wall complies with the IECC in Climate Zone 5.
The prescriptive U-factor requirements for mass walls are shown below in Table 2. NCMA TEK 6-2B, R-Values and U-Factors of Single Wythe Concrete Masonry Walls, lists these values for various concrete masonry walls.
|Table 2: Prescriptive Mass Wall U-Factor Requirements by Climate Zone|
A = U-factors from ASHRAE/IESNA Standard 90.1-04 Table. R-value = 1/U-factor.
B = Insulation is not required, i.e., and uninsulated concrete masonry wall complies.
C = Concrete masonry single wythe exception applies: ASTM C90 concrete masonry, grouted up to 32 in. (813 mm) o.c. vertically and 48 in. (1219 mm) o.c. horizontally, with ungrouted cores filled with insulation, is deemed to comply.
If the building does not comply using the either the prescriptive R-value or U-factor requirements, another option is to use a software tool such as COMcheck. COMcheck determines compliance for the building envelope based on the specifics of the building under consideration and on the project location. Using this option, the designer “builds” a description of the building, entering basic data (size, type of construction, R-value of insulation, etc.) for the building envelope elements (roof, exterior walls, windows, doors, floor, basement and skylight). After the building envelope description and climate are entered, the program displays how close the envelope as entered comes to meeting the specified code requirements. If the envelope fails to comply, it is a typically simple matter to adjust individual elements to bring the envelope into compliance.
COMcheck provides enhanced design flexibility with a little bit of added complexity over the prescriptive tables. Note that using COMcheck is an alternative to complying via the prescriptive requirements (either continuous insulation or U-factor requirements)—the envelope components do not need to meet the prescriptive requirements if the envelope is shown to comply using COMcheck. COMcheck is available for free download from www.energycodes.gov, and includes compliance for various editions of the IECC as well as various editions of ASHRAE/IESNA Standard 90.1.
The final option for energy code compliance is to use a whole-building energy analysis, as discussed above for LEED and other high-performing buildings. Similar to COMcheck, if a building is shown to comply using an approved whole building analysis, individual elements of the building do not have to meet the code prescriptive requirements, nor does the building have to pass when analyzed using COMcheck. The whole building approach provides the most design flexibility, but is also the most complex.
An overview of these code compliance options is available on the NCMA website here.
Regardless of a project’s energy efficiency and sustainability design goals, concrete masonry walls are available to help meet those goals. Various methods are available for energy code compliance and higher-level analyses, and the best approach often depends on the project goals.
When core-insulated single wythe masonry is desired for its aesthetics, durability, economy and low maintenance, energy code options other than the prescriptive continuous insulation requirements should be used, such as using prescriptive wall U-factor requirements or using COMcheck. If the project is LEED-registered, a whole building analysis must be used, and the prescriptive and COMcheck approaches should be disregarded. CMD