Thermal values in ICC and ASHRAE standards
Although differences in prescriptive thermal values can be found between ICC and ASHRAE standards, it is important to recognize the two entities work together closely to make energy codes as consistent as possible. In fact, ICC and ASHRAE jointly signed a Memorandum of Understanding that formally recognized their mutual contribution to advancing building safety and energy efficiency. They also maintain a commitment to exploring ways to optimize codes and standards development.
Due to this agreement, recent energy-related I-Codes formally reference corresponding ASHRAE standards as equivalent paths to code compliance. The 2012 IECC identifies ASHRAE 90.1-2010 as an equivalent code and design approach, while the 2012 IgCC calls out ASHRAE 189.1-2011, Standard for the Design of High-performance Green Buildings. Figure 1 recaps the I-Codes and ASHRAE energy standards, their intended function, and their relationship to each other.
Although ICC and ASHRAE are working closely together to harmonize and support building energy standards, minor differences may occur simply because the two organizations employ separate development processes. For example, while ICC and ASHRAE incorporate many similar approaches to achieve consensus, the very fact their development processes convene at different times and locations virtually ensures some variation.
Figure 2 offers an explanation of how these differences may affect roof and wall prescriptive thermal values. This table identifies the R-value identified for Climate Zone 6 (e.g. Augusta [Maine], Helena [Montana], and Pierre [South Dakota]), illustrating how the minimum levels vary considerably, even between codes that are intended to be functionally equivalent.
Although it would be reasonable to assume the R-35 required by an above-the-code standard such as ASHRAE 189.1-2011 would be higher than minimum code standards such as the 2012 IECC or ASHRAE 90.1-2010 (requiring R-35 and R-20, respectively), it can be quite difficult to comprehend why the R-values in the two minimum code standards differ so significantly.
Unfortunately, the R-value differences between the 2012 IECC and ASHRAE 90.1-2010 appear to be related to procedural problems and timing differences occurring during the development of these standards. In the case of ASHRAE 90.1, a successful appeal by the glazing industry involving prescriptive thermal values for windows effectively delayed the inclusion of prescriptive thermal value tables in the 2010 edition.
Thus, ASHRAE 90.1-2010 was published with a reference to the thermal value tables in the prescriptive edition, so the R-values remained low (i.e. R-20). Although ASHRAE will publish revised thermal value tables with roof and wall R-values much closer to the 2012 IECC, the current discrepancy serves as a good example of the differences that may occur due to separate procedural advancement processes.
Figure 2 also illustrates how the development process for both ICC and ASHRAE may result in not only minor variations in table values, but also significant differences in the basic approach to determining R-value. Additionally, new R-values have been established for polyiso that translate into long-term thermal resistance (LTTR).
LTTR refers to a new way to provide a comprehensive approach to predicting long-term R-value. An updated standard now includes two test methods: ASTM C1303-11, Standard Test Method for Predicting Long-term Thermal Resistance of Closed-Cell Foam Insulation, and for Canada, CAN/ULC-770-09, Standard Test Method for Determination of Long-term Thermal Resistance of Closed-cell Thermal Insulating Foams. This change in the way the R-value of polyiso rigid foam insulation is calculated involves accelerating the aging process to provide an accurate and consistent prediction of product R-value after five years. (This is the equivalent to a time-weighted thermal design R-value for 15 years.) It is important to recognize that this update does not alter polyiso’s physical properties—only the testing method used to calculate R-values has changed.
Figure 3 offers a side-by-side comparison of the old 2010 R-values and the newer 2014 R-values. These newly designated values must also be factored into the calculations, thereby determining a more precise R-value moving forward.
Good information. Very useful.