NOW THAT WE have identified the correct range of winter mean reference temperatures to measure R-value, what exactly happens to the R-value of polyiso roof insulation within this range? The PIMA white paper helps to answer this question, as well, by relying on actual average R-value data for polyiso roof insulation obtained in an independent study conducted by the Rosemont, Ill.-based National Roofing Contractors Association; results appeared in “R-value concerns”, an article published in Professional Roofing’s May 2010 issue.In the NRCA study, samples of polyiso were obtained from roof installations in the field, and their R-value was measured at four different reference temperatures: 20 F, 40 F, 75 F and 110 F. Because the actual average reference temperature range obtained from the PIMA white paper varies from 45 F in Climate Zone 7 to 71 F in Climate Zone 1, the PIMA white paper used the average R-values obtained by NRCA at 40 F and 75 F, which were R-5.4 and R-5.7, respectively. Next, these average R-values obtained at 40 F and 75 F were linearly interpolated for the appropriate reference temperature within the 45 F and 71 F range identified from NOAA average winter climate data. Finally, average R-values for summer conditions were determined for each climate zone, and a full-year average R-value was calculated using the average winter and summer values. These climate-adjusted polyiso R-values and winter mean reference temperatures are shown in Figure 3.
As illustrated in Figure 3, the R-value of polyiso roof insulation does vary among the seven major climate zones—but not by very much. In fact, during the entire year the variation in average R-value for all but the coldest climate zone equals 0.1 R, or one-tenth of a unit R-value or less than a 2 percent variation for a product with a nominal R-value of R-5.6. In addition, it should be noted that these interpolated R-values for winter, summer and annual average R-values shown in Figure 3 do not vary significantly from actual R-values for polyiso roof insulation currently published by manufacturers across North America.
THE INFORMATION presented in Figure 3 begs a fundamental question: What amount of difference in R-value is sufficient to justify significant changes in how we measure and apply nominal R-value within national energy standards and codes? For the building owner and designer, the PIMA white paper suggests that differences of 0.1 or 0.2 R, as shown in Figure 3, may be impractical to impose on the average design professional or code official relying on prescriptive R-value tables to determine the correct R-value to meet the applicable energy code.
But even if this small range of R-value variation for polyiso is used, it would be critical to include one additional variable that unfortunately is missing from most of the available research. And that variable is the actual cost-per-R of the different thermal insulations studied. As an example, if an insulation with a 1 or 2 percent reduction in temperature- related R-value continues to provide a lower overall cost per R, would a recommendation for a slightly more “stable” but more expensive insulation be the best answer for a building owner looking for optimal economic return?
It is important to recognize the R-value of polyiso roof insulation is reduced at some point at lower temperatures. But within any reasonable temperature range associated with typical building operating conditions in almost any climate in North America, this difference appears to be very small. As a result, although the use of a variable R-value for polyiso roof insulation may
be useful within sophisticated energy modeling, it probably offers no value within current prescriptive R-value standards based on a single reference temperature. And, in all cases, a solid economic analysis of the best return on R-value should be conducted to ensure the roof insulation selected actually provides the best ROI for the building owner.
A free copy of the Bethesda, Md.-based Polyisocyanurate Insulation Manufacturers Association’s white paper, “Thermal Resistance and Temperature: A Report for Building Design Professionals”, is available on PIMA’s website.
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