In April, the Polyisocyanurate Insulation Manufacturers Association (PIMA) issued a performance bulletin titled "Measuring the R-value of Polyiso Roof Insulation," which attempts to refute NRCA's recommendation that designers use an in-service R-value of 5.0 per inch when specifying polyisocyanurate insulation.
PIMA's performance bulletin refers to NRCA's recommendation as " an arbitrary unit R-value of 5.0. " The bulletin goes on to briefly explain long-term thermal resistance (LTTR) testing, PIMA's QualityMarkCM LTTR certification program and the results of recent QualityMark verification testing. The bulletin reports results of PIMA's 2015 QualityMark verification testing as an average LTTR per inch of 5.78 for 1-inch-thick product, 5.74 for 2-inch-thick product, 5.85 for 3-inch-thick product and 5.95 for 4-inch-thick product.
The PIMA bulletin also indicates: " It should be noted the LTTR testing conducted under the QualityMark program uses a more severe conditioning procedure than the standard R-value test used by NRCA as a basis for its recommendation. Given the difference in NRCA's recommendation and PIMA's QualityMark program testing results, PIMA suggests that this difference may be attributed to a smaller testing sample size used by NRCA to support its recommendation and a possible lack of experimental controls regarding how NRCA insulation samples were procured and selected. "