During the past several years, use of the long-term thermal-resistance (LTTR) method for determining and reporting R-values for plastic foam insulation has been implemented and is gaining acceptance in U.S. and Canadian roofing markets. Although the LTTR method is viewed by many as a technical advancement, some roofing industry researchers have reservations with the methodology because results may not accurately represent the true thermal performance of the insulation products it tests.
Following is a summary of a research program in which NRCA participated. A work group tested R-values of aged insulation samples and compared results with the products' represented LTTR values.
The technical basis for LTTR comes from ASTM C1303, "Standard Test Method for Estimating the Long-Term Change in the Thermal Resistance of Unfaced Rigid Closed Cell Plastic Foams by Slicing and Scaling Under Controlled Laboratory Conditions," which originally was published in 1995. In this test method, thermal resistance properties of thinly sliced specimens of plastic foam insulation material are tested and, from these data, long-term changes in thermal resistance are estimated. This methodology is reported to account for the diffusion of the blowing agent from the foam insulation's cells over time, resulting in a recognized reduction of thermal resistance of some plastic foam insulations as they age.