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
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...
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