The Polyisocyanurate Insulation Manufacturers (PIMA) have agreed to adopt the Long-Term Thermal Resistance (LTTR) method of determining R-value of polyiso insulation.

LTTR is a 15-year time-weighted average R-value for any foam or blown insulations and is technically supported by ASTM Standard C1289-02 and Canadian Standard Can/ULC S704. The LTTR R-value of 1" of polyiso insulation is 6.0

In a recent decision, the National Roofing Contractors Association (NRCA) Technical Operations Committee has continued to support its long standing recommendation to use 5.6 as the aged or long-term R-value of 1" of polyiso insulation. NRCA contends that there is insufficient data to support the new LTTR values.

Benchmark continues to recommend that any insulation for a project be specified by thickness and not thermal insulation values, to eliminate any possible confusion during the bidding process.

As a specifier, we would suggest it may be prudent to use the more conservative number when determining insulation thickness to prevent condensation, where using the higher value may be appropriate when determining insulation thickness for meeting energy requirements.