In January, North American polyisocyanurate insulation manufacturers implemented updated procedures for testing and reporting the insulation's long-term thermal resistance (LTTR). You should be aware of polyisocyanurate insulation's aging properties and the LTTR methodology, as well as NRCA's opinion regarding whether LTTR accurately predicts polyisocyanurate insulation's long-term R-value.
R-value vs. time
The R-values of closed-cell, polyurethane-type insulation, including polyisocyanurate, are affected by the amount of gas in the foams' cells. Because the R-values of most blowing agents (gases) are greater than that of air, polyisocyanurate insulation's R-value is greatest when there is more blowing agent and less air in the foam's cells.
During polyisocyanurate insulation's service life, air diffuses into the foam's cells and the blowing agent diffuses out of or partially dissolves into the cell's polymer matrix. Each process occurs at a rate dependent on temperature; pressure; and the foam's polymer type, gas type and cell structure. Generally, the inward diffusion of air occurs at a much faster rate than the outward diffusion of the captive blowing agent. Diffusion rates also are affected by the foam's thickness and type of facer sheets.