This month, U.S. polyisocyanurate insulation manufacturers begin reporting long-term thermal resistance (LTTR) values based on updated and revised test methods, resulting in lower LTTR values than previous values. Does this change settle a decades-long debate regarding R-values for polyisocyanurate insulation?
The R-value of closed-cell, polyisocyanurate insulation is affected by the amount of gas in the foam's cells. Because the R-value of most blowing agents (gases) is 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 or partially dissolves into the cell's polymer matrix.