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 ...
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