Most roofing professionals recognize polyisocyanurate roof
insulation loses some of its R-value over time. At the time
polyisocyanurate insulation is manufactured, it has a relatively
high R-value. As the insulation ages and is exposed to varying
temperatures and field conditions, the gas producing the high
R-value in the insulation's cells slowly diffuses and is replaced
with air.
Considering an insulation's actual in-service
R-value—instead of its as manufactured R-value or long-term
thermal resistance (LTTR) value—is important for designers
because the in-service R-value more closely represents conditions
in the...
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