When installed in accordance with manufacturers' instructions, fibrous glass duct wraps will provide specified thermal performance as printed on the vapor retarder facing. This is achieved by determining the "stretch-out" cutting dimension required so that, once installed, the duct wrap is not compressed to less than 75% of nominal (out-of-package) thickness.

Commonly available R-values as published by manufacturers show fibrous glass duct wrap insulations to perform in the R-value ranges shown in Table 6.