A.Bill Woodring, director of technical services at GAF Materials Corp., responds: GAF-Elk’s shingle warranty is still valid when the shingles are installed over a roof deck insulated with SPF foam, which can be an effective method of boosting a home’s energy performance. But foam that isn’t installed according to building code requirements and manufacturer’s instructions can lead to condensation problems, mold growth, and roof deck deterioration. The issue, of course, is that the insulation can retard or block moisture-vapor migration through the roof assembly, where it can then condense in the framing, sheathing, and insulation. Expansion and contraction of the roof deck from changes in moisture levels can lead to buckling and distortion — even splitting — of the shingles.

Condensation problems tend to be more of a concern in cooler climates, since vapor drive from the interior to the exterior of a building is strongest when exterior temperatures and relative humidity are low and interior temperatures and relative humidity are high. The unvented attic guidelines in the IRC (see R806.4, 2009 IRC) are intended to...

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