Q. While the IRC has approved the use of certain “hot” unvented attics, it seems that asphalt-shingle manufacturers still recommend that their shingles be installed over vented roof assemblies. Does this mean that shingle manufacturers will void their warranties if the roof deck is insulated with sprayed-in-place foam?

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 ensure that warm, moist air won’t cool to its dewpoint temperature within the roof assembly and condense. But if I were a designer or specifier, I would also run dewpoint calculations when using SPF insulation underneath the roof deck — regardless of climate zone — to confirm that the proposed roof assembly won’t have a condensation issue.

If an unvented roof is insulated according to IRC guidelines and the recommendations of the sprayed-in-place foam manufacturer are followed — and if dewpoint calculations confirm that condensation won’t occur within the roof assembly — I wouldn’t expect any problems. However, since GAF-Elk does not supply the sprayed-in-place foam insulation, any damage to our shingles — or any other roof-related problems attributable either to sprayed-in-place insulation applied to the roof deck or to a lack of ventilation — is not covered by the terms of our warranty.

Also, before installing asphalt shingles from any other manufacturer, I would contact the company for its recommendations and to confirm that this type of installation won’t void its warranty.