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Q&A Spotlight

First Aid for an Under-Insulated Roof

Insulating and sealing the roof on a family retreat without disturbing the ceiling inside

New roof, old problems. The low-slope roof on this North Carolina getaway has been stripped to the sheathing, clearing the way for an insulated, air-sealed replacement. Photo courtesy VA_Engineer1.

Writing from the mountains of North Carolina, GBA reader “VA_Engineer1” has a pretty good idea of how to re-roof his family retreat. He’s just not sure exactly how to get there.

The 1990s-era roof—shingles and a 1-in. layer of fiberboard that was the roof’s only insulation—has been stripped from the house. Beneath is a layer of 1-1/2-in.-thick tongue-and-groove boards that serve as both the roof deck and the finished ceiling. VA would like to leave it undisturbed. The T&G pine sits atop rafters spaced 48 in. on center.

The building is unoccupied for much of the year, although there is a tenant in a lower story separated from the rest of the house by an insulation layer between the two.

“Primarily concerned with future winter use, keeping heat inside and preventing problems during winter from warm, heated air/moisture vapor moving up into the roof sandwich and condensing to form ice, or to keep sheathing/decking wet leading to rot,” VA writes in a lengthy question posted in the Q&A forum.

VA recognizes that adding a layer of insulation above the roof deck is the best answer, even if it’s not possible to meet current code requirements. One option VA has in mind is to add 3 in. of rigid foam followed by 5/8-in. or 1/2-in. sheathing and then roofing—metal if he can afford it.

But what about those all important details? That’s where we jump into this Q&A Spotlight.

The roof needs an air barrier

Akos has faced exactly the same problem in the T&G roof of a studio (see the photo below). T&G boards may be attractive on the inside, but such a ceiling is notoriously leaky to air, as both Akos and VA…

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