Jan Verschuren has a nicely roofed older house, and a problem to go with it. Cedar shingles have been installed over skip sheathing, making for a roof that’s not only historically correct but one that allows air to circulate freely beneath the roof deck. Verschuren’s next objective is to insulate between the 2×4 rafters, and here’s where he has run into a snag.
Building codes require at least R-38 worth of insulation in the roof. Ordinarily, that wouldn’t be a problem, but Verschuren says that he has only the 3 1/2-inch deep rafter bay to work with. He can’t afford to lose any headroom by adding insulation below the rafters, and he’d rather not tear off the roof to add extra insulation on top of the rafters.
He’s zeroed in on a type of insulation called Spaceloft with a reported insulating value of R-10 per inch. The distributor’s web site describes Spaceloft as “a flexible aerogel composite blanket.”
“So, 3 1/2 iches is all we have, but we’d like to get to an R-50 or thereabouts in our upgrade of this 1925 house built in Climate Zone 4C,” he writes in a Q&A post at GreenBuildingAdvisor. “Spaceloft seems the only option.”
Verschuren thinks he can get nine layers of this material in the rafter bays, followed by one or two layers over the bottoms of the rafters.
There are a few downsides, however: The cost is “exorbitant,” and Verschuren still has to figure out how to detail the installation so it will be vapor-open and able to dry out.
Or, does he have other alternatives? That’s the issue for this Q&A Spotlight.
GBA senior editor Martin Holladay has never heard of Spaceloft, but has two other suggestions. “Three and…