If you convert your vented unconditioned attic to an unvented conditioned attic by installing open-cell spray foam on the underside of your roof sheathing, you may be surprised to discover that your attic is now the most humid room in your house.
Why? We don’t know. Although building scientists haven’t achieved a consensus on the answer, we do have enough information to paint a picture of what’s going on.
Consistent reports from homeowners and builders
Lots of GBA readers have posted reports of this type of problem. Here is a sampling:
A short history of “cathedralized” attics
Unvented conditioned attics were quite rare before the 1980s. Among the early promoters of conditioned attics was Joe Lstiburek, a principal of the Building Science Corporation. Lstiburek’s enthusiasm was rooted in his desire to bring attic ductwork inside a home’s thermal envelope.
During the 1990s, the Building Science Corporation received funding from the U.S. Department of Energy (through the Building America program) to research conditioned attics in Nevada and Texas. After experimenting with cellulose-insulated roof assemblies and fiberglass-insulated roof assemblies, Lstiburek began promoting conditioned attics insulated with open-cell spray foam, especially in Florida.
At a presentation at this year’s Westford Symposium on Building Science in Massachusetts, Lstiburek reminisced. “I spoke with Gabe Farkas of Icynene,” Lstiburek recalled. “We decided, ‘Why not foam the whole damn thing?’ Icynene took this idea to Florida. We created conditioned attics with low-density open-cell foam. That approach became a huge industry in South Florida. Why didn’t we use high-density spray foam? Because Gabe Farkas was the only spray-foam guy who would talk to me.”
Most of these attics stayed dry. “These were conditioned attics,” said Lstiburek.…