Every few months, GBA gets a question from a cold-climate builder who discovers sheathing moisture while installing wall insulation — often in a walkout basement. Here’s the scenario: the builder is installing fiberglass batts in some of his home’s stud bays during cold weather. The next day, he happens to peel back some of the batts and is shocked to discover that the OSB sheathing is soaked. He panics. What should he do now?
At least six GBA readers have posted questions on this topic over the years. Looking for common threads in these cases, I noticed:
Tony Bouchard of Cornish, Maine, was installing mineral wool insulation in the stud bays of a room over his garage. “I noticed a small wet spot on the floor in one corner; it is about the size of a credit card,” Bouchard wrote. “No idea how that got there, so I started poking around and found… the exterior sheathing behind the insulation was really wet. … On the same wall I have a half-insulated stud bay. The open half is dry, but under the insulation the sheathing is all wet.”
David Schreiber of Syracuse, New York, was installing fiberglass batts in the framed stud cavities of a walkout basement. He wrote: “For about a week and a half, the house was heated with a salamander propane heater which dumped a lot of humidity into the house. Plus the humidity from the drywall mud. … Anyway, the basement exterior wall cavities are 2x6s with an OSB exterior. It has been very cold here (-9°F on Friday morning) and with the very humid interior air. … Of course I have condensation on the OSB.”
Brian Lent of Seattle, Washington, was finishing off an area he describes as a half basement. He wrote: “The builder left…
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