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:
- All six cases occurred in cold climates — specifically, Maine, Massachusetts, Michigan, Minnesota, New York, Washington, and Wisconsin.
- In all six cases, the insulation was fibrous insulation (either fiberglass batts or mineral wool batts).
- In all six cases, the walls were not yet covered with drywall.
- Four cases occurred in an area described as a “walkout basement” or “half basement”; one case happened in a room over a garage, and one case occurred in a closet.
- In all cases, there was reason to believe that the indoor relative humidity was elevated, due either to construction moisture (typically, a freshly poured basement slab) or the operation of a humidifier.
Tales of woe
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…