Interior wall sheathing options

A homeowner building a cabin asked whether using plywood instead of drywall on interior walls could trap moisture inside the wall. The wall design, built for a cold climate, layers exterior siding, rigid mineral wool insulation, house wrap, plywood sheathing, wood-fiber batt insulation, and a "smart" vapor retarder (a membrane that changes how easily moisture passes through it depending on humidity) just before the interior surface. The question was whether adding solid plywood as the final interior layer, in place of drywall, would seal in moisture between it and that retarder.

Building science experts responding said no: plain, unfinished plywood is vapor-open enough to behave much like the smart membrane already in the wall, so it won't trap moisture. The bigger concern they raised is air sealing. Drywall's taped and mudded joints double as an air barrier, but plywood panels leave seams that need separate sealing, such as battens and caulk, or a durable membrane placed under the plywood to do that job. They also warned that certain finishes, like polyurethane, can turn plywood into an actual vapor barrier, which could cause problems if the home uses air conditioning for long stretches. Painted plywood with breathable paint was considered fine.

Experts also noted that thick plywood adds little structural benefit, since exterior sheathing already provides wind resistance, and that thinner plywood is easier to work with. For anyone considering a similar interior finish, the exterior sheathing itself can sometimes serve as the main air barrier, simplifying the whole assembly.