Air sealing behind T&G wainscot

A homeowner building a house in the Colorado mountains asked a technical building forum for advice on a wall assembly question that comes up in a lot of new construction and deep energy retrofits: how to handle moisture control on an exterior wall when part of the wall is covered with wood paneling (tongue-and-groove wainscot) instead of drywall all the way up.

The wall in question has rigid foam insulation on the outside of the wood sheathing, plus fiberglass batt insulation inside the wall cavity. Builders in cold climates often add a foam layer outside the sheathing specifically so the sheathing stays warm enough that moisture in the wall doesn't condense on it. Responders pointed out that with this much exterior foam relative to the cavity insulation, the wall is already on the safe side for this climate zone, and a stricter interior vapor barrier is not needed.

The bigger point, though, was a distinction worth knowing if you're ever reviewing plans for your own home: a vapor barrier (which controls moisture moving through a wall) and an air barrier (which stops drafts and air leaks) are two different things, even though people often lump them together. One expert suggested that simply running drywall behind the wood paneling, in the ordinary way, gives the wall its air barrier without extra materials or cost. Another recommended a mid-construction blower door test — a diagnostic that measures how airtight a house is — to catch leaks before the walls are closed up for good.