During the winter, when indoor air is usually warm and humid, most wall sheathing is cold. Under these conditions, we really don’t want water vapor to move from the interior of our homes toward the exterior. That’s why builders in the 1980s installed polyethylene on the interior side of walls.
During the summer, on the other hand, outdoor air can be warm and humid, while our drywall is often cooled by the air conditioning system. Under these conditions, we want to limit the movement of water vapor from the exterior toward the interior. We also want to allow any moisture in our walls to be able to move toward the interior of our homes, unimpeded by a vapor barrier, so that a damp wall assembly can dry out. That’s why an interior-side vapor barrier works against us during the summer.
There are two possible solutions to this dilemma. The first solution is to install an adequate thickness of rigid foam on the exterior side of the wall sheathing. This foam layer prevents inward vapor drive during the summer, while also keeping the wall sheathing warm enough during to winter to avoid condensation or moisture accumulation.
In some types of wall and roof assemblies, it may be appropriate to consider a second solution: installing a “smart” vapor retarder — that is, a membrane with variable vapor permeance — on the interior side of the wall assembly.
The membrane’s vapor permeance rises and falls
When conditions are dry, a smart vapor retarder is relatively vapor-tight (in other words, it has a relatively low vapor permeance). When the air or the building materials adjacent to a smart vapor retarder get more humid, however, the membrane becomes more vapor-open — in other words, its…