UPDATED on March 8, 2016
In the early 1970s, residential builders knew almost nothing about air tightness. The first residential air barriers were installed in Saskatchewan in the late 1970s, when pioneering Canadian builders began sealing the seams of interior polyethylene sheeting with Tremco acoustical sealant. The Canadian builders (and their American imitators) went to a lot of trouble to weave the interior poly around framing members at rim-joist areas and partition intersections.
A few years later, Joseph Lstiburek perfected and promoted the Airtight Drywall Approach (ADA) to air barriers. The ADA system required gaskets or caulk between drywall and wall plates on exterior walls; any electrical box in an exterior wall needed a special flange to allow it to be caulked to the drywall.
These days, some builders approach air sealing like all-out war, and try to make everything as airtight as possible. These are the builders who caulk their sheathing to the studs, and then install housewrap with taped seams, and then caulk their drywall to the studs for good measure. Before the blower-door contractor shows up, they run around the house like a chicken with its head cut off, squirting spray foam into every hole and crack. This approach — let’s call it “if one air barrier is good, three air barriers are even better” — works, except when it doesn’t. (Sometimes it makes more sense to have just one air barrier that is well defined rather than to wage an all-out but somewhat random war against air leakage.)
An exterior air barrier at the wall and roof sheathing
There’s a growing trend among builders concerned about air leakage to establish the air barrier at the exterior wall and roof sheathing. Typically this requires sealing the seams of the OSB or plywood sheathing with tape — either peel-and-stick tape…