Airtight Wall and Roof Sheathingcomments (13) February 7th, 2013 in Blogs
Arguments in favor of exterior air barriers
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 or the proprietary Zip System tape from Huber. (If all insulation is on the exterior side of the sheathing, it's also possible to cover all of the exterior sheathing with Ice & Water Shield - a method called PERSIST.)
Carried to its most logical expression, this approach requires buildings to be framed without any roof overhangs at the eaves or rakes. That way it's easy to seal the transition between the wall sheathing and the roof sheathing. After the air barrier is established and sealed, framers can build rake and eave overhangs as separate elements attached to the exterior sheathing.
Simple and verifiable
Marc Rosenbaum, an energy consultant who works in New Hampshire and Massachusetts, described the advantages of this air-barrier system at a presentation he gave on February 10 at the Better Buildings By Design conference in Burlington, Vermont.
Rosenbaum began his presentation by reminding the audience of the characteristics of a good air barrier. According to Rosenbaum, an air barrier needs to be (a) impermeable to air; (b) continuous; (c) strong enough to resist the forces acting on it; and (d) durable. Rosenbaum doesn't use rigid foam as an air barrier; in his opinion, it's too flimsy to pass the durability requirement.
Rosenbaum explained, "I've tried interior air barriers. I've had Tremco acoustical sealant all over my hands. Airtight drywall is not easy. The approach takes lots of careful workmanship and you can't really test it until you're almost done. Exterior sheathing is easier to seal, and sheathing is durable."
Rosenbaum pointed out that, compared to an interior air barrier, an exterior air barrier:
Summing up, Rosenbaum said, "Why do I like an exterior air barrier? Because of partition walls, floors, wiring, and things that go in and out of the walls."
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