Although building scientists have understood the advantages of airtight construction details for years, few residential plans include air barrier details. That’s nuts.
Ideally, construction documents should show the location of a building’s air barrier, and should explain how the builder is expected to maintain air-barrier continuity at penetrations and important intersections. In a typical house, these intersections might include:
A designer who doesn’t know how to make these areas airtight can hardly fault a builder who fails to intuit details that aren’t even mentioned on the plan. (For more information on air barriers, check out these three resources: Questions and Answers About Air Barriers, the GBA Encyclopedia, and a useful Web page from Oikos, “Advanced Air Sealing.”)
If the day ever comes when most new homes include an air barrier that addresses typical penetrations and the intersections listed above, energy efficiency experts will cheer. That day is a long way off, however.
In the meantime, some progressive builders insist that every home needs not one but two air barriers: an exterior air barrier and an interior air barrier. Although this belt-and-suspenders approach is controversial, it has many strong advocates.
To understand the controversies surrounding double air barriers, it’s important to explain the two positions.
The basic rule of infiltration is “air out always equals air in.” If you have a good air barrier on one side of the insulation, you have stopped both infiltration and exfiltration — so you’re done.
According to this philosophy, it doesn’t matter where the air barrier is located — as long as the insulation is in contact with the air barrier:
If you set a particular airtightness goal — for example, 1.5 ac/h @ 50 Pascals — you can achieve it with any of these methods, as long as you…