In the typical North American home, the air barrier (if there is one) consists of a sheet of polyethylene installed behind the drywall. But some Canadian builders are using what they think is a more efficient approach. According to Gary Sharp, a trainer for Canada's R-2000 energy-efficient construction program, 20 percent of Canadian builders now install their air barrier on the outside of the house. The barrier consists of a standard housewrap like Tyvek or Typar. The only interior air sealing consists of a sheet of 6-mil poly on the ceiling. Any other interior poly serves primarily as a vapor retarder.

Sharp points out that the lack of partition walls on the outside of the house means fewer inside and outside corners to worry about sealing properly. You don't need airtight electrical boxes. And you can do your blower door test at the framing stage, which lets you seal any air leaks before covering the interior walls. "If you do an interior poly air barrier you can't test it until you've installed the drywall," he notes. "Then it might be too late to fix any mistakes."

If you want to try your hand at exterior air sealing, says Sharp, the secret is to carefully plan the air barrier before you start construction. Sharp's system includes a foam gasket between the mudsill and foundation wall, caulking between the sill and the exterior sheathing, a layer of houswewrap with all the seams carefully taped, a bead of caulk between the sheathing and the top plate, and another bead between the top plate and the ceiling poly. The housewrap must be installed before any windows and doors. Windows and doors are then caulked to the housewrap and the space between them and the rough opening filled with a non-expanding foam.

Sharp used this system on his Kanata, Ontario house. The blower door test came in at 1.4 air changes per hour (ACH) at 50 Pascals, which beats the minimum requirement for the R-2000 program. (Although Sharp could have used almost anything as an interior vapor retarder, he decided to install polyethylene just to see how much tighter he could make the house. This brought air leakage down to .9 ACH.)