Walls Above Grade Air Barrier

When the air barrier is on the interior of the wall framing, it can be easily sealed to the polyethylene attached to the underside of the roof framing. However, it is difficult to install a polyethylene air barrier where the floor joists meet an exterior wall because the joists interrupt the continuity of the air barrier. It is good practice to wrap vapour-permeable (and air-impermeable) spun-bonded polyolefin (housewrap) from the inside of the lower wall, around the outside of the rim joist, to the inside of the wall above (Figure 7).

The housewrap is then sealed to the polyethylene air barriers on the inside of the walls above and below. Because it is vapour-permeable, the housewrap will not impede vapour flow at the rim joist. Similarly, the junction between foundation walls and sill plates, the sill plate and joist headers and the rim joists and the subfloor material must be made airtight by installing sealants and gaskets between adjoining elements, or by wrapping the outside of the header with a sheet material capable of making the header airtight.

Figure 7

Air barrier around a rim joist at floor intersection

If the plane of airtightness is located on the interior of the building envelope, the electrical wiring, outlets and switches and recessed light fixtures penetrating through the plane of airtightness must be constructed airtight.

Airtight electrical boxes are recommended for fixtures, outlets and switches located in the building envelope. When polyethylene serves as an interior air barrier component, it should be lapped over and sealed to the electrical boxes. Alternatively, wrap electrical boxes with 0.15 mm (6 mil) polyethylene, then lap over the polyethylene on the wall and tape or caulk the polyethylene wrapper to it. The lap should be at least 100 mm (4 in.) and occur over a framing member. All wires should be caulked where they enter the box to prevent air leakage. It is preferable to avoid locating electrical outlets on exterior walls when possible.

Source : Canada Mortgage and Housing Corporation (CMHC)