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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. A vapour barrier is required for insulation installed between the ends of floor joists in the rim joist space but does not need to be carefully sealed to the joists and floor sheathing 8
unless it also forms part of the air barrier. It is more difficult to achieve an effective air barrier at this location because the materials must be cut and fitted between the joists (Figure 8). Sprayed closed-cell foam is commonly used in this location because it fills the space tightly and acts as both insulation and air barrier. When also serving as an air barrier, the edges of the polyethylene covering the fitted insulation should be caulked to the framing, in order to seal the joints and prevent exfiltration. Rigid insulation is suitable in this location, and some semi-rigid or soft insulation materials that come with a reinforced aluminum backing can also be used effectively. Make the interfaces between windows, doors and skylights and wall/ceiling assemblies airtight by sealing all junctions between the
Placement of air barrier over joist headers
corner wall joint air barrier header wrap insulation along joist parallel to exterior wall
wall studs bottom wall plate (second floor) floor joist vapour barrier and batt or spray foam insulation
Note: Polyethylene is not to be used as a header wrap.