What vapour barrier system works best for additions in Ottawa's climate zone?

Getting vapour control right in an addition is one of those details that either protects the walls for decades or leads to hidden moisture damage. Ottawa has a heating-dominated climate. Through the long heating season, warm, moist indoor air tries to move outward into colder walls. In hot, humid summer weather with air conditioning running, the drive can reverse. Your wall design needs to handle both without trapping moisture inside.

The conventional approach is polyethylene sheeting on the warm side of the insulation, installed continuously over walls and ceilings with seams lapped and sealed. The details are what count: sealing the poly to top and bottom plates with acoustical sealant, using sealed boxes or boots at electrical outlets, and lapping the ceiling layer over the wall layer to make a continuous envelope. In an addition, the transition between the new vapour barrier and the existing house at the connecting wall is the most vulnerable spot for air leakage and moisture problems. Ask how your contractor plans to detail it.

Variable-permeability ("smart") vapour retarders are another option. These membranes are tighter when the air around them is dry, as in winter, and more open when humidity rises, which lets a wall dry toward the inside in summer. They cost more than polyethylene. Ask your designer whether one suits your wall assembly, particularly if the wall has exterior foam or other layers that limit drying to the outside.

If the walls use closed-cell spray foam, the foam itself can serve as the air barrier and, at enough thickness, the vapour retarder. That can remove the need for separate polyethylene. A hybrid approach, a layer of closed-cell foam against the sheathing with batts or other insulation filling the rest of the cavity, is also used. The proportions matter, so the designer should confirm the assembly rather than leaving it to the installer.

Whatever system you choose, air sealing matters more than the vapour barrier material. Air leaking through gaps carries far more moisture into walls than vapour moving through materials. A blower door test after the addition is framed and sealed, but before drywall, can reveal leaks while they're still easy to fix. Ask whether your contractor offers one.

Coordinate vapour control with the exterior side of the wall. Exterior insulation, the sheathing type and the membrane all affect how the wall dries, so the warm-side layer has to suit the rest of the assembly. Your permit drawings show the wall assembly, and the City's Building Code Services can confirm what the submission needs. You can browse addition contractors in the Ottawa Construction Network directory and compare itemized quotes that describe the air and vapour control approach.

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Looking for experienced contractors? The Ottawa Construction Network connects homeowners with qualified professionals:

• Justyn Rook Contracting
• RenoMotion Inc.
• Dump n Dash Hauling
• Pure Flow Water Solutions inc.
• Estra Design