What vapour barrier details are critical where an attached garage meets the house wall in Ottawa?

The most critical detail at this junction is ensuring the vapour barrier is continuous, lapped correctly, and sealed against air movement — not just draped loosely and stapled. In Ottawa's climate, where indoor air in a heated attached garage carries significant moisture, any gap in the vapour barrier at the house wall interface becomes a pathway for warm, humid air to contact cold framing and sheathing, creating condensation inside the wall assembly that leads to mould, rot, and structural deterioration over time.

The Junction Is Both a Vapour Problem and a Fire Problem

At the point where an attached garage meets the house wall, you are dealing with two overlapping code requirements simultaneously, and they must both be satisfied without compromising each other. The Ontario Building Code requires a minimum 45-minute fire separation on the garage side of that shared wall — typically 12.7 millimetre Type X drywall — and all penetrations must be fire-stopped. At the same time, the wall assembly needs a proper vapour barrier on the warm side of the insulation. In practice, the 6-mil polyethylene vapour barrier goes between the insulation and the Type X drywall, and it must be sealed at every edge, penetration, and transition with acoustic sealant or vapour barrier tape rated for the purpose. Stapling alone is not sealing — every seam needs to be lapped a minimum of 150 millimetres and adhered.

Where the garage wall meets the house wall framing, the vapour barrier needs to wrap around the corner framing and tie into whatever air barrier system the house wall uses. This transition is where installers frequently cut corners, leaving a gap at the top plate, bottom plate, or corner stud that allows air to bypass the barrier entirely. In Ottawa's winters, that small gap can allow enough warm, moisture-laden air into the wall cavity to cause visible condensation and frost formation inside the framing — a problem that often isn't discovered until rot or mould is already established.

The bottom plate connection to the slab is equally important. The vapour barrier should extend down behind the bottom plate and ideally be integrated with the underslab vapour barrier if one was installed during construction. A sill gasket under the bottom plate helps here, but the vapour barrier still needs to be sealed to the concrete at the base of the wall using acoustical sealant — not just tucked and hoped for the best.

Electrical boxes, pot lights, and any penetrations through this wall are vulnerability points that require fire-rated covers on the garage side and sealed vapour barrier boots or sealed gaskets on the interior side. Every hole punched through the vapour barrier for a wire or pipe needs to be sealed back to the poly with acoustical sealant. This sounds tedious, but in Ottawa's climate it genuinely matters — the pressure differential between a heated garage and the cold exterior drives air through every gap it can find.

If you are renovating an existing attached garage rather than building new, getting this vapour barrier detail right often means removing the existing drywall to access the framing, which adds cost but is the only way to do it properly. A contractor who tells you they can seal the vapour barrier without opening the wall is not giving you a realistic answer. For this kind of work, you can browse garage renovation contractors through the Ottawa Construction Network directory at justynrookcontracting.com/directory to find professionals experienced with attached garage assemblies.