Does Ontario require a vapour barrier on basement walls and which side does it go on?

Yes, the Ontario Building Code requires a vapour barrier on basement walls, and it must be installed on the warm side — the interior side, facing the living space — of the insulated wall assembly. This placement is specific to cold climates like the GTA, where the goal is to prevent warm, moist indoor air from reaching the cold foundation wall where it would condense and cause mould growth and structural damage.

The standard vapour barrier material is 6-mil polyethylene sheeting, which is installed continuously over the studs and insulation before the drywall goes up. All seams in the poly must overlap by at least 4 inches and be sealed with acoustic sealant or poly tape. The poly must also be sealed around all penetrations — electrical boxes, plumbing pipes, ductwork, and where the wall meets the floor and ceiling. Even small gaps in the vapour barrier allow enough moist air through to cause condensation problems on the cold side of the wall. The City of Toronto Building Division inspects the vapour barrier installation during the insulation inspection, and deficiencies must be corrected before drywall can proceed.

The important exception to the poly requirement is closed-cell spray foam insulation. When closed-cell spray foam is applied at a thickness of 2 inches or greater directly to the foundation wall, it acts as its own vapour barrier and no separate polyethylene sheeting is required. This is one of the key advantages of spray foam in basement applications — it eliminates the labour-intensive step of installing and sealing the poly sheet, and because the spray foam is bonded directly to the concrete, there are no gaps for air to circulate behind it. This dual function is why spray foam has become the preferred foundation wall insulation for premium basement renovations across the GTA.

Open-cell spray foam and XPS rigid foam board do not qualify as vapour barriers on their own at typical installation thicknesses, so if you use these products against the foundation wall, you still need the 6-mil poly on the warm side. This is a detail that some contractors get wrong — check that your contractor understands the vapour permeability ratings of the specific insulation products they are using.

A common and dangerous mistake in GTA basements is installing the vapour barrier on the wrong side — against the foundation wall. This traps moisture between two vapour barriers (the concrete wall acts as a vapour retarder itself and the poly on the cold side), creating a permanent moisture trap that guarantees mould growth. This error was actually common in older basement renovation guides and was done in many GTA basements finished in the 1980s and 1990s. If you are renovating or refinishing a previously finished basement and discover poly sheeting stapled directly to the foundation wall behind the insulation, this should be removed as part of the renovation.

Another important consideration is the below-slab vapour barrier. While not always visible or accessible during a wall renovation, new concrete slabs in Ontario are required to have a vapour barrier (typically 10-mil poly) under the slab to prevent moisture from wicking up through the concrete. Older GTA homes — particularly those built before the 1970s — often lack this sub-slab barrier, which contributes to slab moisture that affects flooring choices.

Getting the vapour barrier right is not a place to cut corners. The materials are inexpensive — a roll of 6-mil poly costs $50 to $100 and acoustic sealant is a few dollars per tube — but the consequences of doing it wrong include hidden mould that can cost $10,000 to $25,000 to remediate and poses health risks to everyone living in the home.