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OBC DIVISION B · PART 5 — ENVIRONMENTAL SEPARATION

Where is a vapour barrier required in Ontario?

Thermally insulated wall, ceiling and floor assemblies must include a vapour barrier to resist moisture diffusing from the warm interior into the assembly. It is installed on the warm side of the insulation.

Code reference — verbatim
Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into wall spaces, floor spaces or attic or roof spaces.
OBC Division B, Subsection 5.5.1. (and 9.25.4. for houses)
What this means in practice

The vapour barrier's job is to stop warm, humid indoor air from pushing moisture by diffusion into the cold parts of the assembly, where it would condense. It must have sufficiently low permeance (Part 9 requires not more than 60 ng/(Pa·s·m²) — polyethylene sheet and certain paints and membranes qualify) and be positioned so any surface it touches stays warm enough to avoid condensation — in practice, on the warm side of the insulation. The vapour barrier is distinct from the air barrier (which stops air leakage, a much bigger moisture transporter), although one material such as sealed polyethylene often serves both roles in wood-frame housing.

Plain language

Indoor air carries moisture. Without a plastic layer on the warm side of the insulation, that moisture seeps into the wall, hits the cold sheathing, and turns to water or frost — leading to mould and rot. The barrier keeps the moisture in the room where it belongs.

Related questions

What are the air barrier requirements in the Ontario Building Code?What performance standards do windows have to meet in Ontario?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.