Does polyethylene have to go on the warm side of the wall in Ontario?
Yes in principle: low-permeance materials like polyethylene must be positioned on the warm side of the insulation, or close enough to it that the sheet stays above dew point. The OBC permits burying it within the assembly only when the inboard/outboard thermal resistance ratios meet the code's tables.
Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.
The familiar rule of thumb, keep roughly two-thirds of the insulation outside the poly and one-third inside, is a simplification of the OBC ratio tables, and the actual required ratio depends on the location's degree-days. This governs common Ontario details: poly behind the drywall with insulated sheathing outside is fine if the ratio works, double vapour barriers are the thing to avoid, and interior foil-faced or poly-wrapped assemblies trapping moisture between two tight layers cause the classic rot cases. Remember the vapour barrier and air barrier are distinct functions even when one poly sheet does both jobs.
The plastic sheet goes on the warm side, just behind your drywall, so moist indoor air never reaches a cold surface inside the wall where it could condense. It can sit deeper in the wall only if enough insulation stays outside it to keep it warm, and never trap the wall between two layers of plastic.
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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.