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OBC DIVISION B · PART 5 — ENVIRONMENTAL SEPARATION

What are the air barrier requirements in the Ontario Building Code?

Assemblies separating conditioned space from the exterior or unconditioned space must include a continuous air barrier system to resist air leakage in both directions.

Code reference — verbatim
Wall, ceiling and floor assemblies separating conditioned space from unconditioned space, the exterior air or the ground shall be constructed so as to include an air barrier system that will provide a continuous barrier to air leakage.
OBC Division B, Subsection 5.4.1. (and 9.25.3. for houses)
What this means in practice

Air leakage moves far more moisture into assemblies than vapour diffusion does, and it drives energy loss, drafts, ice damming and comfort complaints — which is why the air barrier is arguably the most important plane in the building envelope. The code requires the system to be continuous (joints sealed, penetrations sealed, transitions between materials bridged), structurally supported against wind loads, and durable. In houses this is typically sealed polyethylene or a taped sheathing membrane; in Part 5 buildings the air barrier system requires materials with tested air leakage rates and detailed continuity at windows, roof and foundation transitions. Continuity at transitions is the number-one air barrier deficiency flagged in reviews.

Plain language

Think of the building as a windbreaker: one continuous layer that stops air from blowing through walls, around windows, and into the attic. Every gap wastes heat and carries moisture into places it can do damage.

Related questions

Where is a vapour barrier required in Ontario?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.