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OBC DIVISION B · PART 9 · SECTION 9.32 — VENTILATION

What are the minimum openable window areas for natural ventilation in Ontario?

Where rooms rely on natural ventilation, most habitable rooms need an unobstructed openable area of at least 0.28 m2 (3 sq ft), bathrooms about 0.09 m2, and unfinished basements openings totalling 0.2 percent of floor area. Mechanical ventilation can substitute.

Code reference
Rooms and spaces not served by mechanical ventilation must be ventilated by openable windows or openings to the outside, with minimum unobstructed areas set by room type: a general minimum for living rooms, bedrooms and similar habitable rooms, a smaller figure for bathrooms and utility spaces, and an area proportional to floor area for unfinished basements. Mechanically ventilated spaces are exempt.
OBC Division B, Subsection 9.32.2.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

Since new Ontario homes must have mechanical ventilation, these natural ventilation minimums matter mostly for seasonal buildings, unconditioned spaces and older-building renovations. Only the openable, unobstructed portion of a window counts, so a large fixed picture window contributes nothing; check the actual clear opening of casements and sliders. Do not confuse this with the separate bedroom egress window rule of 0.35 m2 in Subsection 9.9.10, which governs escape rather than air. When finishing a basement, previously adequate ventilation openings often get drywalled over and must be replaced by mechanical means.

Plain language

If a room has no mechanical ventilation, its windows must actually open wide enough to air it out, roughly 0.28 square metres of clear opening for bedrooms and living rooms. Fixed glass does not count, only the part that opens.

Related questions

How is the principal ventilation fan sized by number of bedrooms in Ontario?What are the exhaust fan requirements for bathrooms and kitchens 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.