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OBC DIVISION B · PART 9 · SECTION 9.32 — VENTILATION

What are the exhaust fan requirements for bathrooms and kitchens in Ontario?

Intermittent exhaust fans must provide at least 25 L/s (about 50 cfm) in bathrooms and 50 L/s (about 100 cfm) in kitchens; continuously running alternatives may use lower rates. All exhausts must duct directly outdoors, never into attics.

Code reference
Rooms generating moisture and odours require exhaust: bathrooms and kitchens must have mechanical exhaust at prescribed intermittent rates, with reduced rates permitted for continuous operation. Exhaust ducts must discharge to the outdoors through sealed, and where needed insulated, ducts, and total exhaust must be coordinated with supply air so the house is not excessively depressurized.
OBC Division B, Subsection 9.32.3.

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

What this means in practice

The two chronic failures here are termination and duct quality. Fans dumped into attics or soffit cavities saturate insulation and rot sheathing; code requires a dedicated exterior hood with backdraft damper. Runs through unheated space need insulation and slope so condensation drains outward. Kitchen range hoods over gas appliances and high-output cooking may warrant more than the minimum, but very large hoods raise makeup-air and combustion-spillage concerns for fuel-fired appliances. Choose quiet fans, ideally rated 1.0 sone or less, because loud fans simply never get switched on.

Plain language

Every bathroom needs a fan pushing at least about 50 cfm, and kitchens about 100 cfm, with the duct running all the way outside. Venting a fan into the attic is a guaranteed recipe for mould and rot.

Related questions

How is the principal ventilation fan sized by number of bedrooms in Ontario?What are the minimum openable window areas for natural ventilation 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.