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OBC DIVISION B · PART 9 · SECTION 9.18 — CRAWL SPACES

What are the ventilation and access requirements for crawl spaces in Ontario?

Every crawl space needs an access opening of at least 500 x 700 mm (20 x 28 in.) unless serviced otherwise. Unheated crawl spaces require ventilation of about 0.1 m² of unobstructed vent area per 50 m² of floor area, distributed on opposite sides, plus a ground cover to control moisture.

Code reference
Crawl spaces must be provided with an access opening of not less than 500 mm by 700 mm, kept accessible. Unheated crawl spaces require natural ventilation to the outside at a rate of roughly 0.1 m² of vent area for each 50 m² of floor area, with openings arranged to promote cross-ventilation, and the ground must be covered to limit moisture evaporation into the space.
OBC Division B, Subsections 9.18.2. and 9.18.3.

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

What this means in practice

Crawl spaces fail from moisture, so the trio of ground cover, ventilation (for unheated spaces) and drainage does the real work. Vents get blocked by insulation, skirting or landscaping, and hatches get sealed shut during renovations, both common inspection items. The ground cover, typically polyethylene lapped and weighted or sealed, doubles as part of soil gas control. If mechanical equipment or shutoffs live in the crawl space, the access route needs to actually permit service. Cottage-country builds on piers should treat the skirted area under the floor as a crawl space for these rules.

Plain language

You need a hatch at least 500 x 700 mm to get into the crawl space, plastic sheeting over the dirt to hold down dampness, and if the space is unheated, vents on opposite sides, roughly one small vent for every 50 square metres of floor, so air can flow through.

Related questions

What is the difference between a heated and unheated crawl space under the Ontario Building Code?

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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.