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OBC DIVISION B · PART 9 · SECTION 9.13 — DAMPPROOFING, WATERPROOFING AND SOIL GAS CONTROL

Does foundation dampproofing go on the inside or outside of the wall in Ontario?

Dampproofing must be applied to the exterior face of foundation walls below grade, from the footing up to finished ground level. Additional interior moisture protection is required where interior finishes or insulation are applied against below-grade concrete or masonry.

Code reference
The OBC requires dampproofing on the exterior surface of below-grade foundation walls that enclose interior space, extending from the top of the footing to grade. Where walls are unit masonry, parging is required before the dampproofing is applied. Separate provisions address moisture protection on the interior side where finishes are installed against the wall.
OBC Division B, Subsection 9.13.2.

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

What this means in practice

Exterior application is the governing requirement because it stops moisture before it enters the wall; interior-only coatings are not a code-compliant substitute for new construction. On block foundations, the wall must be parged first so the dampproofing has a continuous substrate. When basements are finished, a moisture barrier or appropriate insulation strategy on the interior below grade protects wood framing and drywall. Renovators dealing with existing leaks should note that interior membranes manage water rather than satisfy the original dampproofing rule.

Plain language

The black moisture coating belongs on the outside of the basement wall, where the soil is, running from the footing up to ground level. If you finish the basement, the inside face also needs moisture protection before studs and drywall go up.

Related questions

When is waterproofing required instead of dampproofing for a foundation in Ontario?What are the radon and soil gas control requirements for new houses 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.