The Ontario Building Code | Required Barrier to Vapour Diffusion
9.25.4.1. Required Barrier to Vapour Diffusion
(1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier sufficient to prevent condensation in the wall spaces, floor spaces or attic or roof spaces.
In Plain Language
A vapour barrier is required to control vapour diffusion through the building envelope, as set out in this article, limiting moisture that could otherwise migrate into and condense within wall or roof assemblies.
This is a plain-language summary of the 2012/2017 Ontario Building Code text above, for general understanding only and not a substitute for the Code. Always confirm current requirements against the 2024 edition (O.Reg. 163/24) or a qualified professional before construction.