The Ontario Building Code | Vapour Barrier Properties and Installation
5.5.1.2. Vapour Barrier Properties and installation
(1) The vapour barrier shall have sufficiently low permeance and shall be positioned in the building component or assembly so as to,
(a) minimize moisture transfer by diffusion, to surfaces within the assembly that would be cold enough to cause condensation at the design temperature and humidity conditions, or
(b) reduce moisture transfer by diffusion, to surfaces within the assembly that would be cold enough to cause condensation at the design temperature and humidity conditions, to a rate that will not allow sufficient accumulation of moisture to cause deterioration or otherwise adversely affect any of,
(i) the health or safety of building users,
(ii) the intended use of the building, or
(iii) the operation of building services.
(2) Coatings applied to gypsum wallboard to provide required resistance to vapour diffusion shall conform to the requirements of Sentence (1) when tested in accordance with CAN/CGSB-1.501-M, "Method for Permeance of Coated Wallboard".
(3) Coatings applied to materials other than gypsum wallboard to provide required resistance to vapour diffusion shall conform to the requirements of Sentence (1) when tested in accordance with ASTM E96 / E96M, "Water Vapor Transmission of Materials" by the desiccant method (dry cup).
Section 5.6. Precipitation
5.6.1. Protection from Precipitation
In Plain Language
This clause sets a performance standard for vapour barriers in wall, ceiling, and floor assemblies. The material has to resist moisture diffusion enough, or reduce it enough, that condensation inside the assembly doesn't build up to the point of damaging the structure, insulation, or building occupants' health. It's a performance requirement rather than a specific product mandate: it specifies how well a vapour barrier has to perform, tested to specific ASTM or CGSB standards depending on the material, rather than naming an exact product or thickness.
Common Questions
Q: Does the code require a specific vapour barrier product or thickness?
No. This clause is performance-based: it specifies how well the vapour barrier has to perform rather than naming a required product or mil thickness, verified by specific test methods depending on the material used.
Q: What happens if a vapour barrier is skipped or underperforms?
Moisture can diffuse into the wall assembly and condense on cold surfaces, which over time can lead to mould, wood decay, or reduced insulation performance, which is exactly what this clause aims to prevent.
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.