The Ontario Building Code | Vapour Barrier Properties and Installation
188.8.131.52. 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,
(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