Interior Vapour Barrier
Vapour Barrier is a wall component that is used within the building envelope to control the diffusion of water vapour. Sometimes vapour barriers are referred to as vapour retarders.
Any thermally insulated wall, ceiling or floor assembly must be constructed with a vapour barrier to reduce the potential for vapour diffusion induced condensation. A material can only be considered a vapour barrier if the specified permeability is not greater than 60 ng/Pa.s.m2 when measured in accordance with ASTM E96 /E96M “Water Vapour Transmission of Materials”. The vapour barrier must be located on the warm side of the dew point. Traditionally, this is taken to mean on the warm side of the insulation.
Polyethylene is a vapour barrier material made of a thin sheet of polyethylene plastic that usually has a very low vapour permeance (less than 60 ng/Pa.s.m2). Typically, polyethylene vapour barriers are 0.15 mm (6mil) thick and conform to CAN/CGSB-51.34-M “Vapour Barrier, Polyethylene Sheet for Use in Building Construction”. Polyethylene can reduce the drying potential of the wall where vapour is driven from the outside to the inside. This sometimes occurs in exposed walls clad with materials that can store rainwater, like brick. If polyethylene is installed continuously and is sealed at all joints, it can act as both the air and vapour barrier. For an example of how an assembly with polyethylene, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 12740.
Smart Vapour Retarders, or variable permeance retarders, consist of a nylon material that becomes more permeable as the relativity humidity that it is exposed to increases. Where the wall cavity is relatively dry, like in the wintertime, it acts as a traditional vapour barrier. When there is more moisture in the assembly, like in the summer, a smart vapour retarder may facilitate the drying of a cavity to the inside when vapour flow is outside to inside. For an example of how an assembly with a smart vapour retarder, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 13115.
Vapour retarder paint is a paint coating that has a specified vapour permeance not greater than 60 ng/Pa.s.m2 that controls the passage of water. Compared to polyethylene, vapour retarder paint could allow for better drying potential towards the inside. This vapour retarder paint would be painted onto the gypsum board according to Building Code. This is a less common vapour control approach when compared to polyethylene. Please check with the local municipality. It is important to follow the manufacturer’s instructions when using vapour retarder paint, in particular the minimum number of coats. It must be recognized to not use fewer coats than what has been specified by the manufacturer. For an example of how a wall assembly with vapour retarder paint, and its corresponding information, is displayed on the Effective R Tool, please see Wall 715.
References
Interior Vapour Barrier. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation.
Lstiburek, J. (2011, April 15). Bsd-106: Understanding vapor barriers. Retrieved April 27, 2021, from https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
MemBrain™. (2020, May 26). Retrieved April 27, 2021, from https://www.certainteed.com/building-insulation/products/membrain/
National Research Council Canada. (2015). National Building Code of Canada 2015 s. 9.25.4.2.(3)
National Research Council Canada. (2015). National Building Code of Canada 2015 s. Table A-9.25.5.1.(1) Air and Vapour Permeance Values(1)