Location of the Vapour Barrier
To prevent condensation from forming within building envelope assemblies, materials that act as vapour barriers (vapour retarders), including certain types of insulation, must be located within the assembly so that moisture moving from the inside to the outside does not condense and accumulate within the assembly. This means that vapour barriers should be located on the warm side of insulating materials (Figure 14). An exception to this rule is permitted for walls where no more than one third of the total thermal resistance or RSI-value (R-value) is located on the interior side of the vapour barrier.
This is common in double-wall construction that maintains a sufficiently warm temperature at the vapour barrier to prevent condensation from vapour diffusion in most Canadian climate zones.
Polyethylene film is the most common vapour barrier used in Canadian houses. Other materials serve as vapour barriers such as metal foil, polystyrene insulation, sprayed polyurethane foam, polyisocyanurate insulation, metal and glass.
A vapour barrier must be continuous but does not need to be sealed. (An air barrier needs to be continuous and must be sealed). As a result, sheet polyethylene can only serve as both vapour barrier and air barrier if it is sealed to form a continuous airtight assembly around the perimeter of the house.
Source : Canada Mortgage and Housing Corporation (CMHC)