Moisture, Air Leakage, Vapour Diffusion and Heat Transfer Control
In modern wood-frame houses, continuous insulation is needed to provide energy efficiency and comfort. A continuous air barrier system will restrict air movement into and out of a house, help provide thermal comfort, reduce heat loss and avoid moisture condensation in the walls and ceilings that can cause damage. A vapour barrier prevents water vapour from migrating into the framing and insulation. A sheathing membrane is installed over the exterior sheathing to prevent inward migration of water that penetrates beyond the cladding. The membrane should also allow vapour that has migrated from the conditioned space to dissipate to the exterior.
Source : Canada Mortgage and Housing Corporation (CMHC)