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Understanding vapour barrier

 Vapour barrier is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies of buildings to prevent interstitial condensation and of packaging.

When you add or upgrade insulation you’ll need to add a vapour barrier or vapour control layer (VCL) on the warm side of the insulation and an airtightness layer on the cold side of the insulation.vapour control layer diagram

The vapour barrier or VCL prevent or restrict the amount of water vapour entering the insulation. Failure to do so, if using a manmade insulation such as mineral fibre or fibreglass, would allow interstitial condensation within the insulation, reduce its effectiveness and could potentially cause rot to timbers.

Properly defined, a vapor barrier alone does not control air movement; it controls the movement of moisture. In fact, a vapor barrier is not actually a barrier; it’s a vapor diffusion retarder (VDR). A VDR regulates moisture flow from inside out or from outside in at the molecular level. This moisture control function happens wherever the VDR is used in the structure. Therefore, unlike an air infiltration barrier, the VDR does not have to be continuous, sealed, or free of holes; a perforation in a VDR simply allows more vapor diffusion in that area compared with other areas where vapor diffusion is less restrictive.

Incorrect use of vapor barriers is leading to an increase in moisture related problems. Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior of assemblies prevent assemblies from drying inward. This can be a problem in any air-conditioned enclosure. This can be a problem in any below grade space. This can be a problem when there is also a vapor barrier on the exterior. This can be a problem where brick is installed over building paper and vapor permeable sheathing.