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Ask a pro: Vapour barrier
Vapour barrier I gutted the entire basement and added new insulation. Below grade, there is simply 1 1/2" of white-foam insulation board over concrete blocks. Above grade, there is new Roxul R14 insulation and then the foam board. Do I need to put a vapour barrier over the foam board since I went with the cheaper white-foam insulation? - Dneugebauer via email
Expanded polystyrene (the white stuff) is typically not vapour-impermeable. The polyethylene-film vapour barrier that covers the batt insulation above should continue down over the foam. Technically, the wood strapping that contacts the masonry should also be separated from it by a damp-proofing barrier of some kind. When you get your permit, check with the building department to determine the acceptable damp-proofing in your area. Tar-saturated building paper may be acceptable, and there are a host of liquid roll-on or paint-on membranes that may help your basement wall system to perform optimally.