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Where should vapour barrier be in double basement wall?

mugsy7244 | Posted in General Questions on

Walkout basement in zone 4C (west coast) 2×6 framed exterior wall with r20 fiberglass batt insulation. To eliminate the ‘step’ (many houses have a finished shelf) of concrete foundation which comes up 2′, client wants walls framed from floor to underside joist inside foundation to avoid the shelf, creating a space between the framed exterior wall and the new interior wall. Should the vapour barrier (6 mil plastic be relocated to inside of newly framed wall, and if so, should new wall be insulated? the full cavity be insulated? thanks, dan

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  1. GBA Editor
    Martin Holladay | | #1

    In Climate Zone 4, you definitely don't want to use an interior vapor barrier like 6-mil poly in your wall assembly. Fortunately, the building code does not require a vapor barrier in your wall. If you live in Canada, you may occasionally encounter a building inspector who wants to see an interior membrane of some kind. If you have a building inspector like that, you can install a "smart" vapor retarder like MemBrain.

    For more information on this issue, see Do I Need a Vapor Retarder?

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