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Basement framing: gap or no gap between rigid foam board and framing?

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Posted by: from Mulmur
10/16/2021 at 9:34:23 AM

My basement framing plan involves gluing 2" rigid foam board against the outside concrete basement walls, building a 2x4 stud wall, batt insulation between the studs, and a 6mm vapour barrier over the stud wall. I've been advised to NOT tape the joints between the foam board sheets. My question is should I leave a gap between the foam board and the stud wall, or can the stud wall go up against the foam board? I've seen advice both ways.

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Date/Time10/26/2021 at 2:00:20 PM

Your advice is all wrong. You want a insulation and vapor barrier that does "not" allow "air/moisture" egress/ingress. 1st of all foam board will be the least effective for R value and more expensive then batt.(I prefer Roxul). Framing should always be at least 1/2" min. from the concrete wall if using batt. If you are set on foam board it can be glued tight to the concrete (use styrene rated adhesive). Tape all joints with "tuck" tape. Frame walls in front of the foam board (can be tight) to foam, but as foundations are never straight I would leave a small gap to plumb & square walls. Vapor barrier framing on "warm side" of room. Don't forget about the "joist pockets" at top of walls, this is required by code. If you are going to all this trouble and expense do a better job and frame it with frost walls and use good batt insulation like Roxul. I only use foam board for crawl spaces, small jobs or where space is a issue.

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