AVS Forum Special Member
Join Date: Dec 2008
Location: L.A. - Lower Alabama
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I researched this quite a bit before I made a decision on how to deal with my (basement) theater. In short, you need to call your local building inspector. The requirements for a vapor barrier will differ depending on your location, and it sounds like you know enough to have a good conversation with them about the options you are considering.
With regard to flash and batt, I didn't read the linked article, but that is a method of achieving better insulating performance in your above grade walls. Definitely worth considering there as it is more cost effective than just spray foam and is advertised to achieve better R values than either spray foam OR conventional batts alone. Below grade, I don't think that method provides a thick enough layer of foam to provide a vapor barrier. IIRC, you need about 2" of closed cell foam to act as a vapor barrier, but you need to double check that to make sure. Open cell foam is not a vapor barrier, so that's a non-starter for the basement walls.
You mentioned your exterior water proofing, is it a sprayed on membrane system? Are there multiple layers? One question I could never get a definitive answer to was whether that changes your vapor barrier requirements. Our house has a sprayed on membrane plus a dimpled membrane plus a drainage mat (all on the exterior), so the question was does that provide a vapor barrier on the outside of the house? Our concrete walls are dry! There's much more moisture in the basement air than coming through those walls.
At any rate, if I had it to do over, I would have had 2" of closed cell foam sprayed on our walls, and had them trim it. Even with a vapor barrier on the exterior, and the sprayed foam vapor barrier on the interior, there is no organic material to develop mold. So trapping moisture between is not a problem. Then I would have framed my wall with about 1" air gap between the foam and the 2x's for decoupling. Then used regular fiberglass batts in the wall cavities for sound absorption.