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gregsenne 10-10-2012 09:11 AM

Vapor barrier issue between crawl space and conditioned basement
Ok, I know I have a problem, just not sure what to do about it. Although I've read a million vapor-barrier articles online, I'm still confused.

Between the unheated crawl space and the rest of the basement is that poly sheathing, completely sealed from ceiling to floor. This was originally done to keep the unconditioned crawl space separate from the rest of the basement. It is secured at the top with nails and then caulking and expanding foam, so there is absolutely no air penetration between the basement area and the crawl space. Then, next to this barrier, the basement wall was built. Then, batt insulation was put in the 2x4s, with the kraft paper facing the basement.

The way I see this, is there is insulation with vapor barrier on both sides, which is a problem.

Also, the wall that I am referring to (between the basement and crawl space) is the bathroom wall with a shower on it.

The room is not finished, and there is no drywall. Just studs with fiberglass batts in them.

Unless it's absolutely necessary, I don't want to remove the poly sheeting seperating the crawl space from the basement. If I just remove the kraft paper from the batts, do I have to worry about moisture going through the wall and getting into that insulation, since a shower is on that wall?

What should I do???


Windows on Wash 10-10-2012 07:38 PM

If you don't want to pull the poly, just put some unfaced fiberglass in there.

The poly is a class 1 vapor retarder and should not, if sealed properly, allow any moisture into the living space. Theoretically, moisture that get around via air leaks past the kraft facing, will not be able to dry out but I am not sure that is even and issue.

The vapor resistance of the kraft paper might keep some of the diffusion from the shower moisture but it will not have anywhere to dry to at that point.

gregsenne 10-10-2012 09:12 PM

So I should just take the Kraft paper off of the existing insulation, and hang drywall over it?

I was only concerned because I know my 1st floor has insulation in the outside walls, then the poly barrier, then drywall. And this would be backwards in the basement.

Gary in WA 10-11-2012 11:53 PM

The paper facing will stop most of the moisture trying to go to the crawlspace (doing its job well). The poly sheeting will stop all moisture going either way. Ideally, I would replace the poly with foam board for its insulating qualities will keep the wall cavity below the dew point for no condensation (wall cavity is warm). Because the poly has no insulating value, you run a great moisture risk from exfiltration of the bathroom air condensing on the poly wetting the fiberglass losing 60-70% R-value;

ADA the drywall:

Paper facing has variable perms, it lets more moisture through the wetter it gets (it would still dry to the inside room), poly is bad:

Deep-six the poly with no insulating qualities, and add f.b. The paper facing can be left on IF the f.b. is thick enough, read page 2 and notice the varying thicknesses with varying perm interior vapor retarders, pp. 10-14 for your Zone 5 location: It all hinges on if the cavity is warm enough to prevent condensation due to R-value of the foam for the degree of interior vapor retarder.

Foil-faced (pp.14) is acceptable left exposed in most crawl spaces, check with your local AHJ on fire code requirements to make sure. OR, reuse the poly (over unfaced f.b.) if f.b. is thick enough, still need an ignition barrier on crawl space side.


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