Crawlspace vapor barrier and rigid insulation order of operations
![](https://secure.gravatar.com/avatar/f755064125d8036a6fcf5dc6f25e7322?s=52&d=https%3A%2F%2Fimages.finehomebuilding.com%2Fapp%2Favatars%2F200x200%2Fj.png&r=g)
Hi all,
Ive seen both on the FHB and GBA websites so im kind of confused on which to do. I am in climate zone 5b and have no bulk water issues. My current crawlspace is masonry block foundation walls and dirt floor. I’ll be using a 15 mil poly for vapor barrier, seams overlapped and taped. I’ve seen both rigid insulation on the walls then followed by vapor barrier and I’ve also seen vapor barrier to the walls followed by the rigid insulation.
my plan was to go up the walls by about 1 foot with the vapor barrier. I have approved foam board adhesive for the rigid insulation. I was going to use double sided tape for the vapor barrier. The only thing is, I’ve done a mock run and the foam board adhesive doesn’t seem to dry when applied between the 2” XPS and the vapor barrier likely because they’re both vapor barriers. I was really hoping to avoid having to use mechanical fasteners just due to hassle.
So perhaps I could attach the rigid foam to the foundation walls and then using the double sided tape on the rigid insulation? Just wanted to make sure it wasn’t an issue if the rigid insulation could absorb any vapor from the walls. I suppose it could still potentially dry to the inside a bit.
I’ll be sure to tape the seams of the rigid insulation and then I’ll tie in the rim joist insulation to the wall insulation for continuous insulation and vapor/air barrier system. Thanks.
Replies
Bump
Depends on the specific insulation used. If the insulation does absorb some water, the R value could be reduced a bit.
I did something similar. (foam board to the cmu walls, and heavy poly as vapor barrier taped to the foam, and at joints with an overlap. I also used some soil staples and covered with more tape since my crawl space floor is not exactly even.)
Thanks very much. It’s XPS foam so it’s minimal I just wanted to try to do best case scenario if I could. I will probably buy a small pack of plastic anchors to put through the poly into the foam and tape over them as a slightly belt and suspenders approach