Poly on inside of basement wall
I read this very helpful article by Martin from a couple of years ago
https://www.greenbuildingadvisor.com/article/vapor-retarders-for-basements-and-crawlspaces
He very definitively states that poly on the foundation wall is a bad idea. And I understand why Poly shouldn’t be applied on the warm side of a wood stud wall. But what is the issue on having poly between the foundation wall and the stud wall?
My builder, (who seems to be quite on the ball from a building science perspective, are advocating the following wall system for the basement (from outside to inside):
Dimple Mat
Bitumen-based waterproofing
Concrete foundation wall
***6mm Poly***
EPS (thickness TBC)
2×6″ framed stud wall with mineral wool bat insulation (~R20)
Intello+ smart vapour barrier
Drywall
Paint
What is the risk posed by having the 6mm poly in the assembly in this location? Wouldn’t it just help reduce the amount of vapour drive from the damp foundation wall into the dry basement? And in the winter, if warm moist air from the basement is migrating out towards the basement wall, the majority of it should be stopped by the EPS. And the Intello will allow for inward drying as needed.
So what’s the issue?
And how does the logic change if we replace the EPS with rigid mineral board insulation to reduce the amount of foam used in our project?
Thanks!
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If you have a Prime subscription, in this blog we revisited that topic: https://www.greenbuildingadvisor.com/article/vapor-retarders-for-foundation-walls. The short version is that your proposed assembly is fine. It is important to have the right ratio of EPS to fluffy insulation, to limit condensation; that depends on your climate zone and whether your interior will have excessive humidity.
Thanks -- a very helpful blog post!
So is it critical that I use a non-vapour open rigid insulation though? (Because if I have a vapour open rigid insulation like rigid mineral board I'd have too much vapour potentially hitting the cold poly layer in the winter months and that might lead to excess wetting? Correct?)
Does the presence of the Intello on the inside change anything?
And do I have to specify a vapour open paint? Or is Latex paint okay in the basement?
The problem with basements is that they are "wrong side vapor barrier" by nature. All of the vapor drive is toward the exterior, but the only side they can dry to is the interior. In a normal assembly you put the vapor barrier on the side with the vapor drive and leave the other side vapor open to dry.
In a WSVB assembly you really have to be sure to prevent condensation because it's going to be almost impossible to get that moisture out. So you need to have impermeable insulation so that interior air can't reach a surface cool enough to condense upon. And the warm side has to be vapor open so that whatever drying is possible to the interior can happen.
maxwell_mcgee,
Yes it definitely has to be foam, not mineral wool. Martin explains why in this article:
https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
"Can I insulate on the interior with fiberglass batts, mineral wool batts, or cellulose?
No. Fiberglass batts, mineral wool batts, and cellulose are air-permeable. When this type of insulation is installed in contact with concrete, the moisture in the interior air condenses against the cold concrete surface, leading to mold and rot. That’s why I advise builders that fiberglass batts, mineral wool insulation, and cellulose should never be installed against a basement wall."
I always spec vapor-impermeable foam, specifically polyiso, for foundation interior insulation. EPS is airtight but vapor permeable; with Intello it might be ok, but I prefer to be safe with what I know works. Mineral wool is vapor-open and it's not an air barrier, so placing it against the foundation is not safe. You could use mineral wool on the interior of a foam layer against the wall (or against poly over the wall).
If you have enough foam compared to the fluffy insulation you only need regular latex paint over drywall, a class 3 vapor retarder, but Intello or another variable-permeance membrane would also work. Don't use a vapor barrier paint as it won't allow drying to the interior.