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Basement extruded polystyrene insulation

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  • Dec 17th, 2020 6:07 pm
[OP]
Deal Addict
Jan 1, 2007
1151 posts
171 upvotes

Basement extruded polystyrene insulation

Are most people doing their basement installing extruded polystyrene insulation before framing?

Can you frame directly against the XPS or give an inch or two?
8 replies
[OP]
Deal Addict
Jan 1, 2007
1151 posts
171 upvotes
I was thinking of 1 inch XPS for R5 and frame with Roxul R14.

I am reading that rigid board (and spray foam) is a potential moisture trap from the foundation wall .... hmm.
Member
May 15, 2017
202 posts
181 upvotes
I think if you're going to go through the process of putting foam against the wall, use 2". Foam all of the seams.

1" you still run the risk of condensation forming in the interior side of the foam when you put R14 insulation in front of it. Do some reading on dew point in wall construction assemblies.
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User avatar
Dec 10, 2008
4849 posts
1390 upvotes
Min. 2" XPS or you need a vapor barrier

Or flash-and-bat with 2" of spray closed-cell spray foam and bats.

If I was doing it again, I'd just have someone spray the works of it. Frame out 2x4, 2" off the wall, spray and be done with it.
Let's hug it out
Member
Dec 6, 2020
378 posts
354 upvotes
engmsf wrote: I was thinking of 1 inch XPS for R5 and frame with Roxul R14.
Don't use air permeable insulation (fiberglass, roxul, cellulose) in basements. You'll get mold in the insulation and will have to rip all of it out to get rid of the smell.

Use enough thickness of XPS to get the R value you want/need and be sure to seal all the joints. Yes, this will be eyewateringly expensive.
[OP]
Deal Addict
Jan 1, 2007
1151 posts
171 upvotes
middleofnowhere wrote: Don't use air permeable insulation (fiberglass, roxul, cellulose) in basements. You'll get mold in the insulation and will have to rip all of it out to get rid of the smell.

Use enough thickness of XPS to get the R value you want/need and be sure to seal all the joints. Yes, this will be eyewateringly expensive.
Hmm interesting. If take out builder stuff, I would need 4 inches of XPS to get R22 for code? I can offset the XPS install to hide the joints, but still tape them.

I don't really need a 2 x 4 frame then since no batt is needed? It can be thinner, enough to hold drywall and electrical support.
Member
Dec 6, 2020
378 posts
354 upvotes
engmsf wrote: I don't really need a 2 x 4 frame then since no batt is needed? It can be thinner, enough to hold drywall and electrical support.
You can do 2x4s on their side if you don't want to lose too much space. I wouldn't use anything thinner, however, or you might have issues with the drywall flexing too much, or cracking at the joints.
Member
Apr 17, 2016
344 posts
246 upvotes
Toronto
Recently did my basement, exterior framed with 2x3s, about 1" from wall, and spray foam insulation...I think approx 3" or so.

2x4s are still quite a bit more money and the 2x3s work just fine for these walls. Once the spray foam is in, the wood isn't flexing or moving at all.

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