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Questions about adding eps on top of xps?

592 Views 4 Replies 4 Participants Last post by  carpdad
I want to add more insulation to the stud bays in the unfinished area of my basement after learning that xps eventually looses half of its r value. They sell 3/4 inch eps already sized close to my needs. The rim joist and above ground stud bays have 2 inch xps. The builder left fiber glass and vapor barrier exposed but I suspected it was getting moldy so I replaced with xps and added dehumidifier.

My question is there any issue layering eps with xps? Should the eps go flat against the existing foam or would it be beneficial to leave a air gap? I locked the other sheets in with spray foam and that could easily provide some spacing. Seems it would be a good idea to drywall it as well.

The finished area only has the thin xps against the foundation walls and 2 inch on above ground bays. I didn't realize how beneficial it would have been to have 2 inch xps against the foundation as well. I'm in northern IL.

I can at least improve the unfinished area. I might eventually frame walls against the rest of the foundation but it would only be for insulating properties. I now know that saves a lot on heating costs but does it affect cooling in the summer?

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I'm not fond of air gaps unless needed to take advantage of a radiant barrier (foil surface).

I don't see a disadvantage to using both but this article may help.

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