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Questions about adding eps on top of xps?

602 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 doubt that there is a problem with layering different types of foam but i am no expert on that. But foam in living space needs to be covered with drywall, that is a code thing for your safety.

Mold will not grow on concrete, any of the insulation, or vapour barriers. But if the surface of any of them get cold enough for condensation to form that will leave a little dirt on the surface and eventually mold can grow on the dirt as the food for it.

So when you insulate you want to make sure it is of an R value that will keep the inner surface as warm as the rest of the basement. And have no holes that warm are can get behind or cold can come thru. And with a partly finished basement you want to make sure air can not get behind the insulation where you have stopped.
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