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Questions about adding eps on top of xps?

598 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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Not 100% sure but eps lets vapors and liquids through. XPS is more resistant. There is a video about it demonstrating with colored water. As such, if using foam boards, I'd stick with xps and use it as a drainage plane, vapor barrier and air seal, and don't mix the two.
Rockwool is more moisture resistant but it's not going to solve any water problem, if you have it. Underground bsmt is protected by the ground against temp changes. As such you don't need as much insulation. But if thin and 2" boards were spaced away from the wall and there is gap or room for air to move into the living area, none of it is effective. Same with rockwool. You have to enclose the insulation and stop the air movement in and around the insulation.

I didn't know that xps can lose half the value. I read about some loss over time. This is true for all other insulation. Gas in the double glazed windows, example. Face value of insulation is when it is installed. After that, it is a guess, but it is better than having none.
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