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Basement walls: combining EPS/XPS with polyiso?

kjb034 | Posted in Green Building Techniques on

I’m in climate zone 5A. The builder installed 1/2″ foil-faced polyiso on the upper half of my basement walls, glued to the concrete. There is nothing on the lower half. I have already installed 2″ XPS w/canned spray foam in the rim joists and added back in the “Flame Spread 25” fiberglass the builder installed there.

I am planning to finish my basement at some point and I was wondering what to do with the existing polyiso. My original plan was to do 2″ XPS on the walls + 2×4 studwall with R-11 unfaced fiberglass batts in the cavities. However, after learning about the environmental impact of XPS, I’m now leaning more towards using EPS instead.

I don’t really want to take down perfectly good polyiso (and probably damage it in the process), so I’m wondering if it’s okay to add 1.5″ EPS over top of the polyiso, and do 2″ EPS on the bottom half to bring the full wall to 2″? I may go with thicker EPS since it is so much cheaper than XPS but the same question applies regardless of the total thickness.

Thanks in advance.

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Replies

  1. Dana1 | | #1

    It might be easier to install 1/2" foam on the lower half first, and then 1.5" goods as the second layer. That would end up being more reliably air-tight, since you can overlap the vertical seams of the two layers, and not have a deep lateral seam that goes from the studwall layer to the concrete. But you plan can still work if you prefer to do it that way.

  2. kjb034 | | #2

    That makes sense. Thanks, Dana!

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