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Should I use EPS/XPS or spray foam to insulate basement walls with a brick texture?

Macrocks | Posted in Energy Efficiency and Durability on

I am looking to add a few bedrooms and a bathroom to my walkout basement. It has poured concrete walls on 3 sides, but the poured concrete has a brick pattern / texture to it complete with simulated mortar joints. Since this is an uneven surface, I am wondering if EPS/XPS is an option or if I have to go with spray foam insulation to fill all the voids?

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  1. GBA Editor
    Martin Holladay | | #1

    The most foolproof solution in your case is closed-cell spray foam. If you go this route, I strongly urge you to choose one of the newer formulations of closed-cell spray foam that is made with a blowing agent that has a low global warming potential. For more information on the type of spray foam I'm talking about, see Next Generation Spray Foams Trickle into the Market.

    Closed-cell spray foam is expensive, however, so you may want to go with EPS. (I strongly advise you to avoid XPS, which has a blowing agent with a very high global warming potential. For more information on this topic, see Choosing Rigid Foam.)

    If you do use EPS, you'll need to use a generous amount of caulk or a continuous bead of canned spray foam at the perimeter of the EPS, to prevent air leakage through cracks. (You don't want any warm interior air to contact the cold foundation wall.) You'll also need to tape the seams of the EPS with a high-quality tape. Foil-faced rigid foam is the easiest type of foam to tape, so you probably want to buy foil-faced EPS or foil-faced polyiso.

    For an overview of all of these issues, see How to Insulate a Basement Wall.

  2. ethant | | #2

    Shouldn't we be advising people who are considering XPS to consider local XPS recyclers? Or is recycled XPS not good for basement walls?

  3. GBA Editor
    Martin Holladay | | #3

    XPS works fine on basement walls. Philosophers and ethical advisors may have differing opinions on whether green builders should buy and install used XPS. (It's a little bit like the controversies that arise when animal rights activists discuss the ethics of buying a used mink coat.)

    Personally, I think it's just fine to purchase and install used XPS.

  4. Expert Member
    Dana Dorsett | | #4

    From a design point of view it's best to derate XPS- especially used XPS, to R4.2/inch, it's fully-depleted performance will be after it's blowing agents have all dissipated.

    The drop from R5/inch (it's labeled LTTR) to R4.5/inch can be surprisingly quick, but the long asymptote tail down to R4.2/inch takes a handful of decades or more. With used XPS it's hard to know just how along the curve it already is, so to be conservative assume it's already done.

    Filling all the voids isn't important, and may even be beneficial for draining bulk water incursions. In damp soil areas some people even install dimple mat between the foam and the foundation wall. The key thing is to make it air tight at the seams, but especially at the top & bottom to prevent convective heat & moisture transfer around the foam layer.

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