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Rigid foam insulation over concrete in basement

itsmyname | Posted in General Questions on

I am finishing a basement with poured concrete walls. There is no insulation over the concrete on the exterior. On the inside, I would like to install 1″ rigid foam and cover with 2×4 framing with batts of Roxul between studs. What type and finish of rigid foam would be best — Polyisio, EPS, XPS, or other? I can think of reasons for using each of these but wonder which solution keeps moisture vapor issues to a minimum?

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  1. GBA Editor
    Martin Holladay | | #1

    You can use any of the three products in this application. To avoid moisture problems, the foam layer needs to be airtight -- the idea is to prevent indoor air from contacting the wall. Beyond the issue of airtightness, the next most important feature is a high R-value. You want the interior surface of the foam to be warm, not cold. The higher the R-value of the foam, the warmer its interior surface will be. Thicker is better than thinner.

    More information here: How to Insulate a Basement Wall.

  2. Expert Member
    Dana Dorsett | | #2

    The lifecycle global warming potential of XPS is about 200x that of EPS and polyiso, due to the HFC134a blowing agent, and may be a net-negative for the planet at 2" in a sub-grade application. Cut'n'cobble of XPS into a mostly sub-grade studwall cavity is a colossal step backward for the planet. Foam is not inherently green, foam blown with high-GWP blowing agents is the opposite of green.

    EPS is fine- its blown with pentane (7x CO2 GWP compared to ~1400x CO2 GWP for HFC134a) and is more vapor permeable than XPS or foil-faced iso (iso is also usually blown with pentane post-Montreal Protocol), which can in some instances be important for keeping ground moisture from wicking up to the top of the concrete to affect the moisture content of the foundation sill. (Not an issue if you have a metal or EPDM capillary break between the foundation sill & concrete, or more than a foot of exposed exterior foundation.)

    It's important to use a some foam under the bottom plate of the stud to protect it from ground moisture and keep it warm enough to not take on moisture from humid summer air, if the slab isn't insulated below, with ground vapor barrier.

    The ratio of foam-R to fiber-R matters for the above-grade portion, for managing wintertime condensation issues, and is relative to the average wintertime temperatures in your area. In most of the lower 48 R5 foam/R15 Roxul would still work, but not in in the coolest parts. For US zones 7 & 8 (and most of Canada) R10 foam/R15 Roxul would work.

    If you're concerned about depth, use 2" EPS and turn the studs sideways on the plates (it's not a structural wall-it only has to hold the gypsum) and use split or compressed batts between the studs. Or, do what I did- use 3" reclaimed roofing iso from building demolition recyclers or commercial re-roofing contractors (many will have a stack of scavenged or dinged-up sheets to sell on the cheap) and use 1x furring TapConned to the foundation 24" o.c. on which to hang the gypsum. There are multilple vendors handling reclaimed roofing foam in my area at 1/4-1/3 the cost of virgin stock- cheaper than Roxul.

    If using polyiso on the walls, be sure to keep the exposed bottom edge off the slab, as it can slowly wick moisture from the concrete, which takes forever to dry. Fiber facers on roofing iso are still vapor-retarders, and are fine up against the concrete wall, but the exposed edges are somewhat susceptible.

  3. Morkin_99 | | #3

    In a similar vein, I was considering Roxul but now plan to use 1" XPS up against the concrete, and then frame a 2x4 wall and fill the stud cavities with 2" of rigid XPS (edit: or another foam that is greener based on Dana's comment below), and air-seal using spray foam. Hopefully this passes the green building advisor test!

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