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Is closed-cell foam risky in this assembly?

projectnext | Posted in Energy Efficiency and Durability on

I’m building a new home on the southern border of CZ 7 in Minnesota. I have 3” of Halo Exterra EPS over Zip with 2x battens. The 2 X 6 wall will be filled with R21 fiberglass batts and no interior vapor barrier.

The floor system is 18” trusses, top chord on one end, bottom chord bearing on the other. I plan to spray foam the top chord bearing truss spaces because they have concrete on the exterior. I am considering spraying the exterior wall of the bottom chord truss space as well, but that has foam on the exterior.

The spray foam installers in the area are unfamiliar with the exterior insulation and insist that closed cell foam is the only way to go. They argue that open cell will allow vapor drive through the foam, inside to out. I have read and re-read the cautions on creating a foam sandwich.

Is applying closed cell foam in this assembly too risky? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    There is no risk to installing closed-cell spray foam on the interior side of a concrete foundation wall that also has exterior rigid foam. Concrete does not need to dry out in either direction (especially if you have remembered to install sill seal between the top of the concrete wall and the mudsill, which I assume you have done).

    I'm curious, though, about what type of foundation this is. Is it a crawl space? Basement? Something else?

    If it's a basement or crawl space, then it would be a good idea to extend the spray foam insulation all the way down to the footing.

  2. projectnext | | #2

    Martin-Thanks for your response! It's reassuring to hear I'm on track with the concrete walls.
    Here is where the story gets long. We are rebuilding after a house fire. The foundation and basement floor (w/radiant heat) were undamaged. The new home is a full walkout basement with one level above. I have 8" poured concrete walls under the TCB trusses, step down walls on the side and full wood walls above grade on the walkout side. The sills are all treated lumber and isolated with sealants, EPDM gaskets and sill seal. I have insulated the interior of the concrete walls with 2" XPS covered with 2 X 4 framing. I was careful in air sealing and the exterior insulation is in two layers and all seams are taped.
    The 18" rim on the north and east side of the home is my challenge. With the exterior foam I am unsure of the best way to add insulation to that area. The trusses are on 19.2 centers so adding anything but spray foam becomes a project. Thanks for your thoughts!

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