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Community and Q&A

Open-Cell Spray Foam at Rim Joist

Chip28 | Posted in General Questions on

I think I want closed cell foam, but the only spray foam contractor that I’ve been able to obtain a quote from wants to spray open cell foam in the basement rim joist of my home at the southern edge of Zone 4. It’s a walkout basement and the rim joist is anywhere from 18 inches to 8 feet above grade. Do I need to be worried about the vapor permeability of open cell in my region?

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  1. Patrick_OSullivan | | #1

    Closed cell is more common for rim joists because it functions as an air barrier in what is a typically leaky part of the house. Sometimes closed cell is not ideal (for instance if you had rigid foam on the other side of the rim joist, you'd be sandwiching wood between two low permeable materials), but it's often the better material for this job.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      Aren't both types of foam air-barriers? I thought the distinction between them was their vapour openness?
      Martin's advice is that either can be used in all but very cold climates:

      1. Jon_R | | #5

        Martin from the link:

        > In almost all situations... you don't want to use open-cell spray foam.

      2. Patrick_OSullivan | | #7

        > Aren't both types of foam air-barriers? I thought the distinction between them was their vapour openness?

        At the right thickness:

        At least by me (New Jersey), folks tend to want to apply 2ish inches of foam to rim joists. (I'm not saying this is ideal or sufficient; it's just what I see.) At that thickness, closed cell gets you the air barrier as well as near appropriate R value for basement walls. Open cell doesn't quite cut it.

        1. Expert Member
          MALCOLM TAYLOR | | #8


          Thanks, good to know!

  2. Chip28 | | #2

    Thanks, Patrick. There is no foam or other insulation on the exterior side of the rim joist.

  3. Jon_R | | #4

    Safest to treat it like a normal wall - where a class III or lower retarder is needed on the interior side with open cell (so closed cell is better or at least more convenient).

    1. Chip28 | | #6

      Thanks. I think I'm going to keep trying to find someone who will spray closed cell.

  4. Chip28 | | #9

    Another wrinkle in this question: There is no sill gasket serving as a capillary break, and the sill plate does not appear to be pressure treated. However, because the rim joist is cantilevered beyond the sill plate (i.e. the rim joist does not sit on the sill plate, but several inches beyond it), the outside edge of the sill plate can presumably dry to the exterior.

    Does this change the open/closed cell calculus?

  5. user-2310254 | | #10


    This doesn't answer your question, but I'll note that GBA sometimes advises trying to slightly jack up the foundation and slipping in a capillary break. Martin Holladay suggests this approach here:

  6. Expert Member
    PETER G ENGLE PE | | #11


    The untreated sill and cantilevered floor assembly do not change the open/closed recommendation. We are trying to limit the risk of condensation of interior moisture on the materials exposed to the cold. Open cell doesn't do that. While the exposure to the exterior does allow the materials to dry to the outside, and that reduces the risk of decay somewhat, it is still better if they don't get wet in the first place.

    1. Chip28 | | #12

      Understood, and thanks.

      The house was built in 1980, so the only way I can understand the absence of PT wood and a capillary break is due to the sill being 8 feet above grade in this area (it’s a walkout basement). Jacking up the house is not in the cards, so I’m trying to find the best compromise. I’ll proceed with closer cell as long as I’m not likely to rot my sill plate (which looks great after 40 years).

      1. Expert Member
        BILL WICHERS | | #13

        You're in better shape if you have some ability to dry to the exterior. Closed cell would be safer here. Be careful with exterior details to maintain some drying ability to be safe.

        Note that it's easier than you might think to jack up the house a little bit to slide in capillary break material (I like 1/32" HDPE for this). You only need to open up about 1/8" or so to slide something in. This is much different from jacking your house up enough to be able to replace a rotted sill plate or perform some other type of structural repair.


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