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Contractor wants to spray open cell, but I think I want closed cell

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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Replies

  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

      Patrick,

      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: https://www.greenbuildingadvisor.com/article/insulating-rim-joists

      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: https://www.greenbuildingadvisor.com/article/air-leakage-through-spray-polyurethane-foam

        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

          Patrick,

          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.

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