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Community and Q&A

Capillary break in closed cell spray foam

SamuelH | Posted in General Questions on
I’m planning to have 2″ of closed cell spray foam sprayed on my foundation walls and rim joist. My house has a 6 mil? polyethylene capillary break between the foundation wall and sill. Would it be a good idea to extend the capillary break, so that the foam on the sill/rim joist does not contact the foam on the foundation wall? I’m thinking this might help keep the sill and rim joist wood dry.
My wall assembly is: brick veneer, 1.5″ air gap, a common house wrap, half inch styrofoam with a clear plastic film on both sides, quarter inch pressed fiberboard (looks like sturdy cardboard), then the sill/rim joist. I read here ( that exterior insulation is only a condensation risk in colder climates, and shouldn’t be an issue in mine. Hopefully my understanding is correct. I’m in Nashville, TN, climate zone 4A. The top of the foundation wall varies from 1 to 2 feet above grade. The foundation walls are made of hollow concrete blocks. I live next to a drain easement, so the soil around my house is wet for weeks at a time during rainy months.

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  1. Expert Member
    Peter Engle | | #1

    Closed cell spray foam generally does not wick water, and you don't need to extend the capillary break.

  2. SamuelH | | #2

    At 2" it will be about .7 to .8 perms, so it does have the ability to diffuse some moisture. So you are saying that closed cell foam allows vapor to diffuse through it, but doesn't allow water to wick through it?

    1. Expert Member
      Dana Dorsett | | #4

      That's right. Moisture transfer by capillary draw is a very different process than moisture transfer via water vapor diffusion. Closed cell polyurethane is completely waterproof to liquid water, and doesn't take on moisture in the form of adsorb (which neither liquid nor water vapor) very quickly or easily.

      This is one of the major distinctions between polyurethane and it's chemical-cousin polyisocyanurate, which DOES move water via capillary draw, and does store some amount water as adsorb even without contact with liquid water.

      1. SamuelH | | #5

        That's so counter intuitive. Thanks for clarifying!

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Samuel,

    I think Peter is correct and you do not need a capillary break. Closed cell spray foam, which is commonly used in this application, should not wick water.

  4. SamuelH | | #6

    My installer has mostly finished the spray foam job, but didn’t spray foam the sill plate and top of the foundation wall. I was expecting he would. Is this typical? Wouldn’t it be better if the spray foam were continuous from the foundation wall to the rim joist?

    1. Expert Member
      Michael Maines | | #7

      Samuel, yes, it's now common practice to spray foam the rim joists. Did your installer give you a reason for not including it? If not, I would ask about it.

  5. SamuelH | | #8

    He spray foamed the rim joist, just not the sill plate. So, the vertical part of the wall is sprayed, the top, horizontal part of the wall is not, the sill plate is not, and the vertical portion of the rim joist is. He didn’t give a reason. I was thinking maybe to increase drying potential. Seems like it could cause condensation problems though.

    1. Expert Member
      Michael Maines | | #9

      Samuel, I just noticed that you're in a termite-prone area. Local codes probably require a gap in the foam for termite inspection.

  6. SamuelH | | #10

    We put the termite gap in the middle of the wall, maybe not exactly up to code, so that’s not the reason.

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