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do i still need to do airsealing detail between bottom plate and slab besides sill sealer if going with open cell spray foam cavity insulation?

bLu2021 | Posted in Energy Efficiency and Durability on

Framing tomorrow! Need help making a decision whether I should still take the time to apply acoustical sealant sandwiched between sill sealer and bottom plate and sill sealer and slab foundation if we are going to do open cell spray foam cavity insulation on our 2×4 wall studs? I was just told today that OC spray foam when applied greater than 3 inches is already an air barrier.  Is that true?  Please help, anyone.  I already bought the acoustical sealants and caulking gun but would rather skip this step if it is already unnecessary.

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  1. Expert Member
    Malcolm Taylor | | #1


    What insulation goes in the wall cavities doesn't influence whether you need to air-seal the mud-sill, which is below them. The acoustical sealant doesn't need to be sandwiched under the plate, you can do the joint on the inside or out, and you can do it later - but yes you need to air-seal that area.

  2. Expert Member
    Armando Cobo | | #2

    I guess it all depends how tight of a building envelope you want, we go for ≤1ACH50. As a different option, I typically specify Protecto Wrap sill sealer which is 3/8" thick and has a peel and stick bottom that applies to the concrete forming a really good gasket seal. When Builders use thin sill sealers, I specify to install a bead of sealant on the concrete, then place the sill sealer down, followed by a second bead between the top of the sill sealer and bottom of the plate. That will make sure you have a tight seal.

    1. bLu2021 | | #3

      which sealant do u use for the sandwich on top and under the sill sealer?

    2. Expert Member
      Malcolm Taylor | | #5


      If you are going to use a sandwich with standard sill-seal, I'd suggest stapling it to the bottom plate before standing the walls. Otherwise there is too much back and forth between the two before the walls are trued to know if you end up with an effective seal. That's why I prefer to wait until the walls are up and run the bead at the interior intersection of the slab and plate. The result is observable, and if you are using an interior membrane or poly, you can bed it in the sealant for a completely air-tight installation. The other advantage is this can be done later when the surfaces are clean and dry.

    3. Expert Member
      Armando Cobo | | #6

      Malcolm - We don't seem to have problems with this application, and I think there are many ways to skin a cat, right? The concrete is swept and clean before the first bead is applied.
      Since most of the times we use the Protecto Wrap sill sealer we don't use sealants, and more and more my Builders use Triple Guard’s self-adhesive air and moisture sill barrier on the outside of the wall, sealing off the cold joint between the foundation and frame construction. Redundant? Yes. Effective? Yes, very.
      But since bLu2021 already has the sill sealer, and I imagine it's the thin foam, I gave my solution to his application. I'll also include an old detail...

      1. Expert Member
        Zephyr7 | | #7

        I would think that a heavy bead of a good polyurethane sealant, combined with the stretchiness of a typical foam sill seal gasket would be able to handle a decent amount of wiggling before the seal would fail. You'd probably tear the sill seal gasket material first, which would probably give a visual indication of failure.

        Reading these comments makes me wonder (note I've not tried this) if it would help to use a sealant on the underside of the sill seal to stick it to the foundation wall, and a bead of drywall gasket material on the underside of the sill plate of the stud wall. Since the drywall gasket material acts something like a piece of weatherstripping, it should be able to be rocked around and keep resealing since it doesn't rely on being adhered to a surface to seal. Thoughts?


        1. Expert Member
          Malcolm Taylor | | #9


          Don't see why not, although I don't see what the advantage of having the air-seal in the middle of the plate is when above it will be on either the inside or out. Why not continue it in those planes, which helps continuity?

          1. Expert Member
            Zephyr7 | | #10

            Actually, I like your "wrap the membrane" idea, as it makes a very apparent continous air barrier. I was mostly commenting towards doing it with the sill seal. Lots of ways to accomplish things and all that :-)


      2. Expert Member
        Malcolm Taylor | | #8


        " I think there are many ways to skin a cat, right?"

        Indeed! My reluctance may have something to do with building in a climate where it rains - and when it isn't raining it's drizzling.

  3. Expert Member
    Armando Cobo | | #4

    Protecto Wrap, Prosoco, Titebond, DAP, etc. Check this article:

    1. Antonio Bettencourt | | #12

      Hi Armando, I'm just curious, because I don't have experience with Protecto Wrap. Do you typically have to install termite shields in your work? Is there a way to integrate that with the Wrap? I do mostly slab-on-grade work and put the shield between the slab and the sill plate. So it would block the installation of the Wrap.
      Thanks in advance.

    2. Expert Member
      Armando Cobo | | #13

      From their website... "forming a gasket seal to virtually eliminate any air, moisture and insect/rodent infiltration beneath the sill plate."
      You can also call their Sales and Tech. support in your area:

  4. bLu2021 | | #11

    thanks everyone!

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