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Community and Q&A

Sealing perimeter of basement subfloor

JRoyal | Posted in General Questions on


I’m finishing my basement and following the typical GBA method for the floor: rigid foam installed over the concrete with OSB on top of the foam. The OSB is secured to the concrete with Tapcons. I’m leaving a 1/4″-1/2″ gap around the perimeter to allow for expansion.

My question is around what sealant/caulk to use to seal the gap? A specific brand or type would be much appreciated.


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  1. Expert Member
    Peter Engle | | #1

    You probably don't need to seal the gap. If you are insulating the walls with rigid foam, you can tape the wall foam to the floor foam. If you really want to seal the gap as a part of your air barrier, I would use canned foam (Great Stuff or equivalent).

    1. JRoyal | | #2

      Thanks for the reply Peter. My basement has several load bearing partition walls built directly on the concrete slab when the house was constructed. I went ahead and framed my exterior non-loading bearing walls directly over the concrete as to have everything the same height. So while I do have rigid foam installed against the poured concrete walls (I read GBA after all), there isn't a continuous path between the wall and floor foam. So, I wouldn't be able to tape the two sheets of foam together.

      I thought about using a low expansion one-part foam, like Great Stuff Door and Window, but wasn't sure if I wanted something that had some flexibility in it, like an acoustical caulk or something. I bought a case of the Window and Door foam and a pro-style dispenser when installing the wall insulation, so it would be easy to do.


  2. ssnellings | | #3

    Where possible, I'm a fan of assemblies that you can deconstruct if something goes wrong. This goes double for areas like basements where 'bad things' seem to happen more often.

    So my recommendation would be to use what Conservation Technology out of Baltimore calls a 'gap gasket'. Model BG44 is the one for 1/4-1/2" gaps. You have to call or email them to order, and my understanding is that there is not another vendor in the United States at the moment for these kinds of building gaskets.

    Canned foam or backer rod with caulk (like sealing an exterior window) would be a more typical answer.

    1. JRoyal | | #4

      Hi Sam. Thanks for the info on Conservation Technology. The gaskets look like a pretty decent option. Its amazing in 2020 I will have to call or email them to get it :), but I like the option of being able to pull it out if needed also.

      One question for everyone though; Could backer rod be used as a seal by itself?


  3. Expert Member


    You don't need anything like a 1/4" to 1/2" gap with OSB. You should be gapping the sheets 1/8", and leaving the same amount at the perimeter. That expands the options you have to seal the gap, although I don't see much benefit to doing so. What are you trying to stop by sealing it?

    1. JRoyal | | #6

      Hi Malcolm,

      I am gapping the butt ends of the OSB 1/8" using spacers. The long edges are T&G, so they are spaced automatically. The 1/4-1/2" gap around the perimeter comes from something Martin wrote in answer to some planning questions I had a while back ( I'm shooting for a 1/4" gap, but with the lack of squareness of the walls and my skill level, I end up with anything from 1/8" to 1/2".

      The purpose of the sealing is to make the floor air barrier continuous to minimize any condensation on the slab. I take it from your's and Peter's comment, there might not be much value in that?


      1. Expert Member
        MALCOLM TAYLOR | | #8


        A gap of 1/8" between OSB sheets assumes each will only expand 1/16", so a similar gap at the perimeter only needs to allow for similar 1/16" expansion. A larger space serves no purpose.

        Sealing the gap will do no harm, but remember that the drywall and baseboard trim will effectively cover the gap anyway. I'd run a bead of Big Stretch around the perimeter and be done

  4. Jon_R | | #7

    When it comes to movement, tape is better than a bead of class 100/50 sealant. Canned spray foam is so bad that it doesn't even have a movement rating.

    Do design your floor to dry upwards (how much depends on the foam perms that allow wetting). And think about what will happen when you get bulk wetting (flooding, toilet leak, pipe leak, spill).

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