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How to seal a joint between a crawl space floor vapour barrier and spray polyurethane foam?

seanprokopec | Posted in Energy Efficiency and Durability on

I am currently doing some energy upgrades to a 2 bedroom house built on a crawl space.

The crawl space interior walls and rim joist cavity was previously spray foamed but no vapour barrier was placed on the floor.

I intend to lay 10 mil poly sheet over the entire surface but am concerned as the how I should seal the vapour barrier to the spray foam. I have read that ideally, you want to mechanically fasten the poly to the concrete also adding a bead of acoustic sealant but I am not sure how to do it in this in this particular situation.

If anyone has run into this situation and has come up with an effective method I would be very thankful to hear it!

Thanks

Sean

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The 10 mil poly doesn't need to extend all the way to the foam, but it's better if it comes up to at least the exterior grade. Gluing it to the foundation wall with a bead of polyurethane caulk a few inches from the edge of the sheet, followed by sealing the edge of the poly down with duct mastic after the caulk has set up works pretty well on clean concrete or clean CMU.

    For more uneven or dirtier surfaces it's sometimes better to use a heavy bead of caulk then clamp the edge of the polyethylene in place with 1x furring held in place by through-screwing it to the foundation directly at the caulk line with masonry screws.

  2. seanprokopec | | #2

    Hi Dana,

    The walls have already been sprayed with closed cell polyurethane foam.
    The trouble I'm having is sealing the floor vapour barrier to the already in place spray foam.
    Mostly because of the difficulty of sealing it to such an uneven surface.

    Sean

  3. GBA Editor
    Martin Holladay | | #3

    Sean,
    The amount of water vapor that will escape from a small crack isn't very significant. That said, you can always make a seal if it's important to you. I wouldn't bother -- I would just get the polyethylene close and keep the edge in place with a collection of bricks.

    If you want to attempt a seal, you can use Dana's suggestion of horizontal 1-by furring secured with long TapCons. This method could be accompanied by many tubes of caulk or gobs of canned spray foam.

    But vapor diffusion is proportional to area -- and I doubt if this crack represents more than 1% of the area of your crawl space. So your vapor barrier will be 99% effective.

  4. Greenconfusion | | #4

    Where are you getting the 10 Mil plastic? I have a similar project .

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