Methods for sealing sill plate to foundation?

Discussion in 'Remodel Forum & Blog' started by cacher_chick, May 18, 2013.

  1. cacher_chick

    cacher_chick Test, Don't Guess!

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    My basement was constructed with standard 8" concrete block and the sill plate was set on that with no gasket. Needless to say, is has major air infiltration which I would like to stop.

    I've gotten spray foam underneath it in places, but most of the foam just falls down into the hollow cores of the block. I'm looking for other ideas on how to tackle this problem more effectively.
  2. LLigetfa

    LLigetfa DIYer, not in the trades

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  3. cacher_chick

    cacher_chick Test, Don't Guess!

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    That would seem simple enough if the top block wasn't hollow.

    Working from the basement, the seam between the block and sill plate ranges from 1/16" where the block is solid to 1/4" where the open web of the block is. The backer rod would have nothing to support it where the openings are in the top of the block.
  4. LLigetfa

    LLigetfa DIYer, not in the trades

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    Backer rod comes in a variety of diameters to match the gap needing filling. One generally just inserts it about a half inch below the surface to keep from having to use too much caulk.
  5. cacher_chick

    cacher_chick Test, Don't Guess!

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    I know what backer rod is. With all due respect, I'm hoping for a better solution than cobbling together 3 pieces of backer rod for every 16" long block all way around the house.
  6. LLigetfa

    LLigetfa DIYer, not in the trades

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    That suggested you were inserting it too deep. If you have other reasons for not wanting to use it then fine, but don't give lame excuses.

    My son does high-rise recaulking work and has hammered in miles of backer rod. It is boring and often tedious work when you need to keep switching to different diameters.
  7. cacher_chick

    cacher_chick Test, Don't Guess!

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    Maybe I should have posted a photo. :)

    The sill is 2x8, which has an actual dimension of 7-1/4 x 1-1/2. This causes the block to stick out far enough from below the plate that in many places I can see down into the hollow cores of the block, but there is not enough of a gap to get grout or any similar filler into the cores.

    I was thinking that slowly over a few weeks I could jack sections of the outside joists/wall up to see if I can slide in 8" wide strips of 1/4" thick XPS under the sill. I'm not too excited about that idea either.

    Pulling the siding off and attacking it from the outside might be less problematic in the end.
    Last edited: May 19, 2013
  8. LLigetfa

    LLigetfa DIYer, not in the trades

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    Ja, pic might have helped. Can you nail in a furring strip to close the gap with a narrow strip of closed cell sill plate gasket under it?

    Or maybe hammer in some cedar shingles between the block and the sill and then caulk or foam?

    Or treat the top of the block and edge of the sill with latex bonding agent and then trowel on a stiff mix of modified thinset?
  9. Dana

    Dana In the trades

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    You can cut'n'cobble some 1.5" XPS or EPS onto the CMU ledge, spray foam sealing the rigid foam to the CMU first, and once it's cured a bit, foam seal the gap between the sill & foam. Don't stop there- trim the sill/foam seal flush, then cut'n'cobble foam over the top of the sill and the face of the band joist, sealing them with foam. It's better to leave 1/4" gaps on the seams & edges when tacking the rigid foam in place (use foam-board construction adhesive to get it to stick to the wood), to allow reasonable penetration & expansion room for the expanding foam to make a good seal.

    You can use anybody's 1-part can-foam, or if you have a lot to do, a 12-board foot FrothPack (2-part 1.75lb density closed cell foam) goes a long way, and is available through box-stores for about $35-40.

    Relative to poured concrete foundations hollow core CMU has very low wicking capacity and a great deal of drying capacity for the foundation sill, but they also tend to leak HUGE amounts of air (and not just at the foundation sill or above-grade portion.) Putting sealed rigid foam (or 1-2" of closed cell spray foam) against the foundation wall on the interior has benefit well beyond simple R-value.
  10. DougB

    DougB Member

    Can you stuff some fiberglass insulation in the block - just enough so it will support the foam until it hardens?
  11. cacher_chick

    cacher_chick Test, Don't Guess!

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    Thanks guys for the ideas. Between them I feel better about tackling this without going to extreme measures. :eek:

    I was thinking about investing in a FrothPack, but the local store carries the big 200 bd foot kit, and it was over a buck a foot. I did not know they made smaller ones. My basement is still about 60F, so I will need to wait a couple more months to get to their recommended temperature for application.

    Should I expect a FrothPack to be a one shot deal that is going to have to be used all at once? (Like the small cans of spray foam?)

    If I already have 2" XPS up against the inside of the band, can I just spray over the top of it?

    Should I expect overspray I.E.- does the surrounding area need to be masked off as if we were spraying paint?

    Thanks again!
  12. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    Could you drive in some screws through the bottom plate first? Give something for the spray foam to bite to?

    Maybe drive a screw (3") in every 3" and then spot spray it. Let that set up and then go at it again?

    Perhaps you could use some string and push it throw the gaps. Shoot some foam and then pull back on the string a little. Once you get a few spots set you should be able to fill in the rest.

    Just throwing out an idea - never tried it before.

    Get some pictures up.... JW
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