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Community and Q&A

Sealing gap in sill plate

rockford33 | Posted in General Questions on

So I recently moved into a new house this summer.  After all the dust of moving, painting, etc. has settled (and some leaks sealed during a long, heavy rainy summer), I noticed an area along my sill plate which needs some attention.

The house (at least in this section) appears to have a double sill plate along the top of the basement foundation wall.  Right where the steel support beam in the basement meets the foundation wall, there is a longitudinal gap in the bottom sill plate.  The top sill plate is continuos, but the bottom one has about a 1 foot break right at the end of the steel support beam (like they couldn’t fit a piece of wood there, so they just left it out…).  I can see the bottom support rail for the vinyl siding on the exterior through this gap…

What is the best way to permanently seal this gap?  Originally I had planned to pull off that section of siding, installed a piece of pressure treated wood in the gap (with a sill gasket) to seal it, but the AC fuse box and lines are right above the gap so I can’t pull the siding off or even bend it enough out of the way to get a piece of wood in there.  My next thought is to slide a piece of flashing under the siding wide enough to bridge the gap, then spray foam the hole from the inside.  Does that sound reasonable, and would spray foam wick moisture up from the foundation, so placing a sill gasket would still be warranted?  The gap is above grade by about 2 feet, so water intrusion doesn’t seem to be an issue, just a large area of air infiltration that I want to get fixed before finishing the basement next year.

Thank You,

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  1. GBA Editor
    Martin Holladay | | #1

    Canned spray foam will work, and I don't think you have to worry about wicking. I don't know what this gap looks like on the exterior, but if you think that a piece of flashing can be slipped over the gap to contain the spray foam, that sounds like an acceptable approach to me.

  2. rockford33 | | #2

    Thank you for the reply Martin. You can't see the gap from the outside, the vinyl siding overlaps the concrete foundation slightly. There is also landscaping a few feet in front, so if some of the flashing shows, you'll never see it from the street unless you are back behind the landscaping and AC condenser.

  3. CMObuilds | | #3

    Keep in mind vinyl siding isnt a bulk water barrier, that is why it has weep holes built into the panels, so you want to flash it out under your WRB so any draining water doesnt roll into your foam patch/basement.

    Double sill is odd unless the beam pocket got screwed up or they were satisifying a possible code issue for foundation overhang, but is the bottom sill flush with exterior shearhing?

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