I'm trying to determine how to address a small gap between my foundation wall and the bottom of the house. I believe this gap is being used by vermin to access a crawlspace and travel up the walls.

What would be the best filler for this gap? I'm thinking perhaps caulk, but open to suggestions.

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  • How big is the gap?
    – mmathis
    Oct 24, 2017 at 14:33
  • @mmathis Approximately 1/4" to 1/3"
    – Dave
    Oct 25, 2017 at 14:47

2 Answers 2


It looks like your siding is wood (probably cedar). You don't want to seal this too much - the underside of these shakes/shingles needs to be able to release moisture or your siding may rot sooner. Best option is to seal from the inside, probably with spray foam.

Edit based on comments: because you've indicated this is an inaccessible crawlspace, you should consider two other measures:

  1. Apply pesticides around your foundation, under the edge of the shingles, and at crawlspace vents. These can help control the vermin problem without perfect sealing. (Powder treatments including diatomaceous earth are pretty effective and easy to use for ants and similar critters, but you'll want to target whatever particular vermin you've seen.)
  2. Get access into your crawlspace and inspect there. If the crawlspace has no floor and vermin are digging through the dirt to get inside, extra sealing at this seam would have no effect. However, better sealing from inside the crawlspace could seal your walls and floors for better insulation and critter-resistance. Adding a moisture/bug barrier on the floor may also help. And you may find a bigger issue, like a colony of ants/termites/whatever established in your crawlspace.

I'd encourage you to get access into your crawlspace and inspect there -- it's really the best way to ascertain the cause of your critter problems and enables the most effective solutions.

  • They are cedar. But the crevice is somewhat behind the shingles, so I'm wondering if I can seal the gap without concern for trapping water behind the shingles. There's about a 1/3" gap between the backside of the shingles and the foundation wall top. Unfortunately the crawl space is inaccessible unless I break through the foundation wall -- at which point, I agree, spray foam would be the preferred method of closure.
    – Dave
    Oct 23, 2017 at 22:09
  • I've added another photo, brightening up the image and showing a red zone where the filler may be able to be added. Also worth noting that the crawl space has screened vents, which could allow moisture to escape if necessary.
    – Dave
    Oct 23, 2017 at 22:24
  • OK. I have a very similar setup on my house actually (Boston area, built 1893, cedar siding painted blue). The crevice you've identified appears to be the upper edge of the skim-coat of mortar on your foundation wall. Sealing that will probably do nothing: it's not likely to be a path that leads into your crawlspace or wall. Oct 26, 2017 at 14:16
  • Answer updated with a couple of better options. Oct 26, 2017 at 14:26

Don’t use spray foam to seal the holes. Mice will eat right through it. You want to use copper wire mesh. They can’t chew through that.

  • You can foam up the mesh. That gives air sealing as well.
    – DaveM
    Apr 12, 2019 at 12:50
  • That is true. But I’ve done spray foam before and it didn’t hold up
    – C Fella
    Apr 12, 2019 at 16:01

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