My house doesn't have any gasket between the sill plates and the concrete foundation, so there's some gap between the two (see red circle on the photo, which was taken from the ground facing up). I frequently see spiders in the house, and this gap might be the culprit. I caulked this area from the inside of my crawl space wherever I could reach, but there are areas I can't access from the crawl space (e.g., some are covered by the vapor barrier, some are in a crawl space I can't get to, etc.). I don't think these gaps are huge or anything, but large enough for spiders to get in. Preventing spiders from coming in is my main goal (my crawlspace is vented, so I don't think sealing these gaps will help with the energy bill).

I've read it's usually difficult to caulk or spray foam this gap from the outside, but in my case, the siding is like 2 feet above the ground so it's easy to access. In addition, the siding is quite a bit distance away from this gap because I have OSB sheathing and gypsum sheathing (for fire resistance, as I'm in a high wildfire risk area - see my earlier post (thanks to @isherwood)). So I should be able to caulk or spray foam between foundation and sill plates without touching the siding and Tyvek (I read you're not supposed to seal between the bottom of the siding and Tyvek, so water that gets behind the siding can drip out).

Can I use any urethane caulk or spray foam to fill the gap along the perimeter of the house? (I read urethane is good for concrete) I saw Home Depot has 3M Fire resistant caulk, but I didn't see one made out of urethane. Or should I use a ZIP System flashing tape instead? Thank you.

Photo taken from ground facing up

  • Foam, will compensate for the expansion and stop aliens
    – Traveler
    Jun 2, 2022 at 23:17
  • Did you end up spray foaming? I have a similar gap that I'd like to spray foam. BTW if you use spray foam, use great stuff pro + a pro gun from amazon. Those single use bottles are junk are one time use only.
    – Blaskovicz
    Sep 7, 2022 at 13:28

1 Answer 1


My preference would be Foam.

It will compensate for thermal expansion avoiding cracking.

It is water resistant and it will stop bugs.

Example: UV resistant

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  • The foam that I've used will dry and crumble when exposed to UV. This will, mostly, be protected, but it may be worthwhile looking for one that's rated for UV protection. (Don't know if such a beast is made.)
    – FreeMan
    Jun 3, 2022 at 11:34

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