I have an unfinished basement that always has tons of beetles, spiders and cold air (yes I know most basements have this, but it is more than normal). I finally decided to take a piece of insulation out that sits on top of the foundation between the floor joists and there is a gap between the top plate and floor joists where you can see light getting into the house and a substantial amount of cold air coming in.

Is there any reason I can't fill this gap with something? Also would you guys use expandable insulating foam or something different. Blow are pictures for reference.

enter image description here

  • 3
    What is on the other side? Apart from potential spiders, mice and cold air.
    – Tim
    Nov 13 '20 at 13:24

This is a perfect scenario for spray foam, which both seals against airflow and adds insulation value. Pick up a couple cans and seal all such joints, then overlay fiberglass batts that match or exceed your wall insulation thickness.

Be aware that the standard foam swells to many times its initial size. Use small beads and try to force it into the void rather than just covering the surface. Also be aware that urethane does not come off clothing, skin, and almost anything else. Wear gloves and old clothes, and cover any surfaces or objects you need to protect, including outside the gap. Leave any bulge alone to cure, then tear or cut it off. Wiping it usually makes a mess.

You might also use a silicone or paintable latex caulk on the outside face of that gap, though. I do wonder when there's no exterior covering at that point. Does the wall sheathing not extend onto the sill plate?

  • 7
    my favorite t-shirt still has a dab of faded orange from me spraying a home in the exact same location about 10 years ago. 100+ washes and me picking at it has made it slightly smaller.
    – DMoore
    Nov 12 '20 at 22:08
  • 1
    On the messiness of foam: if you don't toss a can of the foam after use, make sure you put it on something that you don't care about. Whatever was left in the straw will continue to expand and drip.
    – JimmyJames
    Nov 13 '20 at 19:29
  • The OP could also use a piece of drywall or plywood to seal the big gap, then screw, putty, and/or caulk that into place before the foam. It would give it a little more resilience to prevent the foam from spilling out the gap. Nov 13 '20 at 20:31

Isherwood is right on the approach - spray foam or caulk - or both (caulk first). I however want to add... If I had a crack this big I would block it off with a 2x4 or even 1x's. This is mouse heaven and caulk/foam aren't keeping mice out.

  • 3
    There are spray foams that are said to deter rodents and insects, though I can’t speak to their efficacy.
    – canadianer
    Nov 13 '20 at 7:17
  • Are you thinking the mice will likely eat through the spray foam and caulk (but not the 1x's)? Nov 13 '20 at 10:45
  • Mice rarely eat through wood unless they're desperate. They'll rarely eat through silicone, even. Wood is insurance if mice have been an issue before.
    – isherwood
    Nov 13 '20 at 14:16
  • @isherwood - Honestly I would block it not because mice eat through caulk/foam - not sure I have ever seen that but the fact that this crack is big, I wouldn't expect caulk and foam to cover it long-term. Yea it might look look covered but then 6 months later the caulk moved as caulk will do in huge cracks (think temp change in wood) and then you have a little hole in the caulk. Mice don't need much to get through. This general area of the house during home building is a pet peeve of mine with lack of insulation and workmanship. Cracks this big in this spot... totally normal.
    – DMoore
    Nov 13 '20 at 16:24
  • 1
    Fair enough, but I think the photo is deceptive. The light is coming in at an oblique angle and making it look larger than it actually is.
    – isherwood
    Nov 13 '20 at 16:26

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