A few spots on the top of my basement wall have had their tops hacked off for projects (ducts/plumbing/unknown). We had a mouse issue and I'm looking to close these spots up in case they are finding their way in somehow that way.

My main question is should I cap/cover them off again with some sort of tile/adhesive or do I need to fill them?

The first two locations are against the back of the house but there is another room/slab further out above it and it looks pretty dry down in the "void" of blocks. They don't appear to be filled or reinforced. I was planning to just cover these up.

The third location which is on the side of the house is where I'm unsure if I should also fill it in with rocks/mortar/anything recommended? or also just cover it up on the top. It's a difficult spot to get access to but the pics show the outside walls mortar and further into the gap there is some dirt. If you're looking at the first of these "3rd location" pics the wall to the right is where the garage would be.

Loc 1

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Loc 2

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Loc 3

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  • 2
    I don't know about the cinder blocks, but after you take care of those you'll still have to fill in the remaining gaps to keep the mice out. Search online for copper mesh for rodent control. Stuff that into the holes, then cover it with expanding foam. I used that technique at my house, and it's kept the mice out for close to three years so far. If you skip the mesh, they'll just chew through the foam.
    – mrog
    Jul 19, 2018 at 16:12
  • they will definitely chew through foam. With an older house myself, I found "Motomco Tomcat Bait" along the top of the brick works well keeping the mice down. Jul 23, 2018 at 19:04
  • Appreciate the rodent comments - I have begun with the steel wool/copper mesh and also chemical warfare... I am more interested in the void along the foundation wall in the 3rd set of pictures. I don't understand why the void is there and want to know if my inclination to fill it with larger rocks and something like quikrete mason mix would be bad.
    – TEEKAY
    Jul 23, 2018 at 19:28
  • I had some of that Tomcat Bait (the 1 ounce bricks of Bromethalin), but I was a little freaked out by handling the stuff. I was treating it like it was pure cyanide. Looked up the (admittedly thin) information on its toxicology, though, and computed that I would have to eat two whole bricks of the stuff to warrant a stay for observation in an ER. I don't remember how much Fido would need to consume, but I place it where only a poisoned rodent itself could expose him.
    – popham
    Sep 30, 2023 at 18:16

1 Answer 1


If it were my house, I'd get several bags of Quikcrete and take the time to carefully shovel it in there with a small garden hand-shovel and maybe some sand. Quikcrete is cheap, reinforces your foundation from other problems, and fills the air gap. No gap, no problems. Then like you're doing now for rodent control and insulation. Unfortunately, I can't back this solution up with experience.

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