Sealing between rim joist and concrete block in basement – GreenBuildingAdvisor

Last summer we had a wasp nest in our basement. I found where the wasps were entering- there was a sizable hole near a basement window. The hole had been hidden by a chunk of concrete that was loose. Removed the loose concrete, filled the hole with Great Stuff pest block. Now I wait to see if it succeeds at blocking pests.

Then I did what any good homeowner would do – look for the same problem elsewhere. Yes, I found it.

My house doesn’t have a sill plate – the rim joists sit on top of the cement block basement walls. One might imagine the rim joist and cement block meeting like butt cheeks but it’s really more like a thigh gap. Cement was applied to address this but it’s often loose and rarely providing the air seal that was hoped for. Removing the cement with a hammer is trivial, as is pushing a screwdriver through the resulting gap.

Problem identified – what should I do about it?

I already had decided that using a high end spray foam on the joists was likely a good idea. I could continue chipping away at the cement that I’ve judged to be useless BUT maybe there’s a counter argument. Maybe I was to stick with the pest block to seal the gap OR maybe I could rely on the spray insulation.

Anyway, I thought I should stop and think about my next move before acting – any advice?