How I foamed my block wall bays without pulling the framing out

Oct 19, 2026 Last reply: 2 days ago 4 Replies
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Joe_Plumb

One thing that saved me on a similar job: my bays were not anywhere near uniform. Old basement, studs cut in place, every gap a diffrent width. I stopped measuring each one individually and just cut each sheet slightly proud, then shaved it down till it pushed in with light…

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Did almost exactly this in my basement a couple years back, and the framing stayed put. Cut rigid foam sheets so they fit snug in each bay and pressed them right against the block. Don't leave gaps and hope the batts hide them, because air leaking behind the foam is where you get condensation. A bead of low-expansion foam around every edge does the job better than tape on a rough block surface.



For thickness, I left enough depth for a good layer of mineral wool between the foam and the drywall. Check what depth you actually have before buying board, that's the mistake I nearly made. The studs touching the block will still be a weak spot thermally, no way around it without continuous board behind them. But with the foam sealed tight to the masonry, the warm room air never reaches cold concrete, so the condensation risk mainly comes from sloppy edge sealing rather than the stud contact itself. I've had zero moisture issues thru two winters now.



If any bay sits below grade and stays damp, sort that drainage issue first. Foam against wet block just hides the problem.


One thing that saved me on a similar job: my bays were not anywhere near uniform. Old basement, studs cut in place, every gap a diffrent width. I stopped measuring each one individually and just cut each sheet slightly proud, then shaved it down till it pushed in with light pressure. A cheap foam saw made that part painless, clean edges every time. Snug enough to stay put on its own before I ran the sealant bead. Took maybe an extra minute per bay and I had way fewer touchups than on the room where I tried to pre-measure everything.

I did the cut-to-fit thing in my garage workshop, worked fine, but one thing I'd add is test for moisture first if you're not sure about the wall. Tape a square of plastic sheeting down tight for a day or two, see if it's damp underneath. Saved me from sealing up a section that had a slow leak from a downspout. Fixed the grade outside and the test came back dry after that. Cheap and easy peace of mind before you cover everything up.

Same expirience here with the shave-down approach, works way better than trying to measure every bay perfect. One thing that sped me up was numbering each sheet and the matching bay with a pencil as I went, so when I came back to foam the edges I knew exactly which piece went where without rechecking. With a dozen-plus bays down there it saved me a bunch of head scratching.

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