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I'm planning on adding a lot double-gang old-work boxes to a wall that is made of 2" foam and 1/2" drywall on top of 2x4 studs. The tabs on old-work boxes don't reach far enough to grip the back of the foam, so I need to make some clearance right behind the drywall in the upper right and lower left corners so it can grip just the drywall. What might be a good tool or method for removing a chunk of foam that is set behind 1/2" of drywall? The chunk removed needs to encompass the "swing" of the tab, and be at least as thick or slightly thicker than the depth of the tab. The drywall needs to remain intact for the tabs to grip. I'll be doing at least 20 of these boxes, so I'm less concerned about precision and am more interested in just getting it done. Even if that means removing the entire thickness of the foam in that spot, the spaces between the bays are insulated yet.

Any ideas are appreciated!

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I'd try a rotary (drum) rasp of a suitable diameter in a drill. The radius should be slightly larger than that of the swinging part on the box.

You should be able to work it in behind the drywall with the shaft perpendicular to the wall and carve out a dandy space for the flipper-flapper. Just brace your hands well against the wall. There's a lot of leverage there.

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  • That's brilliant. Will need a pretty chunky one (didn't know they made them this big!), but should totally work AND keeps me from cutting into the drywall since the back edge is "safe". Great idea, and my current plan unless someone suggests something that might be quicker.
    – Brandon
    Commented Dec 3, 2021 at 16:10
  • I think you'll find this plenty quick, and even if there are faster solutions they're likely to remove more material than necessary, degrading insulation value. Electrical boxes are common cold points in a wall.
    – isherwood
    Commented Dec 3, 2021 at 16:29
  • I'd have two hands and my stomach on the drill. And maybe only run it backwards. If it decides to bite it's going to be a bad day for you or the drywall.
    – Mazura
    Commented Dec 4, 2021 at 1:47

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