I am insulating my rim joists with 2" rigid foam board. Most of the rim joist is bare, and the foam fits along it nicely. Some sill boxes have some small irregularities (vent pipes, plumbing, etc) that I have worked around easily enough.

However, there are many electrical cables running along the rim joist near the circuit breaker box. For a good 10 feet in either direction, there are enough cables that it doesn't seem practical to get the foam board flush against the rim joist. Yes, I could carefully and laboriously cut little channels for each cable in the back of the foam, but that would take a long time.

Is it okay to simply push the foam against the cables and call it good enough? This will leave some air pockets trapped between the foam and the rim joist because the foam will be off of the joist a 1/2" or so (the width of the cables). Will this cause moisture or other problems? Seems it shouldn't as long as I do a good job sealing around the foam board.

(p.s. I know that spray foam would be the ideal solution, but we are trying to avoid that, mostly for cost reasons)

  • you'll be fine with a gap
    – DrewJordan
    Commented May 22, 2015 at 13:19
  • Do you have a pic of what you are doing? I have found that rock wool is less expensive and superior to foam boards in this situation... and a much easier install.
    – DMoore
    Commented Nov 19, 2015 at 0:04

1 Answer 1


Spray foam would not be either an ideal or recommended procedure to cover up all the wires where they exit from the main power panel. The first moment when some electrical problem needs troubleshooting, you need to add a new circuit or an existing wire needs moving you will be cussing that you ever thought of spraying in foam in this area.

  • 1
    Okay - that's good to know. any thoughts on the rest of it? Commented Nov 23, 2014 at 21:58
  • Yes yes yes. This is exactly why I think that people are absolutely crazy spray foaming attics that carry almost every circuit to the house. Spray foam has its places on exterior walls where you are dealing with one electrical run at a time but putting it near a lot of electric is a mess. Thankfully most of the crap houses that I spruce up have no insulation at all - and that is how bad spray foam is in the wrong places - I would rather have nothing to start with.
    – DMoore
    Commented Nov 19, 2015 at 0:03

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