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Air Sealing Around Electrical Boxes in Ceiling from the Attic

Lindaloowho | Posted in General Questions on

Hi All,

Working on an unfinished attic in Southern Ontario. I just watched Mike Guertin’s video re: the topic above.

I sealed the my standard electrical boxes by butting rigid foam tight to the sides and top of the box, cutting space for the wires. Kept the foam held held in place with acoustical sealant, then spray foamed the rigid foam with Gaps and Cracks. I also spray foamed the spaces in the rigid foam where the wires come out. I did not use fire-rated material. 

In an earlier post I read that there was no knowledge of having to use fire-rated material to insulate around the electrical box, but to keep the space inside the electrical box clear of insulation. 

Can I confirm that this is true? I DO NOT have to use fire-rated caulk, vapour barrier, or leave a space between electrical box and rigid foam or spray foam when dealing with a standard electrical box? 

Thanks for clarifying,

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  1. Expert Member
    BILL WICHERS | | #1

    You would only need to use fire rated materials here if your local codes require it or if you need to maintain an overall fire rating for the entire wall or ceiling assembly. In normal situations, you’re fine with what you did.

    I use red silicone high-temperature caulk for extra safety, but it’s not required. I would use urethane caulk instead of acoustical sealant mainly because it’s less messy to work with.

    The important thing is to keep the insulating materials out of the box itself.


  2. Lindaloowho | | #2

    Thanks Bill,
    Yes, I discovered quickly the mess that acoustical sealant makes. Black and gooey and sticks to doesn’t harden or dry either. I happened up against it in error, a week after applying , when I didn’t have working grubs on. Ruined a favourite shirt and shoes. :(

  3. 1869farmhouse | | #3

    My mentality has always been that an air sealed box is way harder to start a fire in than a drafty one - so a little foam can’t hurt.

    1. Expert Member
      BILL WICHERS | | #4

      Foam inside the box counts towards the box fill though, so it reduces the volume available for wires and splices (the reason the boxes are all rated for how many cubic inches they contain is that the electric code specifies how many wires/splices/etc can be installed in a box of a given volume).

      Most canned foam isn't fire rated, either, which is another issue. If you want to cover the box with foam, just put some tape over all the holes first so that the foam doesn't go into the box. I usually use foil tape for this, but just about any tape that will stay stuck while you do the work is OK.


  4. kyeser | | #5

    I use a combination of Siga Tape and red(fire rated) expanding spray foam. Tape all the holes, then spray foam the box to your sheetrock. I have had good success with this method, as there isn't much air that moves around the boxes. Pretty tight.

    1. charlie_sullivan | | #6

      I like this method. Much less complicated than cutting up pieces of foam.

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