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Do I have to clean out and replace the Great Stuff?

Oak_Orchard | Posted in General Questions on

I have been back-side and edge sealing and also sealing the gaps on the inside of electrical boxes with Great Stuff Door and Window (low expansion foam). Then today I saw a blog that says Dow advises not to contact it with copper wire. I called them and they said its flammable during installation and when cured.

Do I have to clean out the Great Stuff and switch to another product for air sealing electrical boxes? If so, any suggestions.

Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    Yes, you do. You should never fill an electrical box -- even partially -- with spray foam or any kind of insulation.

    The air in the electrical box is there for a reason. Its purpose is to dissipate heat. Any interference with a conductor's ability to dissipate heat can create a fire hazard.

    The electrical code specifies the minimum volume of electrical boxes for a reason -- to be sure that there is enough air around the conductors to ensure safety.

  2. user-723121 | | #2


    Do your air sealing outside the volumetric portion of the electrical box and you will be fine.

  3. user-901114 | | #3

    What can be used in a retrofit situation where you don't want to pull the wall apart?

  4. davidmeiland | | #4

    You mean what type of insulating/sealing material?

    I've had good luck with the airtight electrical boxes from various manufacturers. The Thomas & Betts versions are available locally the my electricians will use them if I have them on the job. The only downside is the lack of a 3-gang and 4-gang size.

  5. user-901114 | | #5

    Thanks David. I wonder if using the orange fireblock foam inside the box is also a no no. Even if you're careful not to take up space in the box.

  6. davidmeiland | | #6

    Stephen, I'm sure the NEC says something to the effect that nothing but rated electrical devices shall be used inside electrical boxes. I wouldn't put foam inside a box, it's just not necessary.

    I have definitely put canned foam around electrical boxes. With the common types, a little bit of it can ooze through the various holes and come into the box--we're talking a tiny amount here. The metal boxes have screwholes in them without screws, and plastic boxes have gaps around the wire openings, so you can get some showing from the inside.

    I don't see how this is any different than what would happen with spray foam wall insulation. That stuff expands more than the Tremco LEF that I use, and no doubt the same thing happens. No direct experience, but that's my hunch.

    What I like about the airtight electrical boxes is the flange and gasket around the front. You don't get an air leak between the box and the drywall. Of course you can caulk or foam that gap with regular boxes if you use them, but if you use an airtight box, you don't need to do anything to seal it.

  7. user-901114 | | #7

    So in a retrofit situation where an old home has outlet leakage after a blower door test, we have two options 1. Rip the walls apart and install airtight boxes or 2. Install gaskets under the covers and use child proof outlet covers. Hmmm. May be an opportunity for a new product. Like maybe an airtight box within an old box.. codes for min space would have to be met. Some of the space could extend beyond the plane of the drywall perhaps.

  8. user-901114 | | #8

    Or, push a can foam straw thru a hole to get behind the box and blast some fireblock foam behind the box from the front. Cutting out any excess that got inside the box.

  9. GBA Editor
    Martin Holladay | | #9


    1. Turn off the circuit breaker.

    2. Remove the cover plate.

    3. Unscrew the receptacles or switches and carefully rotate them 90 degrees.

    4. If possible, insert the nozzle of your caulk gun to the rear of the box and carefully caulk the cable penetrations. This is not always possible (physically), but often is.

    5. Carefully caulk the crack between the drywall and the electrical box.

    6. Put everything back together and turn the circuit breaker back on.

  10. Oak_Orchard | | #10

    Well, I spent more than a day this long weekend taking apart my electrical switch, octagon and outlet boxes, scraping out the Great Stuff we sprayed (sparingly) into each one only two weeks ago. I then used a 50 yr latex caulk to spot plug these same holes. I also back sprayed the Great Stuff on any reachable boxes. It does not adhere well to the bottom back ends of the boxes; the caulking takes less space and does not sag out.

    I avoided PVC caulk for this job as would freeze the wire together and to the box.

    I have tried expensive boxes with baffles and gaskets; they are definitely not airtight and need the same treatment as conventional boxes.


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