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Community and Q&A

The best way to seal electrical boxes?

John Fahey | Posted in Green Building Techniques on

I have a refinished basement in a 1950 house. I’m trying to DIY air seal it. I air sealed all the rim joist I could access in the couple of rooms without walls covered with insulation (rigid insulation).

I noticed a great deal of air coming through the outlets. What is the best way to seal these? I used foam gaskets but there are still a noticeable flow of air coming in when the gas furnace kicks in.

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  1. Charlie Sullivan | | #1

    I would actually prioritize checking what's going on with your duct system, and sealing leaks in that--those leaks are (probably) causing the imbalance that drives that infiltration. Is there ductwork in spaces that are outside your air barrier, such as attic, part of the basement, or garage? If so, start with sealing the joints in those with duct mastic or foil tape.

  2. Expert Member
    Zephyr7 | | #2

    I agree with Charlie about checking ducts. See if you have air returns using the stud cavity as a “duct”, and make sure any vent grilles are clean. No returns should have dampers.

    You can seal the electrical boxes with fire rated silicone caulk (not intumescent fire caulk). You can also use the orange great stuf foam, but I like the fire caulk better. Try not to get a lot inside the box, just seal the holes and the edge of the box where it meets the drywall.

    If you don’t have a sealed combustion furnace (two pipes to outside for both combustion air supply and exhaust), then your furnace is drawing combustion wire from inside. Make sure no natural draft heating appliances (like a typical water heater) backdraft when the furnace is running.


  3. Expert Member
    Peter Yost | | #3

    Hi John -

    Great GBA resource on the issue of air sealing electrical boxes in existing buildngs:

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