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Community and Q&A

Air Sealing Electrical Boxes

Salesi | Posted in General Questions on

I see a lot of information on sealing plastic electrical boxes but not on sealing the standard electrical box with a mud ring and conduit. There are a lot of holes and seams in the metal electrical boxes and I was thinking of using either the aluminum tape used to seal duct work or mastic spread on heavy to seal all the holes and seams. Or is there a better way to seal them? This is new construction to I have access to the back of the box.

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  1. Expert Member
    BILL WICHERS | | #1

    While I haven’t tried it myself, mastic would probably work great for this. Foil tape would work on the flat areas but would probably be difficult to work with around the conduit fittings. I’ve usually used red high temperature silicone caulk myself, but I think mastic would be less work on a metal box since you could just spread it on and seal everything quickly.

    The only issue I could think of is that many metal boxes have a film of oil on them left over from the manufacturing process. This oil film might cause problems with adhesion of the mastic. Wiping the box down with an acetone soaked rag before applying the mastic would fix that, and isopropyl alcohol might also work as a slightly less aggressive solvent.


    1. Salesi | | #6

      Good point about the oil. I need to wipe them down before applying anything.

  2. AlexPoi | | #2

    I don't know where you are located but in Canada you can buy vapour barrier boot made for electrical box. I suppose it's available in the US as well

    1. Deleted | | #3


  3. arnoldk | | #4

    +1 for the vapour barrier boot for receptacle boxes but you can also use airtight plastic box which is what I used when I worked in electrical.
    In both case you will still need to seal with caulking where the wires enter the airtight plastic box or the vapour barrier boot.


  4. jberks | | #5

    As a guy who's spent a fair share of time using tin tape in the most asinine situations, I'd try the mastic first. Tin tape would work great in this situation, but it takes a lot of time and zen like patience peeling the backing, folding it lengthwise to get them in the corners, getting it around the conduit, missing, retaping, peeling it off your fingers. Etc.

    The Canadian vapour boot would be a pain if you already have the conduit in place.

    Mastic with a brush, or silicone with a nitrile glove and just hand spread it, in my world would be so much faster.

    Canned foam is the least best in terms of longevity I suppose. But still way better than nothing and still a very good option. It's also the fastest/easiest if you have a lot to do. Get the low expansion, window and door type of foam and a foam gun, it'll do you well.


  5. onslow | | #7


    Just one thing to add to your potential sealing headaches. I encountered a surprise problem in my former house, which occurred despite sealing the boxes with metal tape. (yes it is tedioius) Code for that home required conduit for all wiring as it appears your's does.

    I don't believe outlets are airtight, based on a surprise water drop falling from a ceiling outlet. I had placed it there in case I went ahead with a projector TV system. I first thought I had a roof leak, but discovered the outlet cover plate was full of water. I then realized heavy condensation had collected on the wiring passing up into the conduit.

    The electrician had run the conduit along the roof joists above the insulation to provide a junction box location for the roof fan, then piped further along before bending down into the back of the box location for the outlet. Warm and relatively moist air was passing through the outlet, up into the conduit where it condensed and tracked back down the wires. Very unexpected and fortunately not creating a short. I had to press sealing putty around the wires and into the conduit to prevent further condensation. A ceiling light box would do the same I believe.

    This may be a one in million situation, but if you think of conduit as very small ductwork, it may help you head off a similar occurrence. In my new house I spent a good deal of time sealing the envelope and elected to not fuss with sealing the plastic boxes and Romex type wiring. I am gambling perhaps, but I don't think I am going to see any significant air exchanges into the wall. All but a few wiring locations are contained inside the conditioned space and should be neutral as far as moisture transfer is concerned. The exit points for the garage, exterior lights and outlets were of course heavily sealed. Stainless wool and sealant combined to prevent mice on top of air. Do check the points where conduit passes through your WRB and garage if a shared wall. Mice are masters at squeezing in along conduit.

    1. Expert Member
      BILL WICHERS | | #8

      Receptacles are definitely not air tight. Switches are likely better, but still not air tight. As far as I know, there are no airtight versions of either device.

      What I’ve done in the past is to stuff some large backer rod into conduit that I think might leak air (or sound — it’s surprising how we’ll sound can carry through a length of conduit). The backer rod, if stuffed in for at least a few inches, helps to block airflow but is still easily removable if you need to pull new wires into the conduit.

      It might also be worth specifying the use of compression couplings in attics instead of set screw couplings. The cost for either isn’t much different, but the compression type are much less leaky. This would help limit stack effect drawing air through conduit from inside the house.


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