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Recommendations for Electrical Box Seal

dfvellone | Posted in General Questions on

Hoping for recommendations for electrical box seals which would facilitate a quality airtight drywall installation.

My electric is completed and I used heavy-duty fiberglass reinforced electrical boxes.

Thank you, Daniel

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  1. Expert Member
    BILL WICHERS | | #1

    If you're using a smart vapor retarder, you can detail it into the box using flashing tape. That's what I've been doing as I renovate rooms at my own house. I detail the vapor retarder as an air barrier. I seal the wire entrances to the box with some canned foam or high temp silicone caulk prior to putting up the vapor retarder, then, after the vapor retarder goes up, I cut around the box, then use flashing tape to detail the edges and tie them into the box. If you're using the fiberglass "hard boxes" that I've often recommended on here, you should have no other holes to seal in the boxes except the ones that actually have wire in them. The nice thing about the fiberglass boxes is that they are completely sealed except for any holes you pop out at install time.

    I'm not a fan of the flanged boxes because the cause they make high spots in the wall behind the drywall.


    1. dfvellone | | #2

      Ok, so just simply detail the vapor retarder with tape. I had seen EnergyBlock, but it still required detailing the block to the box for an acceptable seal.

      How about if I proceed with only the airtight drywall approach (I'm still undecided if I'll use membrain + airtight drywall, or airtight drywall alone.) In that case detailing the joint between the box and drywall cutout with caulk is the best approach? No need for an additional box gasket?

      1. Expert Member
        BILL WICHERS | | #3

        Many would probably use acoustical sealant around the box, but that stuff is seriously messy. I've often injected canned foam using the small plastic tip attachment for the foam gun. Caulk could probably work too, but you'd need to get the drywall edge clean of dust first or you won't get a good seal. The advantage to taping a membrane is that it avoids the need to seal to the edge of the drywall in the cut opening. Most sealants don't adhere very well to the chalky surface of a cut edge of drywall.

        I actually put up a vapor retarder (MemBrain, in my case), then detailed it as an air barrier. I also put the drywall up using the airtight method around the perimeter, since it's just a bead of sealant as you go. I didn't seal the boxes to the drywall though, I relied on the tape seal to the vapor retarder for that. I sealed the exterior sheathing prior to insulation too. It's quick and cheap to run beads of sealant, so some redundancy here is helpful and doesn't add much cost.


        1. dfvellone | | #4

          That's good info. I've been wrestling with the double vapor retarder for some time, but maybe
          the benefit sealing Membrain to a lot of outlet and switch boxes is the deciding factor.
          One more thing I've not heard any feedback on, does the presence of two vapor retarders, Membrain and the airtight drywall approach with vapor retarding paint limit the ability for any moisture to dry towards the living space?

          1. Expert Member
            BILL WICHERS | | #5

            Drywall is pretty vapor open, as long as you don't put a zillion coats of paint on it, or vinyl wall paper. Drywall isn't really a vapor retarder, and it's certainly not a vapor barrier. Airtight drywall is an AIR barrier, not a VAPOR barrier, they are two different things.

            In my case, I have foil faced polyiso on the exterior of the wall, and that IS a vapor BARRIER. Since there is only 1/2" polyiso in most places (I haven't replaced the siding yet, at which time I'll add more polyiso on the exterior), and that's a bit light for my climate zone (northern edge of zone 5), I put in the smart vapor retarder for some extra safety margin. I detailed the vapor retarder as an air barrier for ease of sealing to electric boxes, and because it was generally easy to do -- just a bead of sealant around the wall perimeter prior to hanging the vapor barrier. I sealed the exterior polyiso to the wall too (easy to do with the wall open), and I sealed the drywall around the perimeter too. This gives me a triple redundant air barrier.

            The result is a much comfy corner room (where I'm sitting right now writing this, BTW :-), which used to be pretty drafty. The difference is very noticeable. Admittedly a lot of the leakage was around the bay window, which had essentially no effort made at air sealing during the original installation by previous owners of the house. I was very careful with my detailing in that area too.


  2. dfvellone | | #6

    What perimeter sealant did you use for the vapor retarder?

    Thanks, Daniel

    1. Expert Member
      BILL WICHERS | | #7

      I used a urethane caulk. Probably one from Loctite's PL line. I can go find a tube if you want to know for sure which one, I don't remember off the top of my head.


      1. dfvellone | | #8

        That's not necessary. That's what I use most commonly. Just wanted make sure I wasn't missing anything.

        Thanks very much for the advice.

  3. ERIC WHETZEL | | #9

    We used Bill's approach on our build:

    I also added acoustical putty pads to the back of each standard electrical box.

    Once drywall was up, I used this caulk to seal the drywall to the boxes and the Intello/HF Sealant:

    It's the best low VOC caulk I've been able to find. It cleans up with rubbing alcohol, which is a little unusual, but it holds up well to seasonal expansion and contraction.

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