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Community and Q&A

Air Sealing Electrical Outlet Boxes

brendon89 | Posted in General Questions on

Looking for input on what products high performance builders are utilizing to seal around the wire penetration to there airtight electrical outlet boxes located on the exterior wall? I’m noticing some builders / insulators have been sealing them up with canned spray foam which is combustible if a electrical spark every occurred.

Any product suggestions that have worked well for past projects would be appreciated!

Were located in Climate Zone 5 up in Canada

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  1. user-5946022 | | #1

    Putty pads. They are intended for fireproofing.

    You could also use the fire rated or fire block spray foam. It is usually the orange color version and is sold in the big box stores. Just don't too much inside the box itself.

    1. michaelbluejay | | #11

      What I've read is fire block spray foam "is a lie", doesn't pass unmodified ASTM E814 and ignites at 240°F, well before the studs will. Some caulks have the same problem (e.g., 3M FR 136). Caulk that does meet ASTM E814 is 3M CP 25WB+.

      1. Expert Member
        MALCOLM TAYLOR | | #12

        The best sealing boxes, which also keep any caulk or foam outside the boxes, are: made by sometime GBA contributor Doug McIvers. Unfortunately they aren't approved for use in Canada.

  2. charlie_sullivan | | #2

    You can also buy what I think is the same putty in a 1 lb brick , under the name electrical duct-seal putty. The pads are more convenient--just mentioning the other form because it might be stocked locally if the pads are not, and it's useful to have some on hand for miscellaneous electrical air sealing.

    1. user-5946022 | | #5

      Charlie - that is in interesting product - thanks for pointing it out (have never encountered it). I'm not sure it is the same material of which putty pads are made - standard putty pads contain intumescent firestop material - they are both heat resistant and will swell to block any passage of oxygen when they get hot. The electrical duct seal putty (about $3 for 1 lb) does not seem to have those properties. They do sell pliable blocks of firestop sealant, but those are about $20 block.

      If I'm paying labor to have it done, putty pads are most cost effective. If I'm doing it myself, putty pads are still most cost effective, but I can reduce material cost by 50% by cutting the putty pads into quarters and using 1/4 of a pp across the top of a box and a quarter across the bottom, because when I'm doing it I know I take the effort (not much time, just being conscientious) to ensure the pp covers the entire gap in the box.

      The pp has the added benefit of acting as an acoustical barrier so reduces sound transmission. Along those lines, another alternative is the acoustical putty pads which are sold at the blue big box. They are readily available but I don't think they have any firestop properties.

  3. RILEYG | | #3

    Being in Canada, inspectors still want to see a vapour barrier installed around the electrical box on all exterior walls and exterior ceilings. I use normal metal boxes with the correct sized vapour installed around it. When I nail or screw the box on i do it from the inside of the vapour barrier so holes are only created on the stud side (hopefully this makes sense). After wiring I seal all cable penetrations on the outside of the vapour barrier with PL400. Being in Canada the inspector will want to see a 6mil poly barrier applied over the insulated wall/ceiling. After it goes on you cut the poly barrier around the outside of the electrical box. Pull the electrical box vapour barrier out so the 4 sides are over top of the poly barrier. Use 3m tape or even tuck tape to seal the 4 sides to the poly. If you did it right it is now air tight. Be careful during the drywall stage not to cut the barriers with a roto zip or drywall saw.

  4. Expert Member
    BILL WICHERS | | #4

    Duct seal doesn’t last forever so I’d be reluctant to use it to seal a box inside a wall. Duct seal eventually dries out and fails to seal, usually after around 10-15 years or so in my experience. I only use duct seal in accessible locations, and never for air sealing.

    The reddish putty pads are better, but you have to be careful about molding them to shape and getting them to adhere to the surfaces you’re sealing against.

    Best, in my opinion, for this application is the red silicone high-temperature caulk. This is high temperature silicone, not the intumescent fire stop stuff. This material seals well to everything as you’d expect from a silicone sealant, it’s electrically insulating, and it’s not a fire risk. This is what I like to use to seal small holes on electrical boxes.

    You’re ok using canned foam as long as you keep it outside of the box.


    1. charlie_sullivan | | #8

      Thanks--my experience with electrical duct seal is much shorter than 10-15 years so I appreciate the heads up on the limited life.

  5. brendon89 | | #6

    Awesome, appreciate everybody's feedback!

  6. Expert Member
    Akos | | #7

    I generally use the plastic air tight boxes with gasketed flanges. The wire penetrations themselves are well sealed without any extra attention, the built in foam gasket is good enough. These don't require the poly box around them, you can run the vapor barrier over the box and slit it around the opening. You can seal it around the flange with acoustic sealant, I usually find that drywall will clamp it quite tight without anything extra.

    I've also found that adding support to the other side of the box makes a big difference as the drywall now clamps against the flange on both sides. This also gives a very solid feel for the outlet itself.

  7. neilt | | #9

    Contrary to RileyG (#3) airtight boxes with gasketed flanges are CSA approved for use in Canada.
    They don't require additional poly around them. If you put poly on the wall it can be sealed to the flange.

  8. Expert Member
    MALCOLM TAYLOR | | #10


    If you are detailing the sheathing as at the primary air barrier, as many high-performance houses now do, air-sealing beyond using gasketted boxes isn't necessary. The advantage of having the wires entering the box loose is you can leave a courtesy loop outside in case the stripped wires get damaged.

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