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Community and Q&A

Air-Sealing an Electrical Box

BurkeW | Posted in General Questions on

We are building a high performance house in Des Moines, IA (climate zone 5), and are doing final electrical trim out.  We have a fairly tight house at 1.08 ACH/50 before drywall went up, using Zip as our primary air barrier along with Aerobarrier.  House has been sided and painted.

We have 4 exterior outlets going in, and currently plan to use a flush mount weathertight in use box, similar to the Arlington In-box (  I’ve talked with the electrician about how we might air seal this.  My current plan is this:

– Zip tape around the back of the box to seal off all the unused punch outs.
– Sealant around the wire that will run through punch out.
– Sealant between the box and the siding/sheathing
– Weatherproof seal (some kind of putty) behind the trim plate

I’m curious if others have a tried and true method for sealing these kinds of boxes?  We don’t have access to the inside of the wall any more, so everything needs to be done from the outside.

The alternative is to just use a surface mount box, which isn’t as nice looking, but also doesn’t require a hole in the exterior of the envelope.

Any suggestions?  Our current air leakage, which was measure post Aerobarrier, was around 79 sq/in.  So installing 4 of these boxes in a poor fashion could almost double that number.


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  1. GBA Editor
    Kiley Jacques | | #1

    You’ll find some expert solutions in this Q&A thread.

    1. BurkeW | | #2

      Thanks for the link Kiley! I've read through that thread and found it useful, but it seems to be talking about electrical boxes on the interior side of an exterior wall. In my case, I'm interested in electrical boxes on the exterior side of an exterior wall.

  2. Expert Member
    Akos | | #3

    There are usually small holes in the back side of most electrical boxes. Get canned foam with a small size nozzle and fill the cavity behind the box with SPF.

    You can get a better seal, but very messy, before installing the device box you fill the small pocket behind the box with foam and push the device box into it before it rises and sets. Let the foam expand and seal up and clean up anything that oozes out.

    Either case, make sure to trim any SPF that makes it into the electrical box.

  3. walta100 | | #4

    I used fire rated caulking in and around my electrical boxes.

    It is important to fill as little space in the box as possible as number of cubic inches inside the box is important in keeping the electrical connections cool.


  4. AlexPoi | | #5

    For the wires coming in the box, you can use Gardner Bender Duct Seal available at home depot or some strain relief connector with a seal like this one :

  5. woobagoobaa | | #6

    You've spent a lot of $$$ and effort on your control layers / ACH rating. I'd go with surface mounted boxes and use some of these to maintain your air/water control layers.

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