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Sealing electrical boxes after construction

user-5946022 | Posted in Energy Efficiency and Durability on

So after you have addressed all other issues, and you are down to the last bit of air sealing, and air is coming through the electrical box, what is the best way to seal them?

There seem to be three options, with option 2 being the preferred.  Is this correct?  Below are some questions about using option 2…

a) foam, ensuring that NONE of the foam encroaches on the volume of the electrical box.  Fire rated foam, with the nozzle in the box opening, and trimming away any that expands into the box seems like it would be the quickest way.  Is this code compliant?  The following post seems to indicate anything in the box could be considered a code violation…
and this post unecivocably states it is:

b) From the post below, it seems high temp fire rated caulk is preferred.  Does this caulk stay in place in the hole through which the wires come after the caulk is cured and when the electricians move wires around to terminate the devices?  If you keep it from encroaching on the box, is there any code issue associated with applying this to the knockouts, both the unused open ones and the ones through which wires come?

c) Interior applied putty pad – sometimes called “box inserts.”  Seems like it might be quick, but according to this they are not approved for non-metallic boxes, which many residences have.

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  1. Expert Member
    BILL WICHERS | | #1

    A lot of how you do it depends on how much access you have, and which kind of box. I prefer the white Fiberglas “hard boxes” in walls where I have air sealing to do because these types of boxes have no holes — their knockouts are all sealed until you actually pop them out to bring wires in. With these boxes, a bit of high temperature silicone around the wire is usually all you need to seal the box, and some canned foam to seal the box to the wall.

    For the regular plastic boxes or metal boxes, both of which have lots of holes, you’ll need to do more work with your caulk gun to seal all the holes. If you have access to the back of the box, you can either use the fire stop pads and form them around the back of the box, or you can bury the box in canned foam and just trim away any that seeps into the box through holes. Another possibility is to use aluminum duct tape to seal the baco the box.

    There are lots of ways to do it, just make sure you don’t intrude too much into the interior volume of the box, and you should use fire rated materials for safety.


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