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Looking to use plastic outlet boxes (NuTek One Gang Plastic Device Box - 18 cu-inches) for a home renovation. I've taken the exterior walls down to the studs and will run new 12/2 20A circuits with receptacles housed within the plastic outlet boxes.

This is an old house with 2x4 studs so would like to insulate with spray foam to increase the R-value. Any idea whether plastic device boxes can be used with spray foam insulation? Will the heat generated through the curing process possibly warp or otherwise compromise the plastic? AFAIK, these boxes have a 2-hour fire rating but may deteriorate at the 80-degree Celsius mark.

Also not sure whether the spray foam (polyurethane) may react with or compromise the plastic device boxes. Any insight or NEC / CEC code citations on this topic would be welcomed.

clarification As per @DMoore, what I meant to say is that I've removed the old gyprock from the walls so that I can replace the circa 1950 Sydney Steel plant wiring. "Fortunately" there was no insulation in the walls to worry about. interior work

And yes, the spray foam insulation will be done 'professionally' - aka not me.

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    I can only recommend that when running 12/2, you'll want to use the largest boxes possible go with a 20 or 22cu box. Especially if you're using "Decora" style square faced outlets or switches - the device takes up a lot of room in the box, not leaving much for the wiring & wire nuts. The few extra dimes per box will pay for themselves in ease of installation.
    – FreeMan
    Commented Sep 21, 2021 at 17:17
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    Agree with Freeman... you install the biggest boxes you find within reason. This can't be undone after the drywall is up and makes futureproofing easier.
    – DMoore
    Commented Sep 21, 2021 at 17:33
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    Well, you could always use metal boxes. Classically, 4x4x1-1/2" boxes with a 1-gang mud ring, giving about 26 cubic inches, in a box only 1-1/2" deep. Because of that, you can install 2 of them "back to back" in the same stud wall, even with a short conduit nipple between them for passthru. That means you can keep the switches at a consistent height and spacing in both rooms. Or use my favorite method: 1 circuit per wall with rooms on both sides of the wall sharing the circuit, giving each room access to more than 1 circuit, and saving wire length too. Run wire to 1 box, use the passthru.
    – Harper - Reinstate Monica
    Commented Sep 21, 2021 at 20:03
  • These are exterior walls - hence the need to insulate. The NuTek plastic boxes as spec'd to 18 cu-inches which I believe exceeds the minimum fill rate of 14.0 cu-in for 12-2 wiring. Once I try to do the first connection, I may come back with some expletives and upvote @FreeMan's comments.
    – GlennML
    Commented Sep 21, 2021 at 22:14
  • @GlennML -- why spray foam in the cavities? Are you trying to air-seal without having to take the siding off...?
    – ThreePhaseEel
    Commented Sep 21, 2021 at 23:55

2 Answers 2

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You can use silicone caulk to seal the entries into the box. If the walls are open when the spray foam is to be applied, you shouldn't have to worry about warping.

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You are going to have to figure out how to keep the foam out of the boxes. Once it cures you are fine. You will have to use the low expanding stuff and keep it from entering the back tabs. The fronts we saran wrap and rubber band. The back tabs are not easy to block.

I don't really understand the logistics you have going on here too... You have the walls open? Then how do you spray? If it were me - and I just did this with a house - I am putting in mineral wool insulation on walls and poly boards on the outside. Poly boards, plastic sheathing and a good tape job is a very very good protective barrier. I would never spray foam because I would worry about gaps (I would pay someone to do it with more experience perhaps and that knows how to work around electrical).

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  • My assumption was that the OP was having someone come in to do the spraying for him. Are there DIY options for a "full house" spray? I know there are cans of it for small jobs like around a window or door... Heck, if there's a DIY option, I might just investigate that!
    – FreeMan
    Commented Sep 21, 2021 at 18:31
  • @FreeMan - yes there are kits. I have seen my guys do our basements and honestly I can't see for the life of me how its better than roxul which packs easy. I certainly test my abilities with my external walls. The close cell stuff in my opinion is working the ratios and spray rates. It certainly isn't intuitive on how much you need to spray.
    – DMoore
    Commented Sep 21, 2021 at 18:39
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    @FreeMan The kits I have seen in ads always have 'professional use only" in the ad, from big tire store that sells everything.
    – crip659
    Commented Sep 21, 2021 at 19:15
  • @crip659 - we pay about $300 for the kits that will do a good size basement. They are two part and include gun. We have an varying results in trying to reuse the leftovers. I don't order these and we don't use them often but I can get the exacts.
    – DMoore
    Commented Sep 21, 2021 at 19:22
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    Wow. Interesting, thanks, guys. I had no idea that there were "for pro use only nudge, nudge, wink, wink" kits available to the average home DIYer. Also, I don't think the "big tire store that sells everything" operates on my side of the border, but I'll do some digging.
    – FreeMan
    Commented Sep 22, 2021 at 11:40

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