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Receptacle connection with foam sheathing

rodrob15 | Posted in Building Code Questions on

I’m going to have 2” of rigid insulation on the exterior of my home and am trying to figure out the best way to install receptacles. It seems that one way is to use recessed receptacles (like this one by Arlington; link below). This isn’t preferable to me given I’d be cutting into the sheathing, which makes it a little harder to air seal.

I found a product made by Sturdimount which allow for “the electrical connection to be made over the housewrap”. This would seem advantageous to me since I could just fish romex through my sheathing/foam and into the UL rated box. It’s better from an air sealing perspective since I can use special gaskets for wires. However, I’m unsure on if this would be a code violation since I’d have romex running through foam? Would this configuration be ok if I used Romex rated for damp locations?

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  1. Jon_Lawrence | | #1

    We are using PVC conduit which is taped to the Zip sheathing using Siga's Rissan tape. The conduit extends past the sheathing, through 3" of Rockwool, a 1 1/2" nailer, and 1" Azek board where the outlet box will be mounted. I had a SturdiMount box that I bought for the hose bibb and showed it to my siding guy, but he preferred to make his own mount. See the bottom of the wall in the attached picture.

    1. rodrob15 | | #4

      Thanks Jon, your wall is very similar to mine. I'll have 2'' of mineral wool (not sure which brand right now, having a lot of trouble with pricing, lead time, etc.) and 2x4's used for the nailer/rainscreen.

      Did you run ROMEX through the pvc conduit?

  2. BrianPontolilo | | #2

    Here's how one of our bloggers handled this with no special products. I assume he'll be using some sort of weather-proof cover.

  3. Expert Member
    BILL WICHERS | | #3

    I personally would not use a nonmetallic box for the exterior, especially one like in that link that appears to mount to the foam. I think over time such a box is going to “squish in” and get wiggly and probably also break your seal(s).

    I would do either one or two things as follows:
    1- make a little mounting surface like you’d do to mount a light on siding. Mount a “bell box”, which are the outdoor metal boxes, there and run your cable through the knockout in the back of the box.
    2- my preferred method: mount a metal “deep” box (2-1/8”) on the surface of a stud. If you use a 4” square box, some knockouts will overhang the stud so you can use rear knockouts for cable entry. If using a single-gang box, you’ll have to use a side/top/bottom knockout and bring the cable into the foam a bit. You should protect the cable at that spot using a nail plate. When you install the cover plate, use some PVC trim to shim out from the siding a bit to meet the plate.

    A 4” square box is best here because you can use “mud rings” to bring the device opening up to close to flush with the surface of the siding. You’ll need to do a good job airsealing the box though. I’d use foil tape prior to installing the box to be sure I sealed all the screw holes and other caps.

    Don’t use UF cable in the wall. Just use regular NM (romex) cable. UF is needed for burial. NM is fine where it’s not exposed or buried. Any cable needs physical protection (conduit) if exposed.


  4. rodrob15 | | #5

    Thanks for all the replies guys.... I might not of made it clear in my initial post, but the crux of my question really is if standard Romex is permitted to be run through rigid insulation of any kind? Or if that's considered a damp or wet area, in which conduit would be required or special cable. Adding blocking or conduit are definitely viable options, but they all have their pitfalls to me (i.e. more taping and concerns with air sealing over time). If I am permitted to just run Romex through using a gasket (see link below), it just seems much easier and also this air seal should be rock solid for a long time and I wouldn't have to worry about any leaks through the electrical box itself.

    1. Expert Member
      BILL WICHERS | | #6

      Inside the insulation will not be considered a wet location. NM is kinda sorta expected to be in insulation sometimes, which is a big part of the reason you are only permitted to use the 60 degree rise column in the ampacity chart in the codebook for sizing it.

      In my read of the code, you’re ok running it in your rigid insulation a bit to get to the box. Don’t do something crazy and try to bite through the foam for several feet, just come out of the box and get through the foam to the studwall as quick as you can. I would also put a bit of physical protection (a nail plate) over the part of the foam where the wire is to make sure no random nails might puncture the wire. You’re supposed to use those nail plates whereever the wire is close to the surface in a studwall too.

      I’m making the assumption that you’re going out the back of the box into the studwall and doing your main run in the studwall. If you want to chain a few boxes together in the foam with only one penetration through the studwall, I’d run EMT conduit between the boxes and place that EMT in a notch in the foam. You need to use individual conductors (usually THHN) in the conduit, you’re not permitted to run NM or UF inside conduit.


  5. rodrob15 | | #7

    Thanks for all the reply guys.... I've consulted with a local electrician and he sent me this email, so it does seem that its fine to run romex in exterior insulation:

    "yes you can run romex to an exterior fixture / device. There are specifics as to what type of box and how the wire is secured, but without typing a book, if the romex terminates at the device, and the device is rated for exterior use such as a WR rated receptacle and then the receptacle is covered with a weatherproof cover, then wiring it with romex is fine. Watch your box fill because if you are daisy chaining outlets most exterior boxes ate only rated to hold (2) 12/2 wires. I've actually failed inspection before for having 3 #12's in one exterior mounted box.''

    1. Expert Member
      BILL WICHERS | | #8

      I would add to your electricians explanation, the romex needs to be protected from the outdoors. The romex itself isn’t rated to be in outdoor locations. This just means you need to enter your exterior box from the back so that the romex is always within the structure’s wall and only the device rated to be exposed is on the exterior of the wall.


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