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Electric with rigid foam

user-7525789 | Posted in Green Building Techniques on

I have read so much great information on this site that guided me to how to insulate my complete rebuild of a 1940s house. My 1st story is above grade cinder block Im doing continues 2in interior rigid foam board as I have the house completely stripped down to the block inside. I plan on furring with 2×4 mainly because I have hundreds I salvaged from gigantic construction site where they were throwing them in the dumpster. Plus that gives me a little more depth for electrical outlet boxes.

My question does not involve the insulation itself as I have found bountiful information to guid me but on this topic. I was curious how everyone runs their electrical in situations like this. 

I have a lot of options I can pull everything to the ceiling and drop them down between the furring strips, half the house is over  a cistern so coming from the basement is a hassle. I guess Im wondering what others do because it will cost me almost double the amount of wire to run everything from the ceiling.  I was also wondering if I scored a small tunnel in the foam behind my furring strip and then cover the crossing with a steel plate but Im concerned about what that does to the integrity of my foam board. 

I have lots of electrical experience and understand the code book but nothing really fits into this scenario as you wouldn’t normally build a house this way. Lets just say Im much more fluid with new construction (residential, Im very unfamiliar with block). Any help would be greatly accepted. 


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  1. GBA Editor
    Brian Pontolilo | | #1

    Good morning Eharm,

    I'm sure you'll get some good suggestion on how to run your electrical cable soon. However, this might be a good conversation to have with the building inspector. Code books are one thing, an individual inspector's interpretation of how things should be done in non-standard assemblies is another.

  2. Expert Member
    Zephyr7 | | #2

    This may be a good application for conduit. Using EMT you could run a few main runs, then drop MC whips down to individual device boxes. That gives you armored wiring everywhere, and fewer runs since one conduit run can carry several circuits. If you use the regular NM cable, you do need to protect it with nail plates and the like.

    Another option might be smurf tube (the blue flex stuff, ENT). Any of the conduit systems also have the advantage of allowing for future wiring changes without opening your walls, but remember that al junction boxes need to be accessible — you’re not permitted to bury junctions inside of walls.

    If you’re framing out a wall with those 2x4s on top of a layer of rigid foam, then you can run wiring in the usual way in the wall cavities. It’s common to run “up and over”, going into the ceiling and across then down. It’s hard to recommend any specific cable pathway without actually seeing the site. You just need a viable pathway that goes about where you need it. Try to run the wires along common routes as much as possible to keep things neat and efficient.


  3. user-7525789 | | #3

    Thanks for you input appreciate it. I like the thought of having EMT carrying multiple circuits with the MC whips that can cut down on my wire backtracking, I am trying to stay away from any NM wire is these walls. I believe if I plan accordingly using the method you described I can actually make my junctions in the basement for the majority of the house and just run the whips up that way they unctions are exposed as per code.

    I was curious on thoughts because the "up and over method" will almost double my wire consumption due to the very strange lay out of the old home (sorry its almost impossible to describe or show pictures). As to the 2x4 framing I plan on using them as furring strips not an actual frame to save space. With 2in foam and 3.5in frame and dry wall I am losing 6inches of every room where if I turn them and use them as furring strips its only cutting into the room 4 inches. Doesn't sound like much of a difference but it has major consequences on my kitchen and island clearance.

    Thanks you have given me lots to think about and consider.


    1. Expert Member
      Zephyr7 | | #4

      Glad to help.

      Yes, you can put all the junction boxes in your basement. Junction boxes need to be accessible per code, but it doesn't say "easily" accessible. Locating the junction boxes in tiny crawlspaces is OK, as is locating them in damp basements and anywhere else that someone can get to. You just can't cover them up with drywall, for example.

      I'd recommend using "4-11" (4-11/16") square boxes, 2-1/8" deep, for your junction boxes. Those boxes give you more room for splices, and more useful knockouts (usually 3 concentric knockouts on each side). If they're all going to be in a basement, you might as well use the larger boxes and make your wiring work a little easier.

      If you use your 2x4s on the flat as furring strips, you can use shallow (1-1/2" deep) depth boxes and mud rings which will fit nicely in your reduced size wall cavities.


  4. Expert Member
    Akos | | #5

    If you run the romex behind the studs through the foam, you should have no issues. You only need nail plates if the wires are less than 1 1/4" from the surface, with a 2x4 on flat, the wire is far enough back, I would still put nail plates though. Romex can be run through insulation, putting it in a groove is no problem, wiring gets spray foamed all the time here.

    Your inspector might require some zipties to the studs to secure the wires.

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