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Basement wiring with rigid foam insulation

Joshua Elliott | Posted in General Questions on

I have a concrete full height basement in climate zone 6 built in the 1980’s. The basement is partially within the thermal envelope and I am in the process of insulating and air sealing the rim joists with the cut and cobble method.

After the rim joists I’ll be insulating the concrete walls, and am planning to use 2” polyiso foam, but am open to alternatives. 

my question is this: what is the best way to attach dry wall and allow for outlets without framing a full 2×4 wall inside of the foam? There are a few places where the extra wall thickness would be annoying, so I’m hoping to keep the assembly as thin as possible. Is metal conduit running along the wall too weird for a residential basement? Any other solutions with better aesthetics?

I’m not planning to finish the basement (only drywalling for the sake of the thermal barrier), but don’t want to make things difficult for a future owner who might want to. 

Thanks for your time!

Joshua

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Replies

  1. Expert Member
    DCContrarian | | #1

    How are you installing the insulation and drywall? Just gluing the insulation to the concrete and the drywall to the insulation?

    If conduit is too weird there are surface-mount raceways that are slightly more residential-looking. Slightly.

    How many outlets are you doing? If this is unfinished space there's no requirement. (They also need to be GFCI-protected if that's the case.)

    To run the wire behind the drywall it has to be 1 1/2" beneath the face to be code. What I would do is in those sections use a piece of 1/2" polyiso under a piece of 1 1/2" instead of 2". Cut channels in the 1/2" piece for the cabling, glue it up, run the cable, then glue the 1 1/2" piece over it. I'd run all the outlets as a drop from the ceiling to minimize the cutting.

    1. Joshua Elliott | | #5

      Install: gluing foam to wall, probably attaching furring strips to concrete through the foam, then drywall to furring strips.

      Good to know on the raceways

      The thinner foam with embedded wiring sounds like a good idea.

      I’m thinking I’d either like outlets in places I find convenient for using the basement as a shop (haven’t decided total yet), or as many as would be needed were it converted to finished space.

  2. Patrick OSullivan | | #2

    > After the rim joists I’ll be insulating the concrete walls, and am planning to use 2” polyiso foam, but am open to alternatives.

    InSoFast (https://www.insofast.com) is EPS insulation with a built in plastic stud (for attaching drywall to) and has a built in chase for wiring.

    > I’m not planning to finish the basement (only drywalling for the sake of the thermal barrier), but don’t want to make things difficult for a future owner who might want to.

    I'd worry less about the future owner and more about what you want. :-)

    1. Joshua Elliott | | #6

      That is a very good looking product that I will seriously consider, thanks for sharing it!

      Me vs them: I’m more afraid of this possibility after the former owner had started framing walls and insulating with fiberglass, want to actually pay it forward and not create a headache for the next person. At the same time I realize your point and am probably aiming for a middle ground of a solution I’m satisfied with that doesn’t freak out The buyer/inspector when I sell the place.

    2. Expert Member
      DCContrarian | | #7

      I did Insofast on one wall in my basement.

      The pros:
      * It's easy. My teenage son actually did the installation
      * EPS foam is waterproof.
      * It has channels down the back to direct any water that might get in there
      * You can screw stuff to the plastic studs. I didn't hang wall-mount cabinets but I did screw base cabinets to it and they are solid.
      * It works the way they said it would.

      The cons:
      * It's expensive for what you get
      * The EPS has about half the R-value of polyiso for the same thickness.

  3. Expert Member
    Akos | | #3

    You can strap out the foam with 2x3 or 2x4 on flat. If you notch the foam behind the wood, you can run regular NMD through the notch, no need for conduit.

    For boxes, you can go with surface mount ones but mount them flush with the drywall. These are much shallower (1 7/8") but have more volume than regular 2 1/2 device boxes.

  4. Expert Member
    Zephyr7 | | #4

    You can use 4” square boxes (standard depth is 1-1/2”), and use conduit or MC cable to wire them in channels in the foam. Using conduit or MC avoids the need to keep the wire at any particular depth within the foam for code purposes. Use mud rings on the boxes to bring the devices up flush with the finished surface of the drywall. These are all standard electrical parts that are cheap and easy to get, so you don’t need to do anything fancy.

    Bill

  5. Joel Cheely | | #8

    In one house I had, I used 3 different installation methods: 1) drywall tight to insulation, tapcons to attach drywall. The few places I needed electrical (laundry room), I installed exposed on wood running boards. 2) The same but 1/2" OSB in lieu of gypsum board. At that time, code was not as specific re: thermal barrier requirements. For electrical, surface mounted to OSB. 3 ) traditional 2x4 wall in front of foam.

    I think a reasonable alternative for wiring in first method is to run electrical in baseboard area. Leaving the insulation out there wouldn't hurt. You'd have to figure out best way to box in. If you need to get inspected, you can't have wiring closer than 1 1/4" to face of framing or furring.

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