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2-inch foam on the interior face of the wall: What are my electrical box options?

mangler66 | Posted in GBA Pro Help on

Due to a foundation snaffu, it looks like I will have to move my 2in. EPS foam to the inside of the wall assembly. Although that seems feasible, it will add some labour and material cost (3/4″ strapping everywhere, 3/4″ plywood for the kitchen/cabinet area etc.)

I was specifically looking at the extra pain involved in installing electrical boxes. It seems most of the box extension rings I could find are limited to 1.5″, which is a bit shy of the 2in. I would require.

Are there any manufacturers building electrical boxes specifically with 2″ interior foam in mind? Am I better off using standard boxes and just spacing them off the stud 2in. with a piece of ply? I would love to hear from anyone with past experience with this type of wall assembly.

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  1. GBA Editor
    Martin Holladay | | #1

    Mai Tai,
    Thomas & Betts makes adjustable-depth electrical boxes. That said, I'm not sure of the maximum extension of these Thomas & Betts boxes.

    If you can't use adjustable-depth boxes, the easiest way to proceed is simply to mount the boxes at the correct depth from the start. If you plan to install 3/4-inch strapping to your 2-inch foam, simply attach your electrical boxes to the 3/4-inch strapping.

  2. GBA Editor
    Martin Holladay | | #2

    Mai Tai,
    Here's more information on the adjustable-depth boxes I was talking about. It looks like their maximum extension is 1 3/4 inch, so that probably won't work for you.

  3. mangler66 | | #3

    Thanks Martin, I think I saw those in store, but the box did not say what the adjustment range was.

    I'm looking at 2 3/4" + 1/2" drywall (which I think is implied and not included in the adjustment range).

    Attaching to the strapping may be limiting as to the height of the boxes, especially if I end up with horizontal strapping. Seems like attaching plywood strips to the studs at the right depth and using boxes that are meant to be side mounted may be the easiest/cheapest answer.

  4. user-1097046 | | #4

    Mai Tai,

    Is it too late to explore an exterior foam option? This site is full of people with a wealth of experience in exterior foam applications in both new construction and retrofit. If you can share some details of the current situation, we may be able to come up with a solution.

  5. mangler66 | | #5

    Hi Jon,

    Bottom line is my foundation wall is about 2" too narrow for everything planned (pour mistake).

    See here:

    My options now are fill the extra "ledge" with a dimensional piece of wood (exactly 2x6), or move the foam inside. Concrete contractor already attempted the wood fix, but used old wood and did not use a poly between the wood and concrete, so that will have to be removed. Even with good wood, I am concerned about long term durability of this option, with all my wall bearing on this wood that may be prone to moisture/rot etc. Moving foam inside solves this, and also actually makes the anchor bolt locations "work".

  6. user-1097046 | | #6

    That is a challenging detail. Fortunately it’s only one wall. You might consider a double wall assembly with thick top and bottom plates. If you decide to go with interior foam, the plywood nailers you mentioned would be a good solution.

  7. Expert Member


    The previous thread, and this one, deal mainly with possible mitigation of the error. Who is the general contractor or site supervisor overseeing the subs?

    I'm asking because this seems like a continuation of a sort of fly by the seat of your pants approach that has characterized your previous questions about how to build your project here, and my fear is that if you continue the same way there will be a lot more similar problems.

    You need to get someone on site who understands construction, can oversee the subs, can make sure the drawings are followed, and understands what the options are when errors occur. There is no way you should have to fundamentally alter your building assemblies at this point. You need to recognize something isn't quite working.

    I know this sounds a bit harsh, but I'd urge you to think about what I've written.

  8. mangler66 | | #8


    I don't think it's too harsh. Trust me I am eager to turn this over to my GC. But the agreement was that he would start once the foundation was poured, due to a crazy building spree in this region, and overall lack of time/resources.

    I was on site for the mishap. Did not catch it. I caught the missing door in the garage, kind of caught the misplaced anchor bolts, got them to sleeve the foundation in the right areas (felt like a victory), but missed the extra ledge in the front. So now I am dealing with it. Made me feel a it better to hear that this type of things happens everyday. My current house would be missing a window in the kitchen if I hadn't crashed the building site ten years ago.

    There is still a chance I can fill the 2" wide by 6" high ledge with long fiber reinforced concrete at slab pour and call it a day. The bolts (UCAN Torpedo) currently holding the punky wood could be put to good use, as tie in to the existing poured wall. I was worried about integrity of the small ledge as a second pour, but maybe with long fiber reinforcement and actual bolts holding it to the wall it would be fine. Then it would be business as usual.

    If you were a bit further East Malcolm, I would gladly give you the job :)

    I don't need to tell you it's not easy to find people who know what they are talking about AND care about doing a good job these days.

  9. Expert Member

    Mai Tai,

    Thanks for taking my comments in the spirit in which they were intended. I hate to see new construction projects have to compromise due to errors. I'm glad you can hand things over and relax a bit now.

    If it had been my foundation I would have jack hammered out the top 6" of the wall, epoxied galvanized ready-rod into the wall to act as both reinforcing and anchor bolts and re-poured the top.

    Good luck with the rest of your build.

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