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Community and Q&A

Shallow electrical boxes and exterior 2×4 walls

Cortland15B | Posted in Energy Efficiency and Durability on

Hello all, 

I’ve been lurking and reading for a long time and I’ve yet to find the recommendation of using shallow electrical boxes (17 and 18 cu in) when renovating an older house with 2×4 exterior walls. With a regular box there isn’t even a 1/ 2” behind the electrical boxes for insulation. I think there is a reduced sized electrical box that might give you just over a 1/2” but even still that’s not very much insulation behind an electrical box. Why don’t more people use the shallow electrical box so that you can get a full 2” of insulation behind a box.

This isn’t the super small electrical box that is only 8 cu inches but the bigger box that has a door and an extension to the side and gives a respectable 17 cu inches. They are usually used in shallow basement walls or things like that. They come in remodel and new construction varieties. They would be perfect if they came in draft tight models but I haven’t see any like that. 

The only disadvantage I see is it’s a little harder to air seal but not horrible. I haven’t wired outlets in them so I can’t say if it’s much more difficult to wire but they don’t seem to be that tight. 

What’s everyone’s thoughts? 

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  1. user-2310254 | | #1

    Why not a surface-mounted box?

  2. Expert Member
    BILL WICHERS | | #2

    You have to be careful with those really small boxes. The reason they all specify how many cubic inches they enclose is that the electric code specified how much room you need in a box to accommodate stuff. This means you need a certain amount of volume for every wire, every splice, and the device (“device” here is a switch or a receptacle).

    The super small shallow boxes are usually only good for a light switch. I haven’t actually pulled out my code book to run the volume numbers, but I don’t think you’re really allowed to use one of those boxes for a receptacle. There is a reason why the usual “shallow” box is 1-1/2” deep.

    If you’re using exterior rigid foam, mounting a 4” square box on the structural sheathing and then using a mud ring to get the device mounting flush with the finished outside siding surface works pretty well. For thick foam, you can mount the box over a layer of foam. This is the method I like myself. I don’t like to recess the boxes into the interior stud cavities within the wall due to air sealing and insulating issues.


  3. Expert Member


    To speculate on the Why part of your question: I think renovations of 2"x4" walls usually fit into two categories. Those that don't care about energy efficiency, so the electrical boxes aren't an issue, and those the try to upgrade the wall's thermal performance, and that typically means additional insulation either on the inside or out, so again the box itself isn't that important.

  4. RussMill | | #4

    Our normal method in these situations is to use a 4x4 shallow metal box with a mud ring. The mud ring counts for some space albeit not much.

  5. Expert Member
    Akos | | #5

    A typical 8' section of exterior wall will have at most 2 outlet boxes in it. That represents less than 0.1% of the surface area of the wall. Heat loss there due to lack of insulation is pretty much squat.

    Air leaks through exterior device boxes are another story. You can loose a fair bit of heat through air leaks, should always install air tight boxes in exterior walls.

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