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Community and Q&A

Electrical box for exterior light fixture?

Rocky12 | Posted in Energy Efficiency and Durability on

I need to cutout two holes for my porch lights and am wondering the best way to do so in an airtight manner.

Can I just use round ceiling boxes with flanges (flanges around outside of (flush with) plywood sheathing)?

Or should I just run the electrical wire out a small penetration in the sheathing and put an exterior box around it?


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  1. GBA Editor
    Martin Holladay | | #1

    In most cases, the electrical box in a porch ceiling doesn't need to be installed in an airtight manner, unless (a) your porch is heated or air-conditioned, or (b) there is a conditioned room above your porch.

    If your porch ceiling really is part of an air barrier -- a rare situation, but possible -- an airtight electrical box would be best.

  2. Rocky12 | | #2

    Ok Thanks Martin. So would an electrical box like that one be ok? I would just cut out a hole in the sheathing and place the box in from the outside, leaving the flanges on the exterior side of the sheathing (and then taping over them).

  3. GBA Editor
    Martin Holladay | | #3

    If you use the type of electrical box show in the photo, the circular box (not the flange) needs to be at the same depth as (or a little bit recessed from) the finish ceiling. Make sure that your light fixture is big enough to cover the hole you need to make to install the electrical box.

    If you go to an electrical supply store, you'll find all kinds of electrical boxes. Some of them are specifically designed for remodeling work -- and are easier to attach than electrical boxes designed for new construction.

  4. Expert Member

    You won't be able to get the mounting piece through the hole or secure it. And if you did the flange would be 1/2' proud of the surface.

    Fish a wire through a small hole and use one of these shallow boxes mounted on the soffit:

    Edit: Martin beat me to it.

  5. Rocky12 | | #5

    Thanks guys. I suppose I should of been more clear.

    These porch lights are on my wall, not ceiling. Just like this:

    Is a shallow box still preferable for the wall? Is it enough to hold an exterior light up long term if not secured by a stud. I am thinking I will run a 2x4 across the studs to screw in the shallow box

    Edit: I reread the link you posted and they suggested to mount a 2x4.

    No junction is needed behind the wall though? And by going this route, don't I need to get a box that matches the depth of my siding? Or should I air seal the wire and then install this box over my siding?

  6. Expert Member

    Install your siding, cut out a hole with a 4" hole saw and install the shallow box on your sheathing. If the siding is something irregular, like bevelled cedar, then you may want to install a square mounting plate behind the fixture before siding. You can get these in cedar or composite materials at lumberyards.

    You don't need a junction box on the interior.

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