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Flush-mount electrical panel on exterior wall: should I add insulation?

grmp945 | Posted in General Questions on

Hi all,

I am located in CZ2 and built a small outbuilding/workshop a while back and am finally getting around to adding electrical.

I tried to build tight with zip sheathing, liquid flash, and flashing panels for penetrations.

I would strongly prefer to flush-mount the electrical panel in between the 2×4 wall studs. My concern is that there will be no insulation in that space. Maybe I am overthinking it, but I am wondering if I should add at least a thin piece (1/2 or 3/4″) of XPS foam behind the panel. This would prevent condensation from forming on the interior metal panel in the winter. It would also be beneficial from an insulation standpoint, although I am not really concerned about that given the small size of the building (168 sq ft). However, if I add some foam board behind the panel, then I will need to deal with the interior no longer being quite flush – but I could frame that out with 1/2″ or 3/4″ wood.

Any opinions? Should I not overthink this and just flush mount the panel as normal or should I add a bit of foam board and frame out the interior of the panel?

Thanks for reading 🙂

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  1. charlie_sullivan | | #1

    I think your suggestion of foam behind the panel is the good one, to avoid condensation, or high humidity, in the panel. That will make things corrode faster in there. Unless you are running so much current through that you warm it up and dry it out.

    But not XPS. XPS has a disproportionate climate impact. Polyiso, EPS, graphite-infused EPS (called GPS or Neopor), or, if you must, the new Owens Corning NGX version of XPS is better than the standard XPS or their standard pink foam.

  2. Expert Member
    AKOS TOTH | | #2

    You definitely want foam behind. In similar situations what I have done is run the foam the full height of the cavity and foam the edges. Add in horizontal blocking about 8" above and bellow the panel run your wires through these. You can insulate with batts above and bellow the blocking after the wiring is in.

    Just make sure your code doesn't require either drywall or plywood behind the panel.

  3. Expert Member
    BILL WICHERS | | #3

    Put foam behind the panel, and use polyiso as it's least flammable of the three commonly available types of rigid foam.

    Even better is to mount the panel somewhere where it won't be in an exterior wall at all. When I spec electrical stuff, I strongly prefer to avoid putting any switchgear on the exterior, or even in unconditioned spaces like garages. The reason is that electrical equipment in unconditioned spaces tends to get condensation and dirt inside, and that causes deterioration of the connection points, which is a Bad Thing. In high voltage switchgear, we are actually required to put small heaters inside (there are devices made for this purpose) to keep the interior very slightly above the exterior temperature to minimize condensation inside the equipment.

    If you can't put the panel inside somewhere, and you don't want to mount it on the surface of the wall, then I'd frame out a space for the panel or make a cabinet front to conceal it. Cabinet fronts shouldn't be an issue for the "dead front" clearance requirement in the code since the door can be opened and even removed if needed. You want to get as much insulation behind the panel as possible to keep the interior at room temperature and not something colder where condensation is likely to form.


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