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Adding rigid insulation to inside of solid masonry walls--electrical box details?

vigilant1's picture

We have bougt a house (1950s Cape Cod style) with solid msonry walls (4" CMU inside, 3.5" concrete brick for the exterior wythe).  We're planning to add 2 1/4" of XPS to the inside of the exterior walls, and I'm looking for the most practical way to get the receptacles to the new interior surface.

  Walls will be 1.5" XPS adhered to plaster, 3/4"T x 3" furring strips 24" OC, 3/4" XPS between the furring strips, 1/2" drywall, so a total of 2 3/4" added to the inside of the wall.

  My initial plan was to just use extension rings onthe existing receptacle boxes, but:

      1) I can't find  2 3/4" extension rings 

      2) It's probaby not great to have a metal thermal bridge all the way to  the cold masonry (and resultant condensation in the electrical box/wiring).

    Would you just mount a new, fresh box to the new furring strip, displaced a few inches from the existing box, connect a short length of Romex to the wires in the old box and running through a cover/knockout on the old box to the new box?  (the old box and cover gets covered with XPS)?

Thanks for any tips/ideas.


If you have any splices in (post #215202, reply #1 of 4)

If you have any splices in the old box it must remain accessible.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Keeping old box accessible (post #215202, reply #2 of 4)


Thanks--good point.  I'd need to put a blank metal plate over the old box (with a KO so the cable can exit) and then somehow mount a blank plastic cover plate on top of the drywall--maybe use one of the 1/2" thick plastic mud rings/extenders and trap the flange of that under the drywall.  Then fill the space with some kind of insulation.

  My original idea "felt" incorrect (burying a J-box), I didn't think it through. 

Hi there, You will want to (post #215202, reply #3 of 4)

Hi there, You will want to ditch the old boxes and hope that there is enough slack in the wire to have it reach new boxes mounted to the furring.  As DanH noted, you can't bury the old boxes in the new foam using them as junction boxes..  

The easiest way might be to (post #215202, reply #4 of 4)

The easiest way might be to pull the romex out of the existing box, remoxe that box and mount the new box slightly higher (or lower) depending on where they are fed from. The horizontal displacement of a couple inches should not make that much difference in the 12" wire length from the point of support required away from the box (less than 1/4"). The only issue might be derating the (wire) insulation of that wire because it is buried in thermal insulation.but as long is it is only one cable or if multiple cables are separated you will be OK. per 334.80