Exterior outlet boxes in rigid foam
![](https://secure.gravatar.com/avatar/b2e4a094adc6bea7d52f2beeb965a79b?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fc.png&r=g)
I am hoping to get some advice on the best way to install exterior outlet and light boxes in above-grade 2×6 walls with exterior rigid foam without compromising air-sealing.
My project is new construction in climate zone 5 and I plan on installing conventional insulation inside the walls, osb sheathing, wrb, 2″ rigid foam with taped seams, likely a 3/4″ rain screen, and cement board cladding (in that order).
I am having trouble figuring out exactly where/how to install the exterior outlet boxes though. I have looked into using an in-box that can be adjusted out to be flush with the outside of the rigid foam (or the cladding 4″ proud of the sheathing), but I wonder if it wouldn’t make more sense to install the boxes outboard of the sheathing and wrb in order to minimize disruption to the building envelope (only the romex or conduit would pierce the envelope). I am debating between surface-mounting the boxes directly to the wrb/sheathing, and flange-mounting the boxes to the cladding trim.
I know this must be a common situation for anyone installing exterior rigid foam, but I have been unable to find any definitive details. I would really appreciate any advice on either what I propose above, or on alternative methods.
Thanks in advance!
-Chad
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
You want a solid mounting for the box so that the device (outlet or light fixture, usually) won’t be wiggly. I usually recommend using a “4 square” box and mud rings. The 4 square box is commonly available in 1-1/2” and 2-1/8” depths (a “deep” box). There are many ways to mount the box depending on your wall. You could use the shallower box with a 1/2” piece of foam behind it, for example, and the box screwed into the wall through the foam. You could use a box with “wings” to mount to a stud. Whichever works best for you.
The nifty thing is you can get “mud rings” (usually used for drywall to bring the devices flush with the drywall surface) in many depths from 1/4” or so up to over 2”. Use the mud rings to extend the device opening up to where you want with regards to the exterior surface of your siding. If you use a single-device mud ring, you also get a wide metal surface that you can seal your WRB to.
Make sure to tape the exterior of the box before installation to air seal all the extra screw holes.
Bill
Thanks for the quick reply Bill.
If I understand your suggestions correctly, you are describing mounting the box outboard of the sheathing, right? In my case the WRB will be against the sheathing, so I would seal/flash around the romex/conduit where it comes through the WRB, and also seal/flash the box to the face of the WRB. When you mentioned sealing the WRB to the mud-sill, would that only apply to sealing to the rigid foam, or a WRB outboard of the foam? Do you generally use plastic or metal boxes?
You also mentioned possibly putting 1/2" of foam behind the box. If I did that, could I install the romex directly through the sheathing/foam into the box, or would I need to pass it through a short piece of conduit?
Thanks again!
Let’s assume your stackup is sheathing/2” foam/WRB/rain screen/siding. Cut a hole in the foam to clean a 4” square box. Use a 1-1/2” deep box. Mount a piece of 1/2” foam to the sheathing in the back of the hole, install the 4” box on top. The 1/2” foam is your thermal break between the box and the sheathing. Run your romex or conduit (fat backer rod can be pushed into conduit to help air seal it after pulling wire) out the back of the box, through the 1/2” foam and sheathing. I don’t see any code issues running the romex right through the foam and the sheathing, but if you’re going into a crawl space or basement (something accessible) on the other side of the wall, conduit provides the option to change the wiring in the future if you ever want to.
Now use a suitable depth mud ring to bring the device (outlet or light) flush with the surface of the siding. A single-device mud ring will have a large flat area at the surface of the box, which will be flush with the surface of the 2” rigid foam (1/2” foam + 1-1/2” box = 2”). It’s easy to tape the WRB to this surface to keep a good seal. The mud ring gives a good mounting for your device, the box is solidly mounted to the sheathing, all is well on the wall :-)
I’d use steel boxes and mud rings. You can get plastic mud rings, but I don’t think you can get surface mount plastic boxes.
I've played around with this as well. You gotta get creative when it comes to the penny's (penetrations).
If I can get a good mental estimation of your wall assembly, and assuming the foam is the critical air barrier, I think if you get a 2-1/2" deep pvc box or something similar that sits ideally a little proud of your exterior foam. Then you can cut the insulation to fit that box snugly, screw it to the osb and caulk till you drop around the PVC box to the foam and also within the conduit the wire comes in through into the box.
After furring and when cladding time comes, pick an appropriate outdoor cover and cut the cement board opening to suit it so it sinks in and seals around the electrical box (they usually have a gasket around them that marries to the electrical box). The final assembly won't be flush, but at least it's appropriately air sealed. You can even caulk around the outdoor cover at the cladding level as well, but that would be more for aesthetics.
I personally love the design of a flush exterior outlet, so if you want to achieve that, it might mean going to the electrical Supply with your measuring tape and start measuring things and looking at what you can get for an assembly that works.
On my project I cut a little insulation out, drilled some holes inside of an appropriate box and attached to strapping with screws, adding some caulk to stiffen. I had to secure it while caulk set. Seems pretty rigid now.
Technically, if you drill holes in the side of the box you void the UL listing for the box so it’s no longer part of a “listed assembly” once it’s been installed. This is technically a code violation even though I’ve never seen anyone fail it on an inspection.
From a safety perspective, as long as you maintain clearance between the heads of the screws you added to mount the box and the electrified parts you should be ok. You may also have water intrusion problems, but some caulk on those new holes should take care of that.
Bill
Greendane,
I have 6 plus inches of "outsulation" with stucco and my electricians recommended presetting covered boxes before the stucco went on. Great advice and great looking results. I can't find the exact model I used four years ago, but the Arlington DVBM1W-1 appears to be substantially identical.
After planning locations desired and stud placing, I used an oscillating saw to plunge cut into the foam, first cutting the rectangular perimeter as deep as the blade would go, then cutting from center of the rectangle toward the plunge cut. This will leave a little pyramid shape of foam which you can then cut as desired and flick out with a linoleum knife or whatever has a hooked blade. A bit of messiness below the front surface won't matter.
After checking the knock out location on the box I drilled in through the remaining foam depth and sheathing to a spot close to, but not on, a stud. In some walls it was useful to make a little extension tube off the back of the box using PVC conduit fittings. In my case it just kissed the sheathing making wire feeding easy.
I then checked the fit, taped off unused holes (and the fitting) in the box, then foamed the hole with canned foam pushing the unit into place. I held it true to depth and vertical by tagging it with a few of the very long screws left over from the job. Once the foam had set I took out the screws to make the stucco workers life easier and to minimize thermal conduction. They have remained solid.
The real beauty of these is being able to set them flush to face of foam if you are overly anal like myself. I rabbited the flange depth out of the face foam so there would be no tell tale hump in the stucco. Depending on how your cement siding is hung there may be other models of these boxes that work better for you. They do make one specifically for vinyl siding that has a capture slot to keep things tidy.
Thanks all, sounds like I'm on the right track based on your experiences. I'll just need to make sure to get the sealing details right. In my particular case I am doing "innie" windows so the WRB will be against the sheathing and will also be the primary air barrier. The foam will be a secondary air barrier (belt and suspenders) so will still want a good seal there as well.