GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Water and electrical penetrations in exterior foam

Dave B | Posted in Green Building Techniques on


How do people usually go about running water and electrical penetrations through thick (6″) exterior foam. Just run straight through or run in a conduit? 
What the best way to air/water seal, tape spray foam?
How do exterior electrical boxes get mounted, cut into the foam and recessed ? 

Thanks Dave 
Zone 6

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Zephyr7 | | #1

    For NM cable, which is the usual plastic-sheathed stuff, you can just run it through a hole or slot in the foam. Seal it afterward with canned foam. Not a big deal.

    Mounting a box in foam that thick is more tricky. Normally I recommend mounting a metal box to a stud and using a deep mud ring to bring the device mounting flush with the surface of the siding, but 6" is too deep for that to work. The typical metal electrical box is 2-1/8" deep.

    Your best option is probably going to be to mount a piece of wood (2x4, etc.), in the foam with the box mounted to that piece of wood. Arrange the assembly so that the device mounting location will be at or slightly behind (code is for less than 1/4" behind the surface) the surface of the finished wall, which is probably your siding for an exterior wall. Try to get that piece of wood fastened to something structural in the wall so that your final device mounting is solid and doesn't get loose and "wiggly" over time with use. If you use some 1/2" foam as a shim, that will give you a total assembly of 4-1/8" deep. You could also use a shallow box (1.5" deep) with a mud ring to bring the device flush since mud rings are available in 1/8" increments up to about 5/8", then in larger increments up to about 1.5" or so. You can use the mud ring to get your final device location right and avoid the need for shims that way.

    Bring your wire into the back of the box (be sure to use a clamp for NM cable), or you can cut a notch in the foam and run the wire horizontally. Assuming you're using 2" thick foam sheets to build up that 6" total thickness, try to arrange your box assemblies so that the dimensions work out in 2" increments so that you don't have to cut a recess partially into a piece of foam. It will be much easier to just cut out a full depth section of a sheet or two to make your mounting spot.

    For a water line like a garden spigot, I'd just use one of the freeze-proof assemblies long enough to get the pipe fitting all the way inside and run it through the wall, sealing around the pipe with canned foam. Cut an oversize hole to make it easy to get the foam in around the pipe. A piece of pressure treated plywood can be used to make a solid mounting surface for the spigot, just be sure to mount the plywood through the foam into something structural to keep things solid.


  2. GBA Editor
    Brian Pontolilo | | #2
  3. Dave B | | #3

    Thanks for the details Zephyr7, I will look into this.

    Brian thanks for the link I remember looking at it before, but couldn't find it when I looked again.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |