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Minimizing the penetrations for external electrical fixture/outlet boxes

MStaudaher | Posted in General Questions on

I am building a new home and have used Zip System sheeting as the primary air barrier (no external insulation).  We are about to start siding the home with 5/16″ Lap Hardi and I need to put in the external light and outlet boxes so we can trim around them.  

My question is how best to minimize these penetrations from an air sealing perspective.  We plan to use Forest Trim around these penetrations (1.5″ thickness).  Can I simply mount surface junction boxes and drill through the sheeting for the wire penetration only or do I need to cut larger holes to accommodate the actual junction boxes through the wall?

I have taken a lot of time and effort to limit and seal the wall penetrations and would like to avoid cutting large holes for circular boxes for the lights and the rectangular boxes for the outlets.  

Any suggestions?

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  1. Expert Member
    BILL WICHERS | | #1

    You can do it either way, either mounting the boxes on the surface or cutting them into the sheathing. Depending on what you're doing for siding, one of those two options might be better than the other.

    It will generally be easier to air seal if the box is in the surface of the sheathing with only a round hold drilled behind the box. All you do for this is to mount a cable clamp in one of the knockouts on the back of the box (I strongly recommend using metal boxes here), cutting a round hole to clear the entire clamp using a hole saw. The clamp will extend into the sheathing and end roughly flush with the inner surface of the sheathing. After you run the wire, just fill the hole with spray foam and let it conform to the back of the box, sealing the cable and the entire clamp. You end up with a perfect air seal with little effort this way, BUT you need siding that will have a finished surface at least 1-1/2” out from the exterior surface of the sheathing so that you have room for the box. Thicker assemblies are no problem, either use a “deep” (2-1/8”) box, mud rings, a backer block (piece of wood between the back of the electrical box and the sheathing), or a combination of those things. The goal is to have the face of the box or mud ring between flush and about 1/8” or so behind the finished surface of the siding. It’s better not to try for flush since that leaves no wiggle room if any of the dimensions are off a little.

    If you cut the box into the wall, you’ll have to air seal the back of the box the same way you would for an indoor electrical box mounted in an exterior wall. This is entirely doable, but it’s much more involved. This method doesn’t work as well for thick wall assemblies either, since you have to somehow extend the device mounting location (the face of the box) up close to the surface of the finished siding, and mud rings are the only way to do this. You are NOT permitted by code to mount the device (light, receptacle, etc) to the siding itself, and then just run wires back from there into the box — the box needs to enclose all the electrical connections including those on the device itself.

    I’d prefer the surface mounted box if your siding can work with that. If you’re putting in a rainscreen, that will usually provide enough room for a box. Note that it’s sometimes easier to use 4” square boxes everywhere, and then use either a round mud ring (for lights) or a single or double gang mud ring (for receptacles). You don’t have to use round boxes to get a suitable mount for a round device.


    1. MStaudaher | | #2

      Thanks for the information. I wanted to surface mount the boxes. We are using 5/16 7.25” wide Hardi for the siding. All the exterior outlets and other exterior penetrations will be blocked with 1.5” thick forest trim. If I do the method you described I should end up with the boxes essentially flush with the surface of the trim blocks.

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