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Service Cavity and Electrical Wiring in Ceiling

Vlad Shpurik | Posted in General Questions on

I am getting close to the next phase of construction of my house, which is mechanical/electrical work. The house is framed with standard 2×6 walls with double top plates and conventional roof framing with 2×6 joists, but I followed Hammer&Hand recommendations and installed plywood under the joists for an air barrier. Since I’ll have ceiling service cavity, I would like to avoid running electrical wires through the wall studs as much as possible to simplify installation of the rockwool batts. I’d like to keep service cavity depth to a minimum, 1.5″ ideally. However I am not sure how to do this without having to drop service cavity below the bottom of the double top plates so the wires can be routed through the ceiling and down to the outlets along the studs. Is this a bad idea to begin with?

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Replies

  1. Jamie B | | #1

    Can you provide a link to what you're talking about? I just checked H&H website, and couldn't find what you're describing.

    1. Vlad Shpurik | | #3

      Jamie,

      The detail I've mentioned is described here https://hammerandhand.com/best-practices/manual/5-envelopes/5-2-air-sealing/. Skip to the "Air Barrier: Exterior Wall Sheathing to Interior Ceiling Transition" and "Wall to Ceiling/Roof" sections.

  2. Expert Member
    Malcolm Taylor | | #2

    Vlad,

    I'm not a big fan of shallow electrical service cavities for a few reasons:

    - Most of your wiring is in the walls. That's where the outlets and switches are. Draw out where you actually plan to have ceiling mounted boxes or fixtures, and how they will be fed. You may find you are building a cavity for large areas with no services in them.
    - Check you code. Ours requires 1 1/4" between the face of a framing member and the edge of the hole for the wire. That effectively precludes 1 1/2" deep service cavities.
    - An 1 1/2" cavity limits the type if boxes and light fixtures you can use.
    - To your point about moving from the cavity to the walls: Yes that's a problem. Usually the wires would go up and be drilled through both top plates, but that defeats the p0int of the sealed cavity.
    Unless you have framed the walls higher to compensate, you now have lower than usual walls, meaning that all the lower drywall sheets need trimming.

    If the concern is wires interfering with insulation, you can locate all your horizontal runs on the top and bottom wall plates, and fasten your vertical runs to the edge of the studs, leaving the cavities clear.

    1. Vlad Shpurik | | #4

      Malcolm,

      I've done fair amount of research on service cavities and in general I think there are better ways to achieve air sealing objectives. That's why I did not include them in the walls. With the ceiling though I like the H&H design for reasons other than creating a service cavity. So, it is fairly easy for me at this point to have one in the ceiling, and the depth of it is somewhat is not a big concern since I have 10' tall walls. I have some 2" EPS left, so I can cut that into 3.5" strips and install it under the 2x4 strapping and get 3.5" deep cavity. That will have a double benefit of being able to run oval air ducts for ventilation instead of building a dedicated chase for that.

      If I go with your recommendation and run the wires at the bottom or top plates, that may be difficult to avoid hitting nails that attach studs to the bottom/top plates. Raising those holes ~2" up or down from the plates would defeat the purpose of doing it that way to some extent. Since I'll be installing rockwool batts in the walls, I can run the wires at 48" level and just slit the batts at the edges to go around the wires.

  3. Expert Member
    Malcolm Taylor | | #5

    Vlad,

    I'm not sure what using foam buys you except more depth. It will have no appreciable effect on the insulation of the roof, is yet another layer to install, requires blocking to mount electrical boxes and ceiling grills, and makes drywalling more complicated. If you want a 3 1/2" service cavity, why not use the same 2"x4"s, but not on the flat?

    My comment about height wasn't so much about height, as now not being able to use two five foot sheets of drywall without cutting the bottom ones down.

    I can't see how sliding the batts over wires at 48" is any different than those at the top or bottom of the cavity?

    My comments are all informed by what I'd like to see as a contractor. If you are doing a lot of this yourself, it then becomes whether those efficiencies matter or not.

    1. Vlad Shpurik | | #6

      Right, the foam idea is just to use it as a spacer. It seems to me that attaching 2x4 strapping on flat is easier than on the edge plus it gives more space at the butt joints . As far as the actual depth of the cavity goes, I am only planning to install LED lights in the ceiling, so even with 1.5" it will be more than enough.

      I didn't know drywall is available in 5' widths. I new about 54" wide ones, but not the 5'. I'll check with the supplier next week.

      You are right that fitting insulation around the wires at the top or bottom plates is similar to 48" level, but I am still having hard time understanding how you would run the wires in that case. Do you drill studs at their ends, or a few inches away to avoid hitting nails?

      1. Expert Member
        Malcolm Taylor | | #9

        Vlad,

        "so even with 1.5" it will be more than enough."

        I don't know your codes that well, but as I said in post #2 above, that depth cavity wouldn't fly here. You don't have distance from the face of your strapping for either holes or staples. Worth checking that out before committing to the 1 1/2'" depth.

  4. Expert Member
    Zephyr7 | | #7

    Note that studs on the flat form a 1.5” cavity that is a perfect depth for 1.5” steel 4” square electrical boxes. You can then use mud rings of the appropriate depth to bring the devices up to the surface of the drywall.

    I agree with Malcolm though that this sounds like an awful lot of extra work to just avoid splitting batts around wiring. Normally just drill the studs a few inches up from the ends so you can split the last few inches of the end of the batt to get around the wire. The same goes for wire at the 48” level if you’re not exactly between batts. I find mineral wool batts to be pretty easy to work with around wiring, easier than fiberglass.

    Bill

  5. Doug McEvers | | #8

    I knew some builders that would cut a V notch at the base of the studs (2x6 wall) and run the wiring right along the bottom plate. This made installing the batt insulation easier.

  6. Vlad Shpurik | | #10

    Thanks everyone for your thoughts and suggestions.

    I also would like to mention that the main reason for the ceiling service cavity is to install some LED lightning there and not drill any holes in the plywood that serves as an air barrier. Everything else is a bonus. That's when I started thinking about using the ceiling service cavity to run other wiring to remote locations without having to drill studs. The house has a slab on grade foundation and I won't be running anything in the attic. Ceiling service cavity is going to be build anyway so I though that I might make it a an inch or 2 deeper and used it for other things, like running ventilation ducts instead of building a separate dedicated chase for that.

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