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Please critique my wall design

Stanfo3 | Posted in General Questions on

Hi all,
Looking for opinions…
Climate zone 6
Wall design from interior out:
1) Sheetrock, 2) 1.5 inch service cavity for electrical using horizontal furring strips, 3) 1.5 inch foil faced polyiso that will be taped and sealed to be the vapor barrier, continuous insulation and air barrier (also radiant barrier with 1.5 inch air space from service cavity). Using scissor trusses so polyiso will extend up ceiling creating continuous envelope. 4) 2×4 stud wall with fiberglass batts,  5) fiberboard 1/2 inch sheathing (with supplemental metal diagonal bracing to protect against panel degradation over time) to provide a vapor open environment so wall can dry to the outside. Fiberboard also adds r1.3 to increase continuous insulation above the code requirement of r10.  6) textured WRB to also act as rainscreen 7) siding (not sure what).

Pro’s
1) Horizontal furring strips can be the point of connection for interior walls so no need to create a break in polyiso vapor and air barrier to attach them to exterior walls.
2) Service cavity keeps from having to puncture polyiso.
3) 1.5 inch horizontal air space creates near optimum environment for foil on polyiso to act as radiant barrier (equal to around additional r3 value)
4) Horizontal furring strips make cabinet installation east if placed in the right spot.
5) sheet rock screws don’t penetrate air and vapor barrier.

Cons
1) wiring more difficult and expensive (metal boxes and conduit)

Let’s hear what you think?

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    I think the wall should work quite well. Texured WRB is nor really a proper rain screen, but better than nothing. If you are going with painted siding, I would nail up 3/8" thick strips of plywood with a framing nailer and have a real rain screen.

    You don't need to run wires in conduit, as long as the wire is protected by more than 1 1/4" of of wood where it cross your strapping, so run the vertical runs behind the 2x4 on flat and use zip ties with ears to secure. You can use regular romex.

    Surface mount metal boxes have more volume, if you mount them flush to the drywall surface, they can easy fit into service space cavity. They have about the same volume as standard boxes. Just have to be careful with cutting the drywall opening otherwise you'll need the larger wall plates.

    1. Expert Member
      MALCOLM TAYLOR | | #2

      Akos,

      I think he is proposing fastening the horizontal strapping to the polyiso, so I'm not sure it's possible to run the wires behind. I'm also not clear on whether you can staple wire to the strapping less that 1 1/4" from the surface under the IRC. Maybe someone with better knowledge of that code can chime in.

      Stanfo3,

      As Akos said, it sounds like a well performing wall, with the air and vapour retarder well protected. Minor headaches will be:

      - More backing will be necessary on the corners of exterior walls to compensate for Polyiso.

      - Your framing crew can't work continually, as the interior insulation interrupts the regular construction sequencing.

      - The service cavity is too shallow to include any drains, vents or water supply lines. The plumbing vents through the roof will need some careful sequencing.

  2. Expert Member
    AKOS TOTH | | #3

    The 1 1/4 distance is for protection only where the wire passes through the stud and it can be damaged by drywall screws. There is no requirement for distance from drywall elsewhere. So as long as you are running the wires behind the 2x strapping, you meet code but the wires still need to be secured, thus the zip tie.

    Drill through the 2x strapping and installing a nail plate also works.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      If the strapping is secured to the faced polyiso, how do you run the wires between the two?

      I have a niggling feeling the IRC also has the same setback as our code does for wires attached to studs, not only those penetrating them. I think that's why they don't allow two wires stapled side by side on a 2"x4".

      While there is no explicit setback to drywall, the setbacks and stapling requirements in our BC code (Rule 12-516) for both holes and stud faces effectively maintains the 1 1/4" gap.

      1. Expert Member
        AKOS TOTH | | #5

        I haven't had this specific install inspected but have run wires beside hat channel and the inspector had no problem with it, it could be region specific so always best to check.

        I think the folks that like to strap out their ceilings also run the wires along the strapping by stapling to the bottom trusses.

        I gouged a slot with a long drill bit, might not work well with 2x4 on flat, works fine with 2x on edge.

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