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Proposed wall layers

Thomas_K | Posted in General Questions on

Hello GBA,

I am planning a 1100 square foot single story house in climate zone 4C (coastal Oregon 1/4 mile from the ocean) and I am considering the following exterior wall layers (outside to inside):

1. Hardie plank siding
2. Air gap with 1×4 furring  or Cora-Vent system
3. Water resisting barrier/air barrier with Delta-vent SA
4. 5/8in plywood sheathing
5. 6in studs (with rock wool batts)
6. 1/2in plywood with taped seams
7. 4″in stud wall (or 2×4 turned sideways for 1 1/2in “wall”) for utility chase
8. 1/2in sheetrock

The taped 1/2″ plywood layer is intended to provide a second  air barrier  so that electrical and plumbing does not interfere with the air barrier and insulation. It would also add some additional shear strength. I should also mention that there will be a conditioned crawl space, whole house ventilation (HRV), and ventilated attic.

Questions:
1. Are the two plywood layers permeable enough to dry (inward and outward) if moisture got into the 6in space?
2. Would insulating the 4in utility chase be worthwhile?

THANKS,
Tom

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Replies

  1. STEPHEN SHEEHY | | #1

    Tom- Is there a structural reason for the 1/2" plywood, i. e. special seismic issues? If not, I'd use a membrane as the air barrier between the chase and the insulated wall. That would be cheaper to install.

    Our house has a double stud wall and, like you, we used an air barrier on the outside of the interior studs to isolate plumbing and electrical. After wiring and plumbing, we did install fiberglass batts in that space.

    Did you consider exterior foam?

  2. brendanalbano | | #2

    Why not tape the seams of the 5/8" plywood in layer 4 and use that as your air barrier? That seems like it would simplify the interior layers considerably.

    Have you considered something along the lines of the wall assembly suggested by Oregon Residential Specialty Code 2017 Ch. 11 Table N1101.1(2) Option 1?

    It would be something along the lines of:

    1. Hardie plank siding
    2. Air gap with 1×4 furring
    3. R-5 Continuous Insulation (or more if there is room in your budget!)
    4. WRB (Perhaps using a drainable WRB like Benjamin Obdyke Hydrogap or equivalent is wise here)
    5. Taped Plywood Sheathing (air barrier)
    6. 2x6 Studs with R-21 batt (could be mineral wool if you prefer)
    7. 1/2" Sheetrock

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Tom,
    Before you include a utility chase it's worth planning what will actually go into it, and whether it's worth-while based on a few easily air sealed wires and outlets, and the odd pipe.
    If you do decide to include one, make sure it is deep enough to be useful. 1 1/2" isn't deep enough if it is backed by something solid like plywood.

  4. GBA Editor
    Martin Holladay | | #4

    Tom,
    Q. "Are the two plywood layers permeable enough to dry (inward and outward) if moisture got into the 6 in space?"

    A. Yes. That said, the other posted comments are valid. One of these plywood layers is necessary, but not two.

    Q. "Would insulating the 4-inch utility chase be worthwhile?"

    A. Without insulating the utility chase, your wall has about R-21 (nominal) insulation. If your utility chase is 1.5 inch thick, insulating the utility chase adds about R-5 or R-5.5 of (nominal) insulation. If your utility chase is 3.5 inches thick, insulating the utility chase adds about R-12 of (nominal) insulation.

    Is it worth it? If depends on your goals and your utility costs. If this were my house, I would insulate the utility chase.

  5. Thomas_K | | #5

    Thank you all for your responses.

    Stephen,
    The plywood would add shear strength to the wall besides being an air barrier. We are in Seismic zone D2. Are you happy with your wall?

    Brendan,
    Layer 3 (Delta Vent SA would perform the air barrier function, so no need for taped seams.
    Yes, I am still considering R21+R5 continuous (with Roxul Comfortboard 80) in lieu of the extra interior stud wall and interior plywood air barrier. Would the single exterior air barrier (Delta Vent SA) perform well enough with all the nail holes from furring, insulation, and cladding (plus other penetrations)?
    I am also worried that we would not "get it right" as the exterior insulation makes the details more complicated due to the added thickness.

    Malcom,
    Good comment on the thickness of the chase. Also, the main utility in the chase would be wiring, as I made a point of not putting plumbing on the exterior walls (only the kitchen sink). I guess I was skeptical of being able to seal electrical fixtures in a manner that would work now and for years to come. What is your preferred method?

    Martin,
    Thanks!

  6. brendanalbano | | #6

    With or without the exterior insulation, establishing your air barrier at the exterior sheathing (with your Delta Vent SA, or with taped seams and a conventional WRB, either way should be fine) makes sense to me. If the exterior sheathing is the air barrier, then aren't all your concerns about air sealing electrical fixtures eliminated? As to whether or not the Delta Vent will be compromised by nail holes, I'd guess nail holes are no problem, but if you don't want to guess, you can presumably just contact them and ask.

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