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Community and Q&A

Double stud wall vs 2X8 with strapping

Stephen Watts | Posted in Green Building Techniques on

I am planning to build a slab on foundation “pretty good house” in northeastern Oregon (Climate Zone 5).  I have considered using a double stud wall (outside wall is load bearing) to yield ~R-40 center of cavity using R-4/inch mineral wool batts.  The design from outside to inside would be fiber cement cladding, drainage plane house wrap, plywood sheathing, 2X4 wall, 3.5″ space, smart vapor barrier/airtight layer, 2×4 wall, sheet rock. This provides 10.5″ for insulation.

The concern I have is that having the vapor barrier/airtight layer on the outside of the inside (non-load bearing) wall makes it difficult to conduct air sealing after the sheathing is in place and requires install of batt insulation before the sheathing is installed.  I would rather have the walls and roof constructed and then take my time to insulate and air seal after dry-in.

I am considering 2X8″ walls with strapping to provide 9-3/4″ of space for insulation.  The design from outside to inside would be fiber cement cladding, drainage plane house wrap, plywood sheathing, 2X8 wall, smart vapor barrier/airtight layer, 1″ polyiso spacers between studs and 2X3″ horizontal strapping, sheet rock.  My thought is to use 2X4″ studs, either staggered or in-line with a 1/4″ space between for a thermal break.  The staggered studs would provide more of a thermal break, but would make it harder to use batt insulation.  Another insulation option would be something like Spider Plus blow in fiberglass insulation.  Dense-Pack cellulose is probably not an option because there are no local installers.

The 2X8 wall will use more lumber, have a lower total wall R-value, and require more labor.  Assuming labor after dry-in is free and time is not an issue, does the 2X8 option make any sense?  Is there a better way of providing an air tight layer/vapor barrier with a double stud wall that can be installed after sheathing and dry-in?

I would appreciate any thoughts.
-Steve

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Replies

  1. Shawn Baldwin | | #1

    Stephen,
    You have a lot of different ways to build this wall. My opinion is building with 2x8 studs looks better on paper than in practice. Getting a bunch of straight pieces etc. A lot of lumber too.
    I just built a double stud wall house with double 2x4. I liked the simple construction. My exterior wall was framed traditionally with layout to the outside and load bearing. Then the house was dried in. Then the inside 2x4 wall was layed out to the inside. This offset the studs and drywall started on layout too.
    I did not do a service cavity with horizontal 2x3, but if I did it again I think I would go that route. 1 it would add another thermal break, 2 it lowers or eliminates the amount of electrical penetrations in the interior air control layer and 3 it helps to hold the bulging insulation. (I prefer blown in)
    One man's opinion but it sounds like your on the right track and you'll get great advice here. Good luck with your project.

    1. Stephen Watts | | #7

      Shawn,
      Thanks for providing your experience. I had totally overlooked the fact that the inner wall could be installed after the outer wall and the building is dried in.

  2. Tyler Keniston | | #2

    "The concern I have is that having the vapor barrier/airtight layer on the outside of the inside (non-load bearing) wall makes it difficult to conduct air sealing after the sheathing is in place and requires install of batt insulation before the sheathing is installed."

    I'm having a hard time understanding this. Why can't you dry the house in, as Shawn did, and then build your interior walls with the smart vapor retarder in place? If your smart vapor retarder is a membrane product, you just lap it over the top plate and under the bottom plate to connect with ceiling and floor air barriers. If it's ply/osb, tape the seems while it's on the floor. As an option, you also can put the smart retarder all the way to the inside.

    "the inside 2x4 wall was layed out to the inside. This offset the studs and drywall started on layout too."
    ^Another benefit to double studs that isn't often mentioned (or used, if layout is lined up). I also recently built a double wall and used distinct layouts for sheathing and drywall. Seemed to make sense to me; not sure why not to do this, unless you're connecting the two walls with gussets.

    1. Stephen Watts | | #5

      Tyler,
      Thanks for reminding me that the inner wall can be installed after the outside wall. I understand how the membrane can be sealed at the bottom and top plates. However, I am not clear on how you would seal at junctions between wall sections and in corners.

      1. Tyler Keniston | | #8

        The corners can be a bit tricky. If you're using a membrane type product, it can be turned in and taped to the framing. The abutting membrane is then also turned in and taped to the same framing. This does create a 3-d complexity in the corners, but can be done all from the inside.
        *Utilizing a sealant in key places for the 3-d corner intersections can simplify some of the taping. It's admittedly a bit of that 'origami' many prefer not to deal with, but it can be done well if whoever is doing it is thoughtful about it. Leaving the inner wall shy of the floor joists / truss chords (since its not load bearing anyways) can help with the access in the upper reaches.

        This might be of interest: https://foursevenfive.com/blog/the-double-stud-wall-simplified-low-cost-high-performance/

        Check out their 'project spotlight' with the integrated service cavity.

        Using ply/osb would be a bit different, but you could still probably tape the sheathing to the corner framing and then be sure to tape/seal the abutting framing members.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hey Stephen,

    Tomorrow's GBA feature is "A Case for Double Stud Walls" by Dan Kolbert. Dan has been building and monitoring his system for many years. You will see how his walls are built and I think you'll find the article helpful. Spoiler alert: Dan specs dense pack cellulose and explains why in the article.

    1. Stephen Watts | | #6

      Brian,
      I look forward to reading the article. I understand the case for dense pack cellulose. However, as far as I can tell the nearest installer is in Boise, ID which is ~180 miles away from the build site. If I don't use batts, the most likely alternative is Spider blow in fiberglass.

  4. GBA Editor
    Brian Pontolilo | | #4

    Stephen,

    I got my dates mixed up, "A Case for Double Stud Walls," will go live next Friday. Tomorrow we have an annual tradition to attend to.

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