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Cut-and-Cobble Insulation for a Mooney Wall

Jim12d | Posted in Energy Efficiency and Durability on

Hi Im a builder who works with 1 other carpenter and we only do about 2 projects a year in Montana. We were doing barns and barn style houses; all post and beam and all semi-custom. The spec home builders are making more money and I like the idea of taking a break from interacting with customers. So we are going to shift gears and do a spec home and also switch to stick framing for a very conventional marketable product. The most recent home I built scored 0.89ACH but the wall R value was only about R21. Id like to build a spec home with R30+ and less than 1ACH

We bought some standard 2×6 framed home plan and are budgeting to have a soil/structural engineer review/stamp the foundation (bentonite clay). Ive tried to find spec home plans with double stud walls or any type of energy efficient wall style and cannot find much or have gotten quotes at what I believe to be too high for a small basic spec ($8000+ and would still require local stamp). So without deviating structurally, the Mooney wall looks great. But, it doesnt add a ton of R value and it doesnt look ideal for batt insulation. My calls in the past to insulating subcontractors did not find anyone who dense packs cellulose in this area.

Im wondering about cutting foam and plywood/osb strips to tack onto the interior face of all outside wall studs. Or ripping zip-R sheathing basically. But instead of horizontally with a crosshatch pattern, place them vertically so that it creates a thermal break on each stud but allows an 8in thick fiberglass batt to be used.

I’m seeking any other ideas similar to Mooney wall system that I may have missed in my research. Or builders with experience in double stud wall who can convince me I’m wrong in discarding that idea for this project. In the past I have used exterior foam insulation but I think I would to limit the use of foam on future projects due to carbon cost and microplastic pollution (rasping EIFS installations is a garbage process).

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Replies

  1. Expert Member
    DCcontrarian | | #1
    1. Jim12d | | #2

      Great thanks, exactly what Im looking for!

      1. Expert Member
        DCcontrarian | | #3

        We're here all week!

        Seriously, there's a lot of guys here with a lot of experience in the business of green building -- how to do it cost-effectively and make money too.

        Have you given thought to how you're going to market this house? My experience is that, sadly, most people have no idea that every house is different, they think of them as being manufactured items like cars. When I tell people that two houses of similar size and similar construction can vary by a factor of ten in their energy usage depending on how they're insulated and sealed, the reaction I get ranges from incredulity to outright denial.

        Perversely, many people think an efficient house has to be uncomfortable, they're thinking it's like a sub-compact car. To me it's just as big a selling point for a well-insulated, tight house how much more comfortable it is in all weather than a leaky house.

  2. jollygreenshortguy | | #4

    I was just now figuring out the detailing for a 2x6 stud wall with 2x2 strapping, all at 24" oc.
    I'm looking for a solution that doesn't require foam insulation. I can get a 5.5" R20 cellulose batt and a 1.5" R6 cellulose batt. By my calculations I get a whole wall R value of about R24. It seems to me a really constructible solution because all the exterior detailing, the flashing of openings, installation of doors and windows, ... is done the usual way. There are impacts on the interior but they seem fairly minor, extended jamb trim, blocking for electrical boxes.

    It seems to me the installation of the batts would be a very simple matter. That's about it.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      jgsg,

      It's not a big deal, but on Mooney walls the basic framing for inside corners, interior wall, and wall/ceiling intersections have to be detailed differently to provide backing.

      The thin batts may pose a couple of problems.
      - They won't stay in place just friction fitted the way thicker ones will.
      - Sequencing for the framers, electricians, and insulators gets a bit complex - especially if the is an interior membrane located behind the strapping.

      Mike Smith, originator of the wall, always dense-packed them.

      1. Expert Member
        DCcontrarian | | #6

        "Mike Smith, originator of the wall..."

        Are you familiar with Stigler's Law?

        From Wikipedia:

        "Stigler's law of eponymy, proposed by University of Chicago statistics professor Stephen Stigler in his 1980 publication Stigler’s law of eponymy, states that no scientific discovery is named after its original discoverer. Examples include Hubble's law, which was derived by Georges Lemaître two years before Edwin Hubble; the Pythagorean theorem, which was known to Babylonian mathematicians before Pythagoras; and Halley's Comet, which was observed by astronomers since at least 240 BC (although its official designation is due to the first ever mathematical prediction of such astronomical phenomenon in the sky, not to its discovery). Stigler himself named the sociologist Robert K. Merton as the discoverer of "Stigler's law" to show that it follows its own decree, though the phenomenon had previously been noted by others."

        1. Expert Member
          MALCOLM TAYLOR | | #7

          DC,

          In this case the fault was mine. Mike Smith and Tim Mooney thought up the wall, but as I've discussed it with Mike, I always give him the credit. He is an excellent builder and a really nice guy.

          1. Expert Member
            Michael Maines | | #8

            Malcolm, my recollection was that Mike and Tim came up with the idea independently and flipped a coin to decide who it would be named after. (I'm sure others have also "invented" the system but it was Mike and Tim on FHB's Breaktime forum, 15-20 years ago where it was discussed and made popular.)

      2. jollygreenshortguy | | #9

        Malcolm, this was very helpful. Thanks.
        I'll leave both batts and dense packing as an option for the builder.
        This evening I'll look into the interior framing complications.
        I've also determined that a simple 2x6 stud bay, insulated to R3.6 / inch will meet the u-value requirements for IECC zones 1-3. So the inner layer of insulation can be optional in those zones.
        If I used 2x3 in the wide direction for strapping, electrical could be run on the interior side of the stud wall and still exceed the 1 1/4" coverage requirement. That only leaves plumbing, which I try to avoid in outside walls anyway.
        A quick check shows that 2x3s only cost 10-15 cents more than 2x2s.

        1. Expert Member
          MALCOLM TAYLOR | | #11

          "A quick check shows that 2x3s only cost 10-15 cents more than 2x2s."

          But can't be installed using an air-nailer, or hold up cabinetry. I think the key to any of your assemblies being successful is in making them simple, and not disrupting the usual sequence of construction, or methods most contractors use.

  3. aaron_p | | #10

    Idk how it would compare price/availability in your area, but Tstuds might be a simple option that I think can be swapped without re-engineering anything.
    https://www.tstud.com/

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