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Stick framing with no cavity insulation

user-6792502 | Posted in General Questions on

What if one were to do a traditional stick frame 2×4 or 2×6 construction, but instead of using insulation in the wall cavities, just put 4 inches of rigid foam on the exterior of the plywood sheathing, with an extra layer of 1/2 inch plywood on the outside of the foam for a nailing surface.  This would result in a R-21 assembly with no thermal bridges.  The initial plywood sheathing could be sealed during installation to prevent air transfer, and the taped foam insulation would provide a vapor barrier.  It would essentially be a hot roof on a wall.  I understand that this is basically a homemade SIP, but the material cost would be much less, and possible without a crew and heavy equipment.  After a lot of research this seems like a sound plan but the lack of anyone else employing this method makes me apprehensive.

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    This has been done. I’ve read about some builders in Alaska calling it “outsulation”. There is no reason not to do it, but you’ll have all the usual issues of screwing through the thick insulation layer to hold the assembly together.

    Bill

  2. George_7224612 | | #2

    What you want to do is build a "Perfect Wall" house. Matt Risinger has done it and described it in this video:
    https://www.youtube.com/watch?v=d8i-93ABo3I
    He's done other videos where you can see the interior and some of the mechanical systems. Here is one of those:
    https://www.youtube.com/watch?v=hTwq-qUnr9I&list=PLDYh81z-RhxgCk1qJD6AJktVHZH7-BLSR
    The only difference between his and what you want to do is that he used rain screen strapping for his nailer outside the insulation rather than half inch ply. I hope that you do it and post the results.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    User ...502,

    As others have said, it has been done successfully and can yield a very resilient wall. Many of the complications come when, like Matt Risinger's project, you decide to leave the stud bays exposed.

    - That makes missing the external fasteners more consequential.
    - Running services is exponentially more difficult with exposed cavities. Consider covering at least some of them.
    - The framing itself has to use high-graded lumber, needs much more care with exposed fasteners, and needs a lot of sanding before the finish is applied. The job is much easier if you opt for paint not a clear coating.

    As to the approach in general:
    - Depending on where you are, the thickness of the foam may increase well beyond the four inches you are suggesting. And the complications of fastening, flashing, etc. increase with the depth of insulation.
    - Wrapping a house in thick foam is close to the least green way you can choose to insulate.

    1. nilst | | #5

      Malcolm, I proposed in another thread, a way to get past R40 with a wall assembly similar to what Matt Risinger proposes. ICF forms with a six inch cavity filled with cellulose will get you to R45 and would hopefully solve the structural problems that attaching 10 inches of polyiso with long screws presents. The plastic bracing in the ICF eliminates the thermal bridge of metal screws. It provides continuous vertical nailbase on both sides, every six or eight inches depending on the manufacturer. The bracing is made to withstand tons of wet cement so it should be adequate for rainscreen and cladding. A structural stud wall screwed through the studs to the nailbase of the ICF will anchor the ICF to the frame and provide the rigidity needed. To test the fit and solidity of the ICF I went and got myself some forms to play with and couldn't find any weakness that worried me. Basically it is an updated version of strawbale insulation, substituting ICF for the strawbales. The manufacturers all tout their reuse of waste and benign blowing agents in the manufacture of the ICF. The cost difference between a stud wall insulated to R40 and ICF wall at R45 was not enough to offset the labour savings, and the ease of installation.
      I welcome vigorous and healthy criticism of my idea because I still need to convince building inspectors, friends and collaborators and my wife that this idea will easier to build and affordable and better in the long run than what it would replace.

      1. Expert Member
        MALCOLM TAYLOR | | #6

        Nils,

        You did propose that assembly, and I think it is fair to say it didn't provoke any positive responses. I'm not sure what restarting comments on it here would achieve. Why not bump your original thread and see how you do?

  4. Expert Member
    NICK KEENAN | | #4

    What is your motivation for doing this? The cavity is the most cost-effective place for putting insulation.

    1. user-6792502 | | #7

      No thermal bridges
      No possibility of condensation in the wall cavity
      No fiberglass to hold onto moisture and provide a nesting place for pests
      No insulation being compressed or omitted where utilities are placed

      With how tight the building codes require houses to be these days it is just a recipe for moisture and pest problems to use conventional fiberglass batts. I have remodeled too many tight houses where once you open the wall up there is a can of worms caused by moisture. Houses either need to breathe or have the dew point well outside the wall cavity.

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