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Can the sheathing (in Zone 6) be sandwiched in the thick exterior foam layer?

adamcnyborg | Posted in Energy Efficiency and Durability on

Zone 6 wall strategies require exterior rigid foam to be thick to keep the sheathing temperature above the dew point. A current customer asked that I hit 2012 energy code economically and stop there. A local building inspector recommended this detail: 2×6 wall, 1″ foam exterior to the sheathing, 1.5″ spray foam interior to the sheathing, and blown-in insulation for the remaining cavity. The theory is the potential condensing surface would be the spray foam interior side. The advantage would be siding installation with long fasteners (and maybe a cedar breather) over 1″ foam rather than a strapped rain-screen style detail over 2″ or 3″ foam.

Will that sheathing sandwiched in foam be a liability? Thanks, Adam, owner Tilia Restoration LLC

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  1. GBA Editor
    Martin Holladay | | #1

    Some builders use the approach you describe. I don't like this type of foam sandwich, because if your sheathing ever gets damp, it dries very slowly or not at all.

    That said, you can do it if you want to (and if your local code official approves). Make sure that your wall sheathing is very dry on the day that the spray foam is installed, because it won't dry in the future.

  2. iLikeDirt | | #2

    You're describing a fragile, expensive assembly. You'd be better off saving money by ditching the spray foam and going down to 2x4 framing, and using the savings to triple or quadruple the exterior foam thickness (mineral wool is good too). Then fill the stud cavities with whatever you like. This wall will be cheaper, more robust, and better-performing. I don't see what the problem is with installing siding over furring strips. If anything it should be easier to fasten to wood right there rather than trying to nail or screw through an inch of foam. And of course the furring strips give you a rainscreen gap, further increasing the robustness of the assembly. And if the outer later of foam is foil-faced, you get a free radiant barrier, too.

  3. exeric | | #3

    Totally off topic - Adam, your last name has a very funny association in the movie Glengarry Glen Ross. I wouldn't worry about it though because I'm sure they just picked the name out of a hat. Unless of course you really like talking to salespeople. My last name is kind of strange also.

  4. iLikeDirt | | #4

    I thought the same thing, Eric!

  5. Andrew_C | | #5

    Bonus points to Dana for "get'R done". Copyright worthy.

  6. exeric | | #6

    It occurred to me a misunderstanding could develop from my mention of the movie, the last name, and the salesperson reference. In case anyone doesn't know, in that movie the couple whose last name was Nyborg would have real estate salesmen come to their house just to have someone to talk to. It was not in anyway a reference to anyone here giving advice. It just occurred to me that an individual without knowledge of the movie could interpret my comments that way. In no way was that how it was meant. Sorry if there might have been any misunderstanding by anyone.

  7. adamcnyborg | | #7

    Thanks for the answers. Who knew the useful information would come with puns and trivia? I put strapping over 3" of (reclaimed) rigid foam on my last new build. The labor hours really stacked up; we were running strings on every strapping course to get a flat wall. It sounds like the wisdom of Dana and Nate would be to stick with it, the labor time will come down making thick exterior foam a practical assembly.

    Thanks for the straightforward answer too Martin.

  8. Expert Member
    Dana Dorsett | | #8

    What they said- a 2x4/R13 + R10c.i. wall is both cheaper easy enough to build, and more resilient than 2x6/R20 + R5c.i. .

    For the R20 + 5 case, sheathing can still dry toward the through 1" unfaced EPS or XPS if the siding is back-ventilated (vinyl works, or any other siding needs a rainscreen assembly) at between 1 - 3 perms but not through most foams with facers. At 1.5" 2lb polyurethane on the interior still comes in at about 0.8 perms, which is still in the "reasonable" range, but not great- it's a class-II vapor retarder. The average temp at the sheathing in winter is below typical indoor air dew points, so there will be a slow accumulation through that 0.8 perms, which it will have to get rid of during warmer weather. And it takes three types of insulation installer.

    For the R13+ 5 case the average temp at the sheathing stays above the interior dew point, and doesn't have a winter moisture accumulation problem, and can dry through 3-5 perm standard latex paint on the interior. It then doesn't matter what the vapor permeance of the exterior foam is- there's nothing driving moisture into the sheathing, and it dries at 4-5x the rate if any moisture does reach the sheathing.

    If you're REALLY going to go the 2x6 route, use 1.5" ZIP-R insulated sheathing and 5.5" of open cell foam in the cavity, with a layer of Certainteed MemBrain under the wallboard. At 5.5" of half-pound foam it's even less polymer than 1.5" of 2lb foam, and uses water as the blowing agent instead of HFC245fa. The interior face of the ZIP-R is a vapor barrier, but the OSB is on the exterior and dries toward the exterior. The wintertime performance of ZIP-R is less than advertised in your climate, but not enough so that we really care here (it's a code-min wall, and the shoulder season performance will outperform spec). The MemBrain is tight enough in the dry air of winter that it will keep the open cell foam from loading up with moisture, but the drying rates would be limited primarily by the interior paints.

    (edited to add) It occurs to me that not everyone would know what ZIP-R is:

    Or MemBrain:

    I don't normally recommend ZIP-R for cold climates on price/performance grounds, but if the stated goal is to get to code min cheaply, this'll get'R done quick without having to train anybody on how to mount exterior foam or how to do siding over exterior foam. If you want to cheap out and use vapor barrier latex primer on the wallboard instead of MemBrain, that too will be cheaper, though somewhat less resilient (if the studs ever took on moisture it'll take quite awhile to dry.)

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