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Wall stack-up problem?

Warren_M | Posted in Green Building Techniques on


Zone 5, on border of Zone 6

I am building a single story house north of Chicago, in Zone 5 but just 21 miles south of the Illinois/Wisconsin border Climate Zone 6.

The wall stack from the interior-to-exterior  is as follows; Latex paint Paper faced 5/8” drywall 2×6 stud wall 16” O.C.  with 5 ½” Open Cell Spray Foam (alt 5 ½” of dense pack cellulose) (both insulation types vapor permeable at about  R-20) ½” CDX Plywood Tyvek Commercial D WRB (crinkled drain wrap) 2” single layer of XPS – fastened with screws, edges not taped and not sealed or caulked (R-10 rigid foam) 2×4 horizontal furring strips 3/16” rain screen spacers on furring strips (is the 3/16 gap enough?) 1” vertical closed joint T&G cedar siding, latex paint.

The sidewall assembly design was in mind with the goal of the wall being vapor permeable allowing for drying in both directions.

I intend not to tape or seal the edges of the exterior XPS rigid foam board to allow for somewhat better vapor dispersion.   What’s wrong with this assembly?

Thank you for your help. – Warren_M

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  1. Expert Member
    ARMANDO COBO | | #1

    Nothing wrong with your assembly, IMO. However, if your install your 2x4 horizontal furring strips with a few inches gap between them, there would be enough air running thru that you wouldn't need the 3/16 spacers. I would rather use cellulose better than ocSF.

  2. RussMill | | #2

    If you have an installer, CELLULOSE ALL THE WAY! i havent found a dense pack guy anywhere near me! That wall can last 100 years or more withh cellulose

  3. GBA Editor
    Martin Holladay | | #3

    First of all, can you tell us your name? (I'm Martin.)

    Attention to airtightness is essential. With your proposed wall, it would be a good idea to tape the seams in the exterior rigid foam. (By the way, most green builders avoid the use of XPS for environmental reasons -- either EPS or polyiso would be preferred. For more information on this issue, see "Choosing Rigid Foam.")

    Deliberately leaving unsealed cracks between the pieces of rigid foam is a big mistake. If your wall has a continuous layer of exterior rigid foam, you shouldn't expect your wall sheathing to dry to the exterior -- this type of wall is expected to dry to the interior. In any case, if the wall is designed properly, there will be no moisture accumulation.

    Since you are on the border with Climate Zone 6, you might consider upgrading the R-value of your rigid foam layer -- in Zone 6, you need that foam to have a minimum R-value of R-11.25. For more information on this issue, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

  4. Peter Yost | | #4

    Air tightness always trumps vapor permeability. The difference in overall drying potential if you tape the seams is small (since vapor pressure is a field effect it would be reduced just by the percentage of the wall surface that the tape comprises) whereas air leakage as a point effect can mean even small cracks can move significant amounts of moisture.


  5. Aedi | | #5

    I agree with both Martin and Peter, not taping the rigid foam layer causes more issues than it solves. While you might think of open cell spray foam as an air barrier, it is not continuous, so air can bypass it at areas like base plates and rim joists.

    Not taping the XPS would do very little to the drying capacity of this wall anyway. The crinkle housewrap allows for draining of bulk water with or without taped seams. In addition, not taping the seams makes warm, moisture-laden air more likely to encounter cool surfaces, increasing condensation risks and making the assembly wetter.

    For a well designed wall, drying in both directions is not necessary. However, if it is truly important to you, it might be worth you looking into rigid mineral wool insulation to use in place of XPS -- it's vapor open and drains freely, allowing you to use regular building wrap instead of having to spring for the crinkle stuff.. I'd skip the open-cell foam too, unless you're getting some sort of deal. No matter what, include an air barrier in your wall -- even taping the plywood is an option.

    Finally, you should be able to get away with spacing the 2x6s 24" OC instead of 16" OC. So long as you keep double top plates, it should not affect anything else.

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