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Community and Q&A

Too tight scares me: Exterior XPS over plywood

730d | Posted in Energy Efficiency and Durability on

First post. It has been a dozen years since I was a builder. I feel like I do not know how to build anything anymore.  I am an old guy and as such I have plenty of experience, good and bad. Things can go wrong. I am going to build a new garage, shop bunkhouse at my cabin in cold and hot Wisconsin.  I will insulate and drain under and around my floating slab frost protected foundation.   The interior may or may not be heated or air conditioned always or sometimes.  The interior will be divided into four zones.

The wood walls are what is bothering me.   2*6 walls 9′ – 4 1/2″  tall,  24″ oc.
I know I don’t need 2*6 but I like it as it makes wiring and plumbing and the occasional use of bigger headers easier.  Easier to walk on also.

My go to sheathing and it was very popular here in the Midwest for decades was  Bilt Rite 25/32 fiberboard sheathing.  Strong, vaper permeable, R 2.06 and self gasketing draft  tight.  Much better stuf  than is sold now.  At any rate no longer available.  So I will go to plywood, 1/2″ 4 ply cdx.  I hates osb for all the obvious reasons plus it is heavy, remember, I am old.  I will caulk and tape and seal up the plywood from foundation to roof.

Over the plywood sheathing will go two layers of Jumbo Tex 60 grade d paper.
I don’t love Tyvek. I will use it. My major issue with it is it does not store moisture waiting for a drying environment as paper does.

Next is 2″ of  XPS. This will be caulked taped and sealed as the plywood. Over this is a vented plastic furing and vertical wood siding constructed as a  rain screen.

So the issues are. Won’t there be occasional  frost or moisture build up on the cold side/outside of the cdx and under the xps?  How does it dry/drain  if my foam is pinned tight and  sealed up tight. Remember. Its not a house. It gets heated or cooled, allays, sometimes or never.  Or I might condition it like a house.  If I add a flashing to daylight at  the bottom of the wall, under the wrb/ jumbo tex, does this not hurt the performance of the wall assembly and will it be enough to keep my plywood as new? So am I back to ” houses need to breath ” or should it be sealed tight?

Is it inadvisible to keep the foundation outsulation, xps,  and the wall outsulation xps on the same plane and sealed together.


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  1. GBA Editor
    Martin Holladay | | #1

    Wisconsin has two climate zones: Zone 6 and Zone 7. Here is a link to the map: Climate Zone map.

    Your plan to install 2 inches of XPS (R-10) over a 2x6 wall won't work. What you need is either a minimum of R-11.25 rigid foam (for Zone 6) or a minimum of R-15 rigid foam (for Zone 7). Here is a link to an article that explains what you need to know: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    If you follow the advice in that article, you don't have to worry about "frost or moisture build up on the cold side/outside of the CDX and under the XPS." The rigid foam will keep the plywood from getting cold enough to absorb moisture -- and in any case the plywood will be able to dry to the interior if necessary.

    1. 730d | | #2

      Thanks Martin,
      I understand. Thanks for the links. I will also seal and flash the WRB ,located between the sheathing and XPS, to the out side . This flashing will also be designed to keep soil gas/moisture from rising up into the space between the plywood wall sheathing.

  2. Jon_R | | #3

    I'd also follow Lstiburek's recommendations by adding a Class II vapor retarder (preferably Smart, like Membrain) on the interior side. See Table 3 below.

    Other things being equal, higher perms to the exterior is more robust. So also consider unfaced EPS over XPS. The former also causes less global warming.

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