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Drainage plane between sheathing and rigid foam

Jason Stringer | Posted in Energy Efficiency and Durability on

I am working on an addition project located in Wisconsin. My plan was to use Huber Engineering Zip wall sheathing.

In reviewing Zip walls installation instructions, I noticed that Zip calls for a minimum eighth of an inch drainage plane between the rigid foam and the sheathing. In my wall section I planned to include a drainage plain (rain screen) on the outside of the 3 inches of rigid foam.

Will the recommended Zip wall drainage plane reduce the effectiveness of my foam insulation?

This is not a typical detail in conventional wall sections using traditional OSB sheathing. Let me know if anyone has any tips. Thanks

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  1. John Brooks | | #1

    good question
    Kudos to you for reading the instructions.

    Please don't listen to anyone who tells you to "have at it" without reading the instructions.

  2. John Brooks | | #2

    this comment from John Straube .......
    seems relevant to your question

  3. Aj Builder, Upstate NY Zone 6a | | #4

    Jason in edit mode....what is the siding?

    As is posted below, apparently there is some new thinking. I defer at this point. Two contractor friends in my area have used two layers of taped foil faced foam over CDX for decades successfully. In our cold climate there may be more than one way to detail this it seems.

    Interesting post Brooks.

  4. John Brooks | | #5

    ...Context for my post

  5. Aj Builder, Upstate NY Zone 6a | | #6

    John, speaking of details, that detail you posted would never be built by me. When outsulating with foam, it can be detailed to cover all six sides. Zip must not give a zip about foundation details.

    If you want to really help John, draw that detail completed.

  6. Jason Stringer | | #7

    The siding will be LP composite siding, not stucco.

  7. Aj Builder, Upstate NY Zone 6a | | #8

    Jason, your siding most likely has a detail that they desire. The first attempt at LP siding was crap if used where moisture gets into siding. Huge failures and lawsuits enough to keep the lawyers happy.

    The new stuff is supposedly better. If it were me, I would rain screen between the siding only. Put the foam up in two taped layers. Use enough foam to stop condensation on the inside of the Zip. Detail your drywall airtight.

    Send off a fax to Huber and get final approval from them if you need their warranty. If they still want the rain screen to protect the Zip instead of protecting the LP, then I guess you should do as told to pass the buck on to their warranty.

    I may edit this John so get your screen capture image quick before I do.

  8. Aj Builder, Upstate NY Zone 6a | | #9

    Just googled LP siding... now name "Smart Siding" Marketers for years turn what you read, called spin... so what is the opposite of smart? Stupid! Hardboard siding has been a nightmare. Just thought I would post this search.

    If you look in the search there is a PDF for how to install the siding. It looks older than grandma as I have seen those details since hippies were real hippies. I don't think Joe L, of Straube would approve.

  9. Jason Stringer | | #10

    I think AJ and John are saying nearly the same thing. Sounds like no need for an air gap between sheathing and foam. Further, that rain screen is a good idea between siding and foam.

    Mr Anonymous not really clear what you are adding to discussion, please clarify.

  10. John Brooks | | #11

    Did you read "mind the Gap"?
    If you decide to use ZIP with outsulation... I think that you should follow their instructions and provide a gap. BETWEEN ZIP AND FOAM

  11. Aj Builder, Upstate NY Zone 6a | | #12

    Brooks I disagree. His fail point is the crummy LP siding. It needs to dry so it doesn't disintegrate.

  12. John Brooks | | #13

    I did not say to not ventilate the siding.

  13. Jason Stringer | | #14

    See page 7 of the article, section Foam Sheathing Used with OSB Sheathing. If I am reading that section correctly it states that a gap between the cladding and foam is necessary. Also it states that very little moisture will occur on the outside of the OSB. Further, if the wall cavity insulation is vapor open and there is no internal vapor barrier, then the OSB can dry to the interior. I took that to mean that I don't need a second air gap between the OSB and foam.

    In my case I plan to use cellulose cavity insulation w/o vapor barrier.

    If I were to use closed cell foam insulation in my wall cavity I would then need two gaps one at the outside of the sheathing and another at the outside of the foam.

  14. Brett Moyer | | #15


    Your posts are EXTREMELY frustrating to read. I usually just skip over them but you are posting so frequently I have to read them in order to understand the thread.
    PLEASE follow Martin's advice and sloooooooooooow down.

    Jason, do not follow AJ's advice. He is wrong.

    Huber ZIP roof and wall sheathing combines a structural panel and Weather Resistant Barrier (WRB) in one product. If you are planning to use the ZIP sheathing as your WRB you should (and I think Huber will void the warrantee if you don't) provide a drainage plane (space to allow for drainage) between the sheathing and exterior foam. Doing so will reduce the hydrostatic pressure between the sheathing and the foam. The BSC article that John Brooks pointed out does an excellent job explaining this.

    There are a few housewraps that will provide the 1/8-inch gap: Tyvek Drainwrap, Weather Trek dimpled housewrap, and Greengaurd Raindrop housewrap. However, using these products make the WRB system of the Zip panels pointless. If I were building the home, I would get rid of the ZIPs and use OSB (plywood would be better) sheathing with taped joints to achieve the air barrier, and one of the above listed housewraps as my WRB.

    And to your question about the gap reducing the effectiveness of the foam sheathing- Dr. Joe explains this in the same "Mind the Gap" article John mentioned:

    "But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap. With “crinkly” stuff (draining housewraps) you loose next to nothing. Is it worth it? Yes, in my opinion, the loss in thermal performance is trivial compared to the reduced risk and improved durability."

  15. Aj Builder, Upstate NY Zone 6a | | #16

    Brett, very good post except for when you whack at me (relax, just kidding bro)

    Will adding the space behind foam be a good thing? That I do not know.

    Will two layers of well fitted taped foam work if fitted tight in a cold climate? Yes. We locally have built hundreds this way for decades. That is why I come to my conclusion.

    Building tighter homes with new details is certainly getting to ice cream headache land. This has been an interesting thread.

    And like Brett said with the foam layers, the Zip vapor barrier is not needed. Zip to me was invented to be covered by vinyl siding along with fiberglass batts used inside it. (standard home build found in developments by the millions) In that situation Zip is a great product.

  16. R Miller | | #17

    I bring up this old thread to point out something. While I have seen this same 1/8" recommended by Huber for their zip product, I also see a detail NOT requiring it. It is detail "K-10 Zip System Typical Wall Detail with exterior foam insulation". It also says in the pdf heading... NO GAP so ?? It would be nice if they would make their mind up.

  17. D Dorsett | | #18

    There's a difference depending on how the windows & window flashing are installed.

    If it's an "outie" window where the plane of the glass is roughly co-planar with the siding, the flashing drains out to the exterior of the foam (which would also want some sort of housewrap, properly lapped to the flashing if the foam itself can't be detailed as a code-legal weather resistant barrier) In that type of installation no gap is necessary between the ZIP and foam.

    If it's an "innie" window mount, where the glass is rougly co-planar with the ZIP, the flashing drains onto the ZIP, some amount of space is necessary to allow gravity to do it's work on liquid water. A gap of 1/8" seems excessive, but may be necessary to avoid water droplets spanning the gap, clinging to both the foam & ZIP.

    Products like Tyvek DrainWrap are crinkled to allow tiny drain channels, but the depth of those crinkles/channels is tiny, something less than 1/32". But the surface of the housewrap may not allow the same amount of surface-tension cling that you would have with polystyrene or foil facers and/or the finished weather-resistant surface of ZIP.

    Bottom line, they can have it both ways, but perhaps need to spell out in larger fonts when the gap is necessary and when it isn't. Note the check mark in the lower right corner where it says: " Outie mounted flanged window"

    They also require the gap for any studwall with closed cell foam insulation in the cavity and rigid foam on the exterior, since there is otherwise no or very low drying capacity in either direction for the ZIP:

  18. Andy CD Zone 5 - NW Ohio | | #19

    Any new information on the question of gap or no gap when installing exterior foam directly over ZIP sheathing? Dana's update two years ago points out the critical issue of window flashing in determining the question. It does seem completely redundant to spend the money on ZIP and THEN have to put in a housewrap anyway. I'm going with ZIP as the cheapest and surest approach to both air barrier and WRB (once you count labor costs...) Belt-and-suspenders approach could lead to essentially four WRBs; ZIP, crinkle, foam, housewrap. If anyone has opinions or a tried-and-true practical approach, would appreciate hearing about it.

  19. GBA Editor
    Martin Holladay | | #20

    I don't see any reason why you need to install crinkle-wrap between Zip sheathing and exterior rigid foam. Of course, the foam has to be thick enough to keep the Zip sheathing above the dew point during the winter, as explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Moreover, if you want to use the Zip sheathing as your WRB, remember that all of your window flashing and door flashing has to be integrated with the surface of the Zip sheathing. Think every step through carefully to determine whether this will be easy -- how easy it is depends on where and how your windows will be mounted.

    -- Martin Holladay

  20. Walter Ahlgrim | | #21

    If you look closely you will find Huber provides 2 very different drawing one for walls with interior vapor retarder (barrier) with the drainage channel (vent).

    Any wall with 2 vapor barriers is a recipe rot and water damage.

    Please see the 2 attached photos I highlighted.


  21. GBA Editor
    Martin Holladay | | #22

    The Zip System advice is consistent with everything we've always said here at GBA: If you install rigid foam on the exterior side of the sheathing, you don't want an interior vapor barrier.

    The crinkly wrap is necessary if someone specifies closed-cell spray foam between the studs. But it's much better to avoid the problem in the first place by specifying cellulose or mineral wool between the studs.

    -- Martin Holladay

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