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Community and Q&A

Additional WRB Over Zip Sheathing

scottwoodward | Posted in Green Building Techniques on

I’m finally in the midst of framing and using the ZipSystem for sheathing. I had not planned on putting on an additional WRB layer, but a few of the framing guys suggested it and I also read this earlier GBA thread that suggests it’s recommended (https://www.greenbuildingadvisor.com/question/would-it-be-ok-to-put-housewrap-over-zip-sheathing-due-to-high-winds-and-rain-at-the-beach).

Huber doesn’t exactly recommend it and in a few instances, it can actually void the warranty as is with the case with peel and stick WRB (https://www.huberwood.com/uploads/documents/technical/documents/Housewrap-over-ZIP-System-sheathing-Tech-Tip-Wall-ZIP-System_2021-08-03-194820_dijx.pdf).

In my construction, I’m putting on two inch polyiso on top of the Zip sheathing with the rain screen on top of the polyiso and then Hemlock cladding, which at this moment I intend to treat with pine tar.

What are folks out there doing with respect to a second WRB in addition to Zip sheathing in situations that don’t involve stucco or other scenarios where a second WRB is absolutely required?

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Replies

  1. James Someone | | #1

    Have done it before on a waterfront home in Cape Cod and elsewhere, I prefer a house wrap/plywood over zip system. I've seen to many guys and gals do a poor job taping zip to have confidence in the system, problematic with window/door head flashing details also, Im not convinced its the best approach. Thats my opinion and I won't be using zip constructing my own house.

    Almost every coastal, New England projects spec zip system and closed cell spray foam insulation for walls. Thats all I can remember for the last 10 years. Another topic for another time.

  2. Expert Member
    Malcolm Taylor | | #2

    Scott,

    - Where are you thinking of putting the second WRB?
    - What plane are your windows and doors in?

    1. scottwoodward | | #3

      Hi Malcom,

      Originally, I thought the second WRB could only go right on top of the Zip sheathing, but after reading Martin's 2011 article I see that there are two options -- on top of the sheathing or on top of the polyiso. I am installing my windows (and doors) in "innie" configuration so based on that configuration and following the logic in Martin's article, I presume a second WRB would be on top of the Zip sheathing. However, I certainly wouldn't want to do anything to jeopardize the Huber warranty. I also see that some polyiso, if installed correctly, can be considered another WRB.

      Martin's 2011 article: https://www.greenbuildingadvisor.com/article/where-does-the-housewrap-go

  3. Expert Member
    Malcolm Taylor | | #4

    Scott,

    Usually I'd say a second sheet WRB might be good insurance, but with innie windows, exterior foam, and a rain-screen I don't think it adds much.

  4. NYNick | | #5

    I think it's overkill. If care and attention to air leak detail is utilized installing your Zip system, and then your Poly Iso sheathing, you should be fine.

    I'm guessing your framing guys are so used to WRB's that they think you should use one.

  5. Kyle Bentley | | #6

    Using a 2nd WRB almost practically negates any of the benefits Zip sheathing provides, at a much higher cost.

    If a good job is done on the ZIP taping, and the seams of the polyiso are taped, you should be golden with the rain screen.

    1. James Someone | | #7

      Exactly.

  6. Expert Member
    Michael Maines | | #8

    I'm a fan of Zip sheathing--it's high quality OSB and the coating seems to do its job. I trust acrylic-adhesive tape when it's properly installed. But I also like belt-and-suspenders when there is risk, and having seen rain come through Zip panels and plenty of improperly-installed tape, I spec an inexpensive WRB over the Zip sheathing so flashings can be mechanically lapped. It does void aspects of the warranty, but have you ever tried to make a warranty claim on something like failed sheathing? Good luck collecting.

    1. James Someone | | #9

      Michael,

      If you have seen rain come through Zip what is so great about the product?

      If you need to cover Zip with a redundant WRB to protect your interest, why not just spec plywood with taped seams and a mechanically lapped WRB tied into the window/door RO's or a peel n stick WRB over plywood/OSB and save on the tape.

    2. Expert Member
      Malcolm Taylor | | #10

      Michael,

      I agree in general, but in this case isn't the exterior foam providing as belt and suspenders approach - especially when coupled with a rain-screen cavity?

      1. Expert Member
        Michael Maines | | #15

        Malcolm, I missed that detail--I agree, when there is exterior foam I'm ok with the Zip system as the WRB.

        James, the leaks were presumably due to over-driven nails and/or improperly installed tape. I use a lot of different sheathing materials and they all have pros and cons. The Zip system is airtight, flat, void-free and, most importantly, gets the building dried in quickly.

  7. scottwoodward | | #11

    An additional sub-question -- by using polyiso on the exterior insulation to achieve the required 11.25 exterior R value for Climate Zone 6, I'm now a bit concerned about creating a potential vapor sandwich with the interior insulation and required vapor barrier (either kraft faced batt or vapor barrier over mineral wool). What triggered this question is reading this article (http://blog.lamidesign.com/p/usa-new-wall-info.html), which appears to be very helpful. Here's the relevant excerpt:

    "Now there has been a very strong trend for applying foam layers to the exterior of a wall to create thermal break and add more insulation value. So why mess with mineral wool when you could just do foam? The answer is permeability. The foam layers create a strong vapor retarder layer at the exterior of your wall, so if you hope to put a vapor retarder at the interior side of your wall you will be creating a double barrier wall which is a no-no. A wall configured like this will not allow the interior of the wall to dry if any moisture gets in, and mold is the almost certain result."

  8. Patrick OSullivan | | #12

    > I'm now a bit concerned about creating a potential vapor sandwich with the interior insulation and required vapor barrier (either kraft faced batt or vapor barrier over mineral wool).

    To be clear, you absolutely *do not* want to install a vapor *barrier* (e.g. poly) inside, particularly in this configuration. Kraft batts are likely fine as they are a class II vapor *retarder*, not a vapor *barrier*.

    See here: https://www.iccsafe.org/wp-content/uploads/proclamations/TN06-Vapor-Retarders_pdf.pdf

    1. scottwoodward | | #13

      Thanks Patrick!

  9. Shawn Baldwin | | #14

    Scott,
    Ill throw in my two cents. The Zip does a great job as the air control layer and vapor control layer. I like to add a layer of rolled product, typar/tyvek, to act as a drainage plane and intigrate flashings. You said you are using foam, then rainscreen. So you have a drainage plane behind the rain screen and need to kick the water out with flahings above windows and doors etc.

    Using a rolled product here makes that easier to do because you can weave them in. Otherwise you would be relying on taping flashings to the foam or making flashings to go behind the foam.....got to pick a drainage plane to work from. But that rolled product is not the air and vapor control, so dont get as fussy with it in regards to sealing with tapes and such. All this "wrb" stuff should be thought of as seperate control layers. Just my two cents.

    Shawn

    1. scottwoodward | | #16

      Shawn,

      Do you mean a product like this? https://www.mtidry.com/products/rainscreen-drainage-planes/sure-cavity. And if so, you would recommend it for wood siding?

      1. Shawn Baldwin | | #17

        Scott,

        I have not used that product, there are a lot of them. It looks more like a masonry application and the fabric stops mortor from clogging the drainage mat.

        I will say that its easier for me to weave flashings into the Typar and then put the drainage mat/rain screen over that separately. So that way your not fighting with the drain mat while detailing flashings.

        In your situation though, going over two inches of foam, arent you using a verticle strapping with long screws? Or something similar for your rain screen?

        Shwwn

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