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Downsides to Zip R-sheathing over existing board sheathing?

Patrick OSullivan | Posted in General Questions on

As part of an addition/renovation of my 100 year old colonial style home in NJ, I will be taking the existing portion of the structure down to the existing board sheathing as well as replacing all windows. I will be having cellulose blown into the existing balloon framed cavities from the outside.

My original plan was to then cover the board sheathing in a self-adhered WRB–either Henry Blueskin VP100 or Pro Clima Adhero. I am now considering putting Zip R-sheathing (thickness TBD) over the existing sheathing, to match what I’m using for the addition.

The question: Are there any downsides I should consider with having a second sheathing layer? Window opening depths are a non issue since I’m replacing them (with new construction windows) anyway, and replacing all jamb extensions.

I am ruling out removing the existing board sheathing. I have already scope crept the project enough that I don’t want to add another demo step, nor do I want to play structural Jenga and try to keep the existing structure braced properly while there’s no sheathing on it.

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Replies

  1. Patrick OSullivan | | #1

    I missed another post (https://www.greenbuildingadvisor.com/question/zip-r-over-existing-sheathing) addressing a very similar question. The main difference between that post and mine would be the climate zone; I'm in 4A and that poster was in 5.

    I still welcome any comments/questions.

  2. GBA Editor
    Martin Holladay | | #2

    Patrick,
    Your plan is fine. Installing new OSB sheathing over old board sheathing is a routine approach when installing new siding on an older house.

    If properly fastened, the new Zip sheathing will help brace the walls. If the sheathing seams are sealed with Zip System tape, you'll end up with an exterior air barrier, which is a good thing.

  3. GBA Editor
    Peter Yost | | #3

    Hi Patrick -

    Particularly with ZIP-R, your drying potential for this wall assembly will be very limited to the exterior and so best practice would be to configure every layer/component to the interior of your sheathing as Class III or vapor open. (ZIP sheathing is a Class III vapor retarding material but with the added rigid polyiso insulation, it moves to a Class II vapor retarding material).

    Peter

  4. Deleted | | #4

    Deleted

  5. Brian Reading | | #5

    I am doing a similar renovation as Patrick but in climate zone 6 (mid-coast Maine). I'm considering the Huber Zip-R12 over existing wood sheathing of a 1930's house and then applying cedar shingles on top of that. What would your recommendation be for a rain screen/weep under the cedar shingles? Any red flags for such an assembly?
    Thanks,
    Brian

    1. Expert Member
      Michael Maines | | #6

      Brian, Zip R-12 in midcoast Maine (climate zone 6) provides condensation resistance for framed walls up to R-20, but you probably have more like R-13 in your framed walls so you should be safe. The most popular rain screen product for cedar shingles is Benjamin Obdyke's Slicker Classic, a spun-bonded plastic. It can be a bit squishy when installing shingles; you could upgrade to their version with a fabric facer, meant for masonry but it's nicer for shingling as well. Keene and Mortairvent have similar products. This type of rain screen provides a capillary break and allows for a small amount of air flow behind the shingles which helps them dry. Alternatively, you could use a thinner "drainscreen" product such as Obdyke's Hydrogap, but it mainly allows liquid water to drain, so the thicker products are better.

      1. Brian Reading | | #7

        Thank you Michael, your thoughts are very helpful!

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