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Taped Zipwall or sheathing with Ice & Water Shield on a REMOTE/PERSIST roof?

Brad Hardie | Posted in General Questions on

So any thoughts on what is better or more practical?

I’m getting ready to start sheathing a roof (which will have all of it’s insulation outboard of the WRB and sheathing, with a cold roof on top). It is a R-66 EPS roof.

On one part of the project (a separate building, same construction technique (REMOTE/PERSIST), we used barnboard, then 1/4″ plywood (to hide the ice & water shield above it) from showing through any knots or such from below (exposed cathedral ceiling), then R-66 of EPS above, with 2×4 furring and then plywood and then metal roof.

On the first building we used Prosoco R-guard system on the exterior walls, doors and windows. Prosoco makes a fantastic product, but the MVP and Joint & Seam Filler was just too labor intensive to apply and finicky to get consistently just right. With that said though – I LOVED using the FastFlash and Airdam products for the windows and doors. It has done such a great job, so……..

On the second building in this project we decided to use the Zipwall 7/16″ sheathing on the exterior walls. I am certainly glad I did! I’m so impressed with the Zipwall/Huber products. The Zipwall tape is AMAZING. I can’t believe I haven’t used the Zipwall previously. We will be constructing the roof the same way as the first building, but because I’m so impressed with the Zipwall, and with colder weather on the way, I thought to just use the Zipwall 7/16″ sheathing (taped obviously) over the barnboard and not worry about ICE & Water shield over the Zipwall (except at the valleys and eaves).

I know the Zipwall is supposedly waterproof, and it has performed well, but a good ICE & Water Shield will truly protect the roof. I’m not worried about covering up the whole roof in ICE & Water shield, because of the REMOTE/PERSIST strategy. If I went the way of ICE & Water shield, I could just use OSB or 1/4 plywood over the barnboard again.

Zipwall is twice the cost of OSB, but it has the WRB already on it, which as I mentioned earlier is performing excellently. The Zipwall route ends up being about break even, after the ZIpwall tape, when compared to OSB with ICE & Water.

Any thoughts, or wise advice?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    The lowest level of roof sheathing on a PERSIST roof does not need to be waterproof. I just needs to be a good air barrier.

    Taped Zip sheathing will work just fine for this application.

  2. GBA Editor
    Martin Holladay | | #2

    Brad,
    One more point: If the insulation is on the attic floor, and the attic is vented, that's a REMOTE roof.

    If the roof insulation consists of rigid foam above the roof sheathing, that's a PERSIST roof.

    For more information on the PERSIST approach, see Getting Insulation Out of Your Walls and Ceilings.

    For more information on the REMOTE approach, see REMOTE: A Manual.

  3. Brad Hardie | | #3

    Martin,

    Yes, the REMOTE manual does specify that when building "new" construction the insulation goes on the attic floor, via a raised heel truss. It also specifies the approach used as I described in renovations. Obviously, REMOTE is a manifestation of the PERSIST methodology. In their (the CCHRC) videos on their website, they also show using the PERSIST method on the roof, but describe it as "REMOTE".

    If I am correct, the PERSIST methodology specs a vapor impermeable WRB at exterior side of the sheathing, both on the walls and the roof, not a vapor permeable WRB. Seems most vapor impermeable material is also waterproof, no? I suspect that the original spec was born from the fact that Grace ICE & Water Shield was most readily available, and the newer air barriers, which have some vapor permeability were not available.

  4. GBA Editor
    Martin Holladay | | #4

    Brad,
    There are many variations on the PERSIST approach.

    However, in all cases, basic roofing principles apply. The roofing is the waterproof layer.

    Under the roofing is a backup layer (the roofing underlayment) that is at least nominally waterproof.

    As Joe Lstiburek always emphasizes, the most important thing to remember when installing rigid foam above roof sheathing is to have an excellent air barrier at the level of the lower sheathing. Taped Zip sheathing will certainly work. And, as you point out, taped Zip sheathing is waterproof.

  5. Brad Hardie | | #5

    Martin,

    Thanks for your feedback and advice.

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