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What’s the best shell assembly for my application?

GBA Editor | Posted in Energy Efficiency and Durability on

I’m in climate zone 5, planning to break ground in spring 2011. I’ve pored over GBA’s available data/blogs numerous times.

My future wall assembly, in to out, will be: 1/2″ paperless drywall, 2x6s filled with cellulose, Huber Zip System sheathing, 4″ foil-faced rigid foam, 1/2″ strapping with vinyl siding. I’m shooting for a blower door result of 1 ACH before drywall install (with spray foam at strategic spots!).

Here are my questions:
1. Is StoGuard over Huber wall overkill/not recommended?
2. Or will I be fine just tar-papering the plywood?
3. Is ANY barrier needed/required at the exterior rigid foam surface before strapping?


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  1. Chad Statz | | #1

    I am going to assume you have ruled out SIP wall components for some reason. If not, please consider them if you want it tight, as we regularily get .05 or less ACH in homes. To address your question directly, you must achieve a secondary drainage plane behind siding. If you are confident it exists without Stogaurd than go ahead. Do I understand you are putting 4" foam outside of plywood. Just be darn sure the wall assembly (insulation cavity) is ultra dry stays dry throughout. I don't know the perm rating of of Huber plywood, but installing foil face foam and taping seams will dramtically lower exterior wall perm and minimize air infiltration followed by housewrap (taped). If content with stud framing, caulk seal every plate to plate, plate to floor, and stud to stud corners. During or blower door tests with IR cameras, these are the weakest links in stud framing.

  2. GBA Editor
    Martin Holladay | | #2

    1. The Huber Zip System creates an effective air barrier, so there's no need for StoGuard. The StoGuard system has a different way of sealing seams between sheathing panels, using a proprietary Sto mastic plus fiberglass mesh tape. Huber uses Zip System tape to seal panel seams. You need to choose one system or the other, not both.

    2. The whole point of using the Zip System is that you don't need to install asphalt felt over the sheathing. The Zip System sheathing, properly taped, becomes your WRB. If you are nervous about the system, there's nothing wrong with covering it with asphalt felt. But it isn't necessary.

    3. Every wall needs an air barrier and a WRB. The Huber Zip System can be both, if you follow Huber's directions. However, if you distrust the longevity of Huber Zip System tape, feel free to install a different WRB. Just be sure that the WRB you choose is integrated properly with the window flashing and door flashing.

    It's also possible to use rigid foam as a WRB -- again, if you follow certain recommended details. (This will be the topic of one of my upcoming blogs). That said, many builders feel better installing housewrap on top of their exterior rigid foam. It's your choice.

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