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Question about lavardera USA new wall with a twist.

Eagleeyeshawk | Posted in Energy Efficiency and Durability on

So I think I understand the New USA Wall that Greg Lavardera advocates.

I live in climate zone 4 in Nashville TN. I am not having much luck finding builders who do exterior rigid foam or MW insulation. I think the better New USA wall may  be a good compromise. 

Question: how is the wall changed if I choose to zip R-6 sheathing instead of standard ZIP? I know the system calls for smart interior vapor retarder in zones 5,6,7. With normal zip, I was thinking i was exempt from using interior retarder since I’m in zone 4. But if i use zip R, the wall has to dry inside. Would a smart vapor barrier make more sense in this assembly? 

I am also  worried about the dew point and the sheathing getting cold, but i think Martin said in zone 4 i do not have to really worry about it as it never gets cold enough. Am I correct this applies to zip R as well? 

Amer

 

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Replies

  1. Eagleeyeshawk | | #1

    I think I have figured out the answer. Basically if I do Zip R sheathing and have an interior vapor retarder I will be setting up a vapor barrier sandwich which is a no no. So I think it may be best to go without the interior vapor barrier.

    1. BlueSolar | | #2

      Good call. No way would I install an interior barrier. The New USA Wall doesn't make much sense on that score. You've already got drywall and paint in a typical build, and I seriously doubt that adding a plastic barrier or retarder is going to be a net win. I don't think it's realistic to expect significant drying when a purportedly "smart" vapor retarder is added to painted drywall.

      The tape with Zip bothers me. It's really wide and covers a lot of sheathing surface, which is itself covered in their WRB. Layering tape on top of the WRB seems hacky and can't have the desired properties in those locations. So I wouldn't expect those areas to breathe outward given the double layer. It's not clear why embedding the housewrap on the OSB should be more effective than just wrapping the housewrap over the OSB – it would appear to be identical physics. They should just make the entire wall in one seamless OSB and WRB piece instead of all the 4 by 8 boards – I'd love to just order walls from a factory.

      1. Eagleeyeshawk | | #3

        Thank you for your response. Good to see a gba reader confirming my thoughts.

        After reading through lavarderas forum posts on Mineral wool, I suspect he would argue that most drywall has lots of penetrations including light switches, electric outlets, etc etc and an interior retarder is needed. I now think this is a bad move with zip R

        You point about the tape is interesting. I had not thought about that

        I imagine you are correct in that the physics are identical between factory made OSB and WRB and field applied OSB and WRB. The difference would be the latter you are relying on a trades dedication to detailing on that day.

        Thanks for your time.

        Amer

  2. Expert Member
    Malcolm Taylor | | #4

    Amer,

    You may get more responses if y0u link to the Lavardera wall you are discussing.

    1. Max E | | #5

      I think op is referring to this one
      http://blog.lamidesign.com/p/usa-new-wall-info.html

  3. Eagleeyeshawk | | #6

    Max,

    You beat me to it! Thank you for the reference.

    Amer

  4. Expert Member
    Malcolm Taylor | | #7

    Thanks Max,

    The New USA Wall is really just standard wood frame, with exterior permeable insulation, and a service cavity. There is a lot of information about how those walls perform on GBA once you eliminate the branding. It is a different animal if you substitute ZIP-R, as you can't rely on outward drying.

    Here are some links that may be useful:
    https://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf

    https://www.rdh.com/wp-content/uploads/2017/07/Guide-for-Designing-Energy-Efficient-Building-Enclosures-Web.pdf

    https://www.greenbuildingadvisor.com/article/how-to-design-a-wall

  5. Eagleeyeshawk | | #8

    Malcolm,

    Thanks for weighing in. I agree with you. I think the differences are small but manageable for most builders. Particularly the 2x2 horizontal strapping creating the insulated service cavity. I like the additional benefit of reducing thermal bridging along the studs

    The reason I had been thinking about zip R is because i haven’t been able to find a builder who knows how to do exterior rigid foam insulation. But I wanted to make sure my wall could dry to the inside as long I didn’t place Athar vapor retarder that Mr Laverdara was advocating for.

    Thanks for looking,

    Amer

    1. Expert Member
      Malcolm Taylor | | #10

      Amer,

      My feeling is the service cavity is only useful if you have an interior membrane it is protecting from damage. If the exterior sheathing and drywall are your air-barriers, there aren't enough electrical boxes to justify a dedicated space. If you want it to decouple the studs to reduce thermal bridging, I'd use a Mooney wall instead.

      A variable perm vapour-retarder won't inhabit drying to the inside the way poly would. I don't know enough about your climate to know if it's useful, or necessary.

  6. Eagleeyeshawk | | #9

    Malcolm,

    Thanks for the links. I’ll check them out.

    Amer

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