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Community and Q&A

Double-Stud Wall vs. Zip Sheathing Assembly

bauchin | Posted in General Questions on

I am designing a house for northern Vermont Climate Zone 6 and am trying to decide between two different wall options:
1) Double 2×4 stud wall (11 1/2″ cavity) with rain screen and dense packed cellulose insulation
2) Zip Sheathing R-9 Insulated Panel 2″ – 4’x8′ on 6″ stud wall framing with rain screen – either spray foam or dense packed insulation in wall cavity

In addition to understanding which is better suited to Climate Zone 6, I am considering the following:
– Total cost
– Skills of local VT contractors in acheiving a tight envelope with either system . . I need to understand if either of these systems might be beyond the capacity of local “traditional” contractors to execute
– Difficulty of construction/detailing
I would appreciate any feedback anyone might have to offer

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  1. mr_reference_Hugh | | #1

    Hello bauchin

    A few minutes ago, I provided a reply about double stud wall assemblies. I refer to a BS&Beer assembly competition. They critique typical double stud walls and explain how to overcome the challenges. You can check at the very bottom of this other Q&A.

    You can also look at the wall assembly competitions to see what the expert judges were saying on other wall assemblies. I believe they had one with the ZIP R? sheeting.

    ****Source of information 2021 competition****
    This explains the context of why this critique of wall assemblies was taking place.
    Original webpage :

    Detailed critiques of the 2021 wall assemblies
    Source document:

    ****Source of information for 2022 competition***

    Ignore the year 2021 in the URL, this is for June 2022.

    Detailed critiques of the 2022 wall assemblies

  2. Expert Member
    Michael Maines | | #2

    Zip R-9 in climate zone 6 does not provide enough R-value in the foam layer for condensation resistance. If you include a layer of closed-cell spray foam on the interior, it is safe. But it comes with a much larger carbon footprint than simply using a double stud wall and cellulose insulation. Vermonters I know are very familiar with double stud construction.

    1. frontrange | | #3

      I thought the reason for minimum exterior insulation values was to prevent condensation in the sheathing. Zip-R sheathing is on the outside of the foam layer, so I don't think that's a concern.

      1. Expert Member
        Michael Maines | | #4

        You don't want condensation inside your wall cavity either.

        1. frontrange | | #5

          Is that really a thing? I've lived in several houses with no exterior insulation and never had condensation in the walls.

          1. Expert Member
            Michael Maines | | #8

            Yes, it's really a potential problem. When you add exterior insulation (either inside or outside the sheathing) you usually slow the wall's ability to dry to the exterior, which can lead to moisture accumulation in the wall. And just because you haven't seen condensation in your previous walls doesn't mean it wasn't there. It's likely not as big a problem as building nerds tend to make it, but it's a potential problem easily dealt with in the design phase.

          2. frontrange | | #9

            I see, it's more about the exterior foam reducing the drying potential and prolonged moisture accumulation.

  3. bauchin | | #6

    Thank to all those who responded. From the replies, it seems that the double stud option is preferred unless I use Zip R Sheathing with at least 2 1/2" rigid on outside (R 12) to prevent condensation issue in Climate Zone 6. Another factor that I have to deal with is that due to the sloping site, I have to have a full height basement for at least the down hill portion of the site. I think that the double stud wall option makes this easier as well because there is no offset of the exterior wall for the zip system rigid insulation. Thoughts? And thank you all!

    1. andyfrog | | #7

      Note that the main concern with double stud walls is bulk water intrusion. Just building a vapor open assembly doesn't mean that there is enough energy flux to dry it out before it gets wet again. So when you detail your exterior WRB, make sure to do it extremely well.

  4. mr_reference_Hugh | | #10

    bauchin, I found this really good article on double stud walls.

    I thought I would share another great article that talks about the risks associated with double stud wall and the optimal design. This is from Allison A. Bailes III, PhD who references Joe Lstiburek from Building Science Corp (BSC) and the research by BSC. More help when talking to building officials.

    The article mentions using OSB in the middle. It also refer in more general terms to "sheet goods". Using sheets of rigid insulation in my view qualify as sheet goods.

    This article also suggests sheeting on the exterior, include the option for a gypsum product which is more vapour open than other options.

  5. mr_reference_Hugh | | #11

    bauchin, I just found this article (I think its an article) from Joe Lstiburek from Building Science Corp (BSC). It gives his opinion on using double stud walls, which is different from his opinion on how to build double stud walls.

    I was in fact not aware of his view on double stud walls. Here is his quote from 2012.

    "There are in fact some beautiful Passive Houses. But double walls are tricky to construct, increase the risk of moisture damage and in my view they don’t make a lot of sense. Give me a single advanced frame 2x6 wall with lots of insulating sheathing any day. I did the double wall thing 30 years ago before there ever was a Passive House. It was dumb then. It is dumb now."

    Taken from footnote #3 at the bottom of the "article".

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