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2 x 8 Zip R wall

user-4885540 | Posted in General Questions on

In thinking about building a slightly better than pretty good house, I came across a Build Show video with Matt Risinger and Steve Baczek where Steve was walking thru a house he designed with the following wall assembly:

1.  Siding
2.  Vertical furring strips
3.  Asphalt paper
4.  Zip R9
5.  2×8 on 24 inch centers
6.  Closed cell spray foam (2 inches or so)
7.  Roxul R15 batts

He doesn’t say exactly where other than outside of Boston.  Would guess climate zone 5 to 6.  The wall he describes doesn’t fit nicely within the 5 Walls that Work, but it seems like a really interesting and very buildable approach to constructing a high performance wall.  With the caveat and assumptions that we get the air sealing details nailed, where does this wall fall short if it does at all?

To me it would seem that the first condensing surface for outward vapor drive would be the closed cell spray foam and that would keep the sheathing part of the Zip safe as any moisture that hits the closed cell could dry inward thru the Roxul and then Drywall.

Here is a link to the video for those that aren’t familiar with it or haven’t seen it.

https://youtu.be/N7BqObGA6ss

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Peter.

    Steve's walls rarely do fit into any sense of common assemblies, and they're typically very high-performance from an R-value and air sealing perspective.

    The closed cell spray foam is unfortunate from an environmental perspective, but I guess he needed it to get to his target R-value. Dew point temperature control will be specific to your climate zone, as you know, and I think you are right that you should consider the inside of the closed-cell foam in this case, your first condensing surface.

    I've talked to Steve quite a bit about his assemblies and he is mostly concerned with excellent water management and air sealing details. He says that if he gets that stuff right, vapor drive is not much of a concern.

    Attached is a illustration of the project you are talking about (I believe) and it shows his wall-to-roof details. We published it in FHB. Note that the additional WRB was used for the black color; this is an open-cladding wall.

    Also, we have an article coming up next week on working with ZIP R-Sheathing that you may find helpful. Note for now that a ventilated rainscreen is a must when using closed-cell inside of ZIP R.

    1. brentwilson | | #2

      I'm looking forward to that article about Zip R Sheathing!

  2. Expert Member
    AKOS TOTH | | #3

    Thats around an R33 wall. Pretty expensive for what you get.

    I'm not sure why you would complicate the assembly with SPF.

    Skipping the SPF, going with regular R28 mineral wool batts, same assembly is R35 and probably 1/2 the cost per sqft.

    If you want even better, skip the zipR, a standard 2x8 wall, R28 batts, with fiberboard sheathing and 1.5" of polyiso is R36.

    1. MattJF | | #4

      Akos, are you breaking the foam/fluffy code ratios for zone 5? People seem to get away doing so all the time to some extent. Are you using an interior “smart” vapor barrier of just pretty tight drywall?

      Including ccSPF + zipR seems like a lot of money to spend to ultimately still install a furred rain-screen and end up with cold sheathing. At the point where a rain screen is going up, full on exterior polyiso seems like a pretty reasonable option. I would think the cost of longer screws and window bucks if needed would be less than the approach taken.

  3. Expert Member
    AKOS TOTH | | #5

    There are no ratios to break. With enough exterior foam, you can skip the interior vapour barrier.

    If you put less foam, just put an interior vapour barrier/retarder and the wall will work just great.

    I'm in the land of double vapour barriers (zone 5) where the standard construction is 2x6 wall with R5 rigid insulation. No issues with this.

    You can read more about it here:
    https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed

    For a bit of extra insurance, you can always build the wall with one of the more permeable rigid materials (unfaced EPS or permeable plyiso).

    With 1.5" of foam and 1/2" strapping, you can still nail up the strapping with standard nails, no need for screws. If your WRB is above the sheathing, window install is straight forward, about the only thing you need is a bit longer trim.

    The question is weather even my budget 2x8+rigid wall is worth the cost. Except in colder climate with high energy cost, there is very little money/energy saved by going much above R25 effective wall assembly. Good to run the design through something BEopt.

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