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Wall Assembly with Zip System Sheathing, Exterior Insulation, and Rainscreen

gagejfoley | Posted in Green Building Techniques on

Hi All,
I am working on a new residential build for my personal home just north of Boston. I am interested in wall R value around +/- R-30, without breaking the bank. A few questions as a homeowner trying to drive a high performance build:

I was thinking 2×6 wall, zip system, exterior insulation, 1″ furring strips, and fiber cement siding for the wall assembly.

The 3 builders I spoke with do not have experience with exterior insulation and tend to tell me they “fill wall cavities spray foam” these days for high performance (helps them meet MA blower door sealing code).

Any recommendations for the combination of cavity insulation and exterior insulation for this assembly/climate zone (targeting R-30ish)? XPS vs EPS vs RW on the exterior?

Any recommendations to find a clean detail showing a similar assembly? I want to use this to show builders and gauge their comfort level. I am sure I have come across a few, but find myself drowning in BSC and GBA articles.

Thank you!
Gage

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Replies

  1. Ian Watson | | #1

    If you're using ZIP I don't really see the benefit of exterior insulation. The ZIP system has insulation that will act as your thermal break. If you're looking for R30 you could fill your studs with R22 Rockwool (or the R24 if you can find it) and then buy whatever ZIP thickness gets you the total R-rating you want.

    ZIP is also an air barrier, so if you detail other parts of your building well I don't think you'll have trouble with your blower door. It won't be a super-high-performance building assembly, but you'll meet your targets while keeping it pretty simple.

  2. Joseph Dziedzic | | #2

    Getting the water control details for windows and doors right when exterior insulation is used can be challenging for someone unfamiliar with the process. ZIP R-9 will get you close to R-30 but check to see if the builder is comfortable with using it, as the panels are 2" thick which will possibly require longer nails to attach the panels.

  3. gagejfoley | | #3

    Thank you both for the quick response - unless I am misunderstanding, both of you are recommending ZIP-R for the exterior as a means of reaching the target.

    I was under the impression that I need the exterior side of the air barrier (sheathing) insulated with at least 30% to keep the sheathing warm enough that we will not have any interior condensation (sublimation)?

    See table 3: https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-120-understanding-walls

  4. Walter Ahlgrim | | #4

    Have you considered the Zip+R system? It is a lot easier to get the contractors on board with the +R system.

    The hard thing about getting exterior insulation is convincing anyone that it is their job to install the foam board. The framing crew that’s not wood I not touching it and the siding crew says not my job.

    I have not looked up the details for you zone but I am guessing 2x6 wall near Boston needs 2 inches of foam to avoid condensation problems.

    You may find these two articles interesting.
    https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
    https://www.greenbuildingadvisor.com/article/five-rules-for-wall-design

    I see large amounts of spray foam on new construction plans as a red flag for an easy fix by someone doing sloppy work with zero concern for wasting your money.

    Walta

  5. gagejfoley | | #5

    This may have been a pretty simple oversight on my end.

    Is the zip R insulation vapor impermeable? That would explain what I am missing here (the face of the zip foam would be the condensing surface, not the back side of the zip osb)?

    1. Kyle R | | #7

      Correct, zip R uses polyiso which is a class 1 vapor retarder

  6. Eric Whetzel | | #6

    Your best options for a high-performance wall are likely going to be either continuous insulation, as noted in your original question, or a double stud wall.

    You can read about my experience with continuous insulation here:

    https://kimchiandkraut.net/2019/07/11/passive-house-continuous-insulation/

    You may find some of the links useful as well.

    On GBA itself, I found these articles to be especially helpful:

    https://www.greenbuildingadvisor.com/article/a-case-for-double-stud-walls

    https://www.greenbuildingadvisor.com/article/two-views-of-double-stud-walls

    Beyond cost considerations of the wall assembly type, the biggest hurdle will be finding a builder --- or at least a framing crew --- who is willing to get the details right, regardless of whether the assembly is CI or Double Stud.

    1. gagejfoley | | #9

      Eric - great read. The "PGH" is a good way to describe what I am trying to accomplish. Your notes regarding telling contractors ahead of time are interesting to read. I will try to rephrase some of my discussions with them as an opportunity to try what might be coming.

      Also - great info from the ProTradeCraft videos. Worth a quick shout out here.

  7. Expert Member
    Akos | | #8

    The budget high R value wall that is as close to a standard build as possible is a 2x8 24" OC wall insulated with high density batts (either mineral wool or fiberglass).

    This has an assembly R value of R24 which is pretty close to your target without any of the issues of double stud or exterior rigid. Going from 16"OC 2x6 to 24"oc 2x8 means the lumber cost is about the same, a bit more labor as there are no precut 2x8s.

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