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Community and Q&A

Zip wall insulation ratio question

Erik Lobeck | Posted in Energy Efficiency and Durability on

I am considering using insulated “Zip System R-sheathing” on a project to save some labor$$. From a building science standpoint I am somewhat unsure. I am in Zone 7.

Ideally, I would have something in the neighborhood of a 2/3 out to 1/3 in or 1/2 out to 1/2 in insulation ratio to avoid condensation problems. The Zip wall product places the foam (1 1/2″, claimed R-6.6) on what at first glance seems to be the inside of the wall plane….so theoretically on the “wrong side” with regard to the ratios (more stud cavity insulation).

Or is it perhaps okay due to the hydrophobic character of the bonded insulation being the (hopefully not) condensing surface?

Long story short: Anyone have any input if this would be a safe wall assembly (2×6 w/blown in glass or cellulose w/ R6.6 Zip R, rainscreen outside, all taped obviously)? I am trying to achieve something in the neighborhood of R 30, maybe R-35.

And….has anyone sprayed closed cell foam against the foam of a R Zip panel (chemical compatibility)?

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  1. Aj Builder, Upstate NY Zone 6a | | #1

    I think Zip R-sheathing is a good choice in my zone 6a.

    Your zone and R 30 or more I would use it and do a double stud wall.

    My feeling is that Zip R-sheathing is fully protected from moisture damage.


  2. Aj Builder, Upstate NY Zone 6a | | #2

    If you don't do a double stud wall then you should do exterior rigid insulation as per this site's main page video.

  3. GBA Editor
    Martin Holladay | | #3

    Huber makes Zip-R sheathing in two thicknesses. The 1.5-inch product is rated at R-6.6.

    To avoid problems with moisture accumulation and condensation in your stud bays, you want to follow published guidelines for the minimum R-value of foam sheathing. These minimum R-values can be found in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Since you live in Zone 7, Zip-R sheathing doesn't have enough R-value to meet minimum requirements for this application -- even for 2x4 walls. In your climate zone, the minimum required R-value for foam sheathing on a 2x4 wall is R-10. On a 2x6 wall, you need at least R-15.

    The only way you could make this Zip-R sheathing product work is if you add additional rigid foam on the exterior side of the Zip-R.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    It never makes sense to use Zip R-sheathing with additional exterior insulation or additional interior closed cell insulation. I am a builder and I just wouldn't do such as the benefits of Zip R-sheathing are lost.

    Either one uses zip R-sheathing to save from using rigid foam or one just uses standard Zip sheathing with two layers of well fitted and taped rigid sheathing or neither.

    Also though Martin does not condone it, I see no problem using Zip R-sheathing in an application where it might just have some weather where condensation forms on it. Why? Well condensation is happening in all the homes built to date (millions) that have no exterior rigid insulation. The problem with this condensation is having typical low cost OSB rot. Zip R-sheathing will be wetted on the face of it's foam. The encapsulated OSB that is also made with better than standard OSB is safe from moisture via any small amount of winter condensation. Lastly winter temperatures on average for example my area much higher than the few times of our coldest temperatures which means less condensation than the coldest temperatures could create.

    Choose Rigid or choose Zip R-sheathing. Don't do both. I see no problem with this where I am in zone 6a,

    As I stated above, if you do want to follow the calculated minimum rule Martin advocates then use standard Zip.

    I think time will tell the Zip R-sheathing being so well protected on both of it's sides is one of the best choices one could ever make for a safe rot free sheathing material to build with no matter what the rest of the assembly is made up from. (in my zone 6a)

    Standard OSB is low cost sheathing not worth using in insulated building construction. I have no problem using it for uninsulated detached buildings such as a garage or shed. Use Zip, use Zip R-sheathing, use plywood for a healthier longer lasting home. The extra cost is worth it.

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