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Cork spray with specific wall assembly

Eva7 | Posted in General Questions on

We are building a steel frame tiny home with cathedral ceilings (no attic). It has Zip System R-Sheathing on the outside walls, and rockwool insulation between the steel studs.  The roof differs from the wall assembly in that it includes OSB zip system with layers of polyiso rigid insulation foam board ON TOP of the zip system, instead of against the steel studs.

So, my question is: Will using cork spray over the interior lightweight paneling walls/ceiling cause any issues?? We wanted to use cork spray as a way to improve the energy efficiency of our home but aren’t sure if our unique assembly would suffer instead of benefit.

Thank you

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  1. Matt F | | #1

    What is the thickness of the coating and what is the R value?

    Google shows me products that look more like paint.

    If it is not more than 1/4" thick see this:

    1. Eva7 | | #4

      Typically, thermal cork spray is applied at 3/16 of an inch. It is claimed that it gives a U-value of 0.068, which is similar to about R-14. It is not a paint as far as I know, and the article doesn't mention cork, so I am guessing the thermal cork spray is a different category. It is not marketed as the sole source of insulation, more like a protective additional layer.

      1. Expert Member
        Akos | | #5

        Total hogwash.

        Cork is between R3 to R4 /inch. To hit R14 in 3/16" you would need vacuum insulated panels. At best you are looking at R 0.6. There is no magic when it comes to insulation, you need space to get R value.

        Steel walls generally come in between R7 to R10 no matter the amount of insulation, so with the R6.6 exterior insulation puts the inboard/outboard insulation ratio near 50%. With that amount of insulation, you won't have condensation issues no matter the interior surface finish. Spraying cork on it won't effect anything.

        No matter the interior finish, it is a good idea to get a warm side decent air barrier.

  2. Steve Knapp CZ 3A Georgia | | #2


    Where do you live? Climate zone matters. What is the r-value of the ZIP sheathing (or is it regular ZIP without the integrated foam)?

    1. Eva7 | | #3

      I live in climate zone 5 (Northern Maryland). The ZIP sheathing for the walls has 6.6 R-value (because it is made of rigid foam board glued to OSB). The roof Zip system OSB is regular for roof; has no foam glued to it, so we plan to add at least 3" of rigid foam board on top of the sheathing.

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