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Rockwool on Exterior of OSB With 4-Inch XPS on Interior of OSB?

davgonz90 | Posted in Green Building Techniques on

The layout: Steel Studs  -> 4-Inch XPS -> ZIP Sheathing -> 1-Inch Rockwool Comfortboard -> Furring Strips -> Metal Siding

Im in Zone 1 and 2. 

Will this layout work? Any tips to keep in mind? 

Thank you.

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  1. Expert Member


    What's the advantage you are looking for by adding the rock wool?

    1. davgonz90 | | #3

      I’m experimenting with building an ultra-high R-value passive tiny house. It’s only going to be about 70 SQ feet, but I’m using it as practice for building a larger home in the future.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Sequencing the materials the way you have adds an awful lot of complexity. As Jon said, foam is usually situated outside the sheathing to add strength, make the fastening easier, and keep the sheathing dry. Adding rock wool to the exterior gives you nothing that more foam thickness wouldn't, but also means you have nothing to fasten the furring to except the Zip. That's usually adequate without exterior insulation, but problematic with it.

        1. davgonz90 | | #6

          Thanks for the reply. Aside from structural/installation complexity issues, are there any issues regarding condensation or rot in a structure like this? To your knowledge?

          1. Expert Member
            MALCOLM TAYLOR | | #7


            No the assembly should be very safe - although again, the exterior rock wool doesn't help.

  2. Jon_R | | #2

    The sheathing is often a needed structural component and doesn't function properly when separated from the studs by 4" of foam. Consider studs->zip->insulation.

    1. davgonz90 | | #4

      Thanks for the reply. Aside from structural considerations, are there any condensation or rotting issues I would need to worry about with a layout like this?

  3. Jon_R | | #8

    If I didn't have to worry about code and was building a tiny house, I'd experiment with DIY SIPs. Ie, glued OSB/polyiso foam/Zip.

    1. davgonz90 | | #10

      I’ll definitely look into that, thanks Jon.

  4. charlie_sullivan | | #9

    Note that regular XPS has very high global warming impact. If you must use XPS, the new Owens Corning NGX has much lower impact. You can special order it from Home Depot.

    But you can also switch to polyiso or EPS or GPS (graphite EPS)

    1. davgonz90 | | #11

      Great points Charlie, thank you.

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