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Community and Q&A

Wall design

Andy Zbojniewicz | Posted in Lakesideca Techniques on


I am building a home in Grand Rapids, MI and wanted to clarify whether my wall design sounds reasonable. We plan on doing from in to out: gypsum/drywall, blown in fiberglass with 2 x 6 wall, tyvek, 2 inch Rockwool, furring strips, Hardie siding. I saw some designs having a smart vapor barrier like certainteeds membrain between drywall and studs, is that necessary or if not necessary, recommended? Thanks.


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    It looks like Grand Rapids is on the border of Climate Zone 5 and Zone 6. If you go ahead with your plan, it's OK to use 2-inch mineral wool (call it about R-8, depending on what product you use). But if you switch to rigid foam -- and if you determine that your house is in Zone 6 -- be aware that the minimum R-value of the continuous exterior insulation would be R-11.25.

    For more information on minimum R-value requirements for continuous exterior insulation, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    With semi-rigid mineral wool, you can get away with a lower R-value, because mineral wool is vapor-permeable. But the tradeoff is that mineral wool is a fussier material to install, since it is squishier than rigid foam. For more information on installing exterior mineral wool, see "Installing Roxul Mineral Wool on Exterior Walls."

    For an overview of different wall design options, see "How to Design a Wall."

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Q. "I saw some designs having a smart vapor barrier like Certainteed's MemBrain between drywall and studs. Is that necessary?"

    A. In your climate zone (either Zone 5 or Zone 6), building codes require the installation of an interior vapor retarder. Options include vapor retarder paint or a product like MemBrain. In general, though, attention to airtightness matters more than your choice of vapor retarder.

    For more information on vapor retarder requirements, see these articles:

    "Vapor Retarders and Vapor Barriers"

    "Do I Need a Vapor Retarder?"

  3. Andy Zbojniewicz | | #3

    Great, thanks very much Martin!

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