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I’m not sure that my building inspector understands the science

RedStag | Posted in Green Building Techniques on

I live in climate zone 4A (near Philadelphia, PA) and I am building a 2400 sq ft addition on the back of my house. From the outside in, my wall assembly is vinyl siding, rainscreen, 2″ XPS rigid foam, zip-system sheathing, 2×6 stud framing with Roxul insulation. My building inspector says that I must have an interior vapor barrier. To the best of my knowledge (and research) with the exterior XPS on the wall, the interior of my wall should be vapor-open. I’m not sure that my building inspector understands the science behind leaving the interior vapor-open. Will a “smart” vapor retarder solve my dilemma? Is there an exception with the 2007 IRC that I can look to?

Thanks Ian

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  1. GBA Editor
    Martin Holladay | | #1

    Tell your inspector to look up Table N1102.5.1 in the 2007 Supplement to the IRC.

    More info here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Will a smart vapor retarder solve my dilemma?"

    A. Yes. As building scientist John Straube has said -- I quoted him in my article, Smart Vapor Retarders for Walls and Roofs -- sometimes “the smart material really solved a building official problem, not a building science problem.”

  3. RedStag | | #3

    Thanks for the information. 1 more question...My township follows the 2009 IRC, will the 2007 table you referenced still apply?

  4. Dana1 | | #4

    IRC 2009 has the same wall prescriptives for vapor retarders in SECTION R601:

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