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Community and Q&A

Vapor barrier with mineral wool insulation

paulNed | Posted in Energy Efficiency and Durability on

I am building a garage apartment home in southern Vt zone 6. I have caulked (from the inside) everywhere that I think air may come in from the outside. I am using r23 rock wool 5 1/2 batt insulation. Should I put up a vapor barrier over the rock wool?

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  1. Expert Member
    Dana Dorsett | | #2

    In zone 6 you'd still want a Class-II or tighter vapor retarder on the interior side of walls, independent of sheathing & siding type. Air tight drywall with vapor barrier latex primer is safer/better than 6 mil polyethyene, and a "smart" vapor retarder such as Intello Plu$ or Certainteed MemBrain is a bit safer than vapor barrier latex.

    But the most important thing is air tightness to the conditioned space/interior, and fitting the batts carefully to prevent easy convection inside the cavity.

    It's OK but not necessary in zone 6 to install vapor barriers for VENTED roofs. But unvented roofs are a whole other topic.

  2. Jon_R | | #3

    > everywhere that I think air may come

    This is a case where testing is better than thinking.

    1. paulNed | | #4

      That is correct jon everywhere that I think air may come in. like cracks and crevices and places where you can see lite coming through. That's a good test. If light shines through then air will come in through that same spot.

      1. Expert Member
        Dana Dorsett | | #5

        Light only travels in straight lines- air can follow very convoluted paths. The daylight test is only going to catch a tiny fraction of the air leaks.

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