GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

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?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  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.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |