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Rock wool insulation and smart vapor barrier

Smith6 | Posted in Green Building Techniques on

With my new house construction I was planning on using open cell foam and have asked some questions a few weeks ago, but as I have read more, I am more interested in rock wool insulation batts. First, because of the DIY aspect, and second because there is at least some potential that it is a bit easier on the earth. So, I have questions about the best installation method for my cathedral ceiling and 2 x 6 walls in zone 3.
First, cathedral ceiling is 12/12. Will have soffit to ridge vents. 2 x 12 rafters on 24″ center. Looking at R30 rock wool batts (would like R38 but with vent baffle space not sure I can make additional rock wool batts fit). Questions with ceiling are: Do I need a smart vapor barrier? How will this smart barrier work if I use latex paint on gypsum on the ceiling? Is the roughly 4″ gap from the outside face of the batts to the exterior roof sheathing a big concern..and if so what is the recommendation to deal with this gap? I don’t want to use spray foam in this process. And since the larger vent space is potentially there, would I need an actual vent baffle like smartbaffle in place?
Second, 2x 6 walls, 16″ on center, 9′ height. Looking at R 23 5.5″ rock wool batts. Do I need a smart vapor barrier like MemBrain and if so, again, how does it work if you use latex paint on gypsum board?

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    This article will explain the options for vent baffles: Site-Built Ventilation Baffles for Roofs.

    In Zone 3, the code calls for a minimum of R-38 roof insulation, so you may want to consider installing a layer of continuous rigid foam on the underside of your rafters, followed by furring strips (strapping).

    More details on this type of roof here: How to Build an Insulated Cathedral Ceiling.

  2. GBA Editor
    Martin Holladay | | #2

    There is no requirement for an interior vapor barrier or an interior vapor retarder in Climate Zone 3.

  3. Smith6 | | #3

    I guess if needed I could use the high density R38 fiberglass in the ceiling that is designed to allow for venting space even though that is not my preference, instead of the rigid foam and furring strips. So with unfaced Roxul or other unfaced products, interior vapor barrier is not needed for zone 3...correct?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "So with unfaced Roxul or other unfaced products, interior vapor barrier is not needed for zone 3...correct?"

    A. That is correct. What you do need, however, is an air barrier at the interior side of the mineral wool, so make sure that your drywall is installed in an airtight manner. (Don't install any recessed can lights.)

    If you are creating your own ventilation baffles, I also recommend that you pay attention to airtghtness when installing the baffles.

  5. Deleted | | #5


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