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Community and Q&A

Mineral wool and vapor barrier in zone 4A home?

BJHuffine | Posted in Energy Efficiency and Durability on

I really appreciate this community and everyone’s patience helping me better understand our best approach with this new home.  I hope this doesn’t sound too dumb, but I feel compelled to “make sure” I don’t end up with a serious uh-oh-oh.  I’m at the bottom of the 4A climate zone in Tennessee next to the Alabama/Georgia intersection.   We are considering using mineral wool in this new home and typically, we don’t worry with vapor barriers here.  However, as I was reading an article in GBA (link below), the vapor barrier was mentioned several times for the cooling season.  If I install mineral wool in this climate zone, is a vapor barrier necessary?

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  1. Expert Member
    Dana Dorsett | | #1

    >"If I install mineral wool in this climate zone, is a vapor barrier necessary?"

    No. Air-tightness of the exterior sheathing and interior wallboard matters a lot more than the vapor retardency of those layers. A "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) behind the wallboard can offer a lot of resilience, and possibly worth it if you're the type to lie awake worrying about it.

    If the siding is a brick or stone veneer it may be worth reducing the vapor permeance of the exterior sheathing to reject the intense moisture drives when the sun hits rain/dew wetted masonry, but not with a true vapor barrier, unless it's on the EXTERIOR of the structural sheathing. Even a half-inch of foil faced polyiso would protect both the exterior sheathing and the studwall cavities from exterior moisture drives, as well as offering an R3 thermal bridge over the framing fraction.

    I'm assuming the proposed wall is 2x6/R23?

    1. BJHuffine | | #2

      Thanks Dana. We're using Zip sheathing on the exterior. I plan to use a 3mm rain screen between hardi siding (a mix of board and batten and cedar shake) to reduce any hydrostatic pressure and make way for better drying of the assembly. Then the mineral wool would indeed be R-23 in a 2x6 framing on 16" centers. The only stone will be a natural stone veneer on the bottom foundation wall portion, which is poured concrete.

      Btw... you said "Air-tightness of the exterior sheathing and interior wallboard matters a lot more than the vapor retardency of those layers". That reminded me that I've seen somewhere that the wallboard can be sealed once put up, and I think before the plaster is applied. I believe I've also read that latex paint can serve as a good barrier. So, with that said, would it make more sense to ensure they are sealed well and painted in lieu of a vapor barrier? If so, what's the best approach for sealing the wallboards then?

  2. Expert Member
    Dana Dorsett | | #3

    With ZIP properly detailed with tape & goop it's pretty air tight, and is sufficiently vapor retardent to not present a summertime humidity problem for the studwalls of air conditioned home. Without the WRB facer the OSB runs less than 1 perm when dry, and only 5 perms even with a fairly high moisture content (which it will never have).

    Standard latex paint on wallboard or plaster runs about 3-5 perms, which is just what the doctor ordered for an interior side vapor retarder in a zone 4A climate.

    For a primer on detailing drywall as an air barrier, see:

    It's not rocket-science, but rather goop-science + better/tighter electrical boxes. In a zone 4 climate it's more forgiving than in a zone 6+ climate, but giving it some amount of effort will be worthwhile. Polyurethane sealants are a lot nicer than some of the acoustic sealants people were using in decades past.

  3. BJHuffine | | #4

    Thanks Dana. Sounds like I'm going the right direction here. You mention the "Polyurethane sealants are a lot nicer than some of the acoustic sealants..." What are the major breakthroughs and products worth considering? It seems every time I look up sealants online, acoustic still pops up due to its drying and cracking properties (or lack of). And I'm sure there's good and bad products out there, but it would be nice to have a couple ideas to benchmark from.

    Also, good article. Definitely a more focused approach to air sealing.

  4. Expert Member
    Dana Dorsett | | #5

    >"What are the major breakthroughs and products worth considering? "

    Purpose made very low expansion foam sealants for batt insulation such as Knauf EcoSeal or Owens Corning Propink ComfortSeal are a lot less stinky-sticky than generic acoustic sealants, and are a lot more flexible and penetrates cracks & seams better than generic water curing can-foam/ gun-foam. Proseco R-Guard Joint & Seam Filler (which is a caulk, not foam) is also pretty good stuff.

    I'm not sure any of these products qualify as "...major breakthroughs..." , but it's incremental progress to be sure.

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