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Barrier on inside of mineral wool batts in Zone 6?

MaineMatt | Posted in Energy Efficiency and Durability on

I’m converting a garage into a family room, and I have it down to the studs right now and am getting ready to insulate.  I’ve furred out the studs to make a 2×6 wall, and on the outside I have LP Smartside siding – tyvek – 3/4″ board cladding.  

I’m technically in Zone 6, but I’m close enough to the coast that it’s really Zone 5 ish.

I like the idea of mineral wool batts, but I’m finding a lot of contradictory information about whether or not I should have a barrier on the interior of the installed batts.  Membrain?  Poly Sheet?  Just put up drywall and call it good?  Everyone local I’ve talked to tells me to just put up faced fiberglass and forget about ‘all that new stuff’.

The rest of the house is 2×4 construction with ~50 year old fiberglass batting, so this is going to be the best insulated room in the house no matter what I do, but I still want to do the best I can in terms of efficiency/longevity.

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  1. Expert Member
    Dana Dorsett | | #1

    Tyvek over plank sheathing isn't very air tight. Depending on how leaky it actually is, it's likely to be worth using half-pound polyurethane (open cell foam) instead of rock wool. Be sure to lay a bead of polyurethane caulk on all doubled-up framing seams (jack studs, headers, top plates, foundation sills, etc) for air tightness before putting up the drywall. But if it's too small a job the pros would have to charge too much.

    Even in zone 5 you'd need at least a Class II vapor retarder ("vapor barrier latex" primer on the wallboard) or smart vapor retarder (kraft facer or MemBrain is fine) with that stackup. Had there been a rainscreen gap between the siding & sheathing you could skip the vapor retarder and use just standard interior latex primer, but without it there's too much risk of wintertime moisture build-up in the plank sheathing.

    How many square feet of wall area are we talking about?

    If it's less than 600 square feet of wall area you might use a 600 board foot DIY foam kit and give it a 1" shot of closed cell foam for air tightness instead. That would be sufficient exterior R to use compressed R19 in the remaining 4.5" (~R15 at that thickness) and still have sufficient dew point control at the foam/fiberglass boundary to be able to use standard latex on wallboard or kraft facers on batts as the interior side vapor retarder. If it's <300 square feet of wall a smaller DIY kit would be cheaper (if higher cost per square foot.)

    The center-cavity R-value would be the same R22-R23 that rock wool delivers, but it's substantially more air tight, and has somewhat less moisture risk.

    A down side to DIY foam kits is the HFC245fa blowing agent used, making it one of the less-green insulation options, though only half as damaging as XPS. But given the likely high leakage of the sheathing there may still be a rationale for that environmental hit. The stuff is pretty expensive cash-wise too- usually over a buck a board foot even for the 600 board foot kits by the time shipping and a Tyvek suit for the installer is added in.

    I'm hopeful the DIY foam kit vendors will have HFO blowing agent versions soon. If the incoming US administration takes climate change seriously (seems possible, even likely) and are able to get the US to sign up to the Kigali amendment to the Montreal Protocol they will really have no choice but to stop selling kits that use HFC245fa.

  2. MaineMatt | | #2

    Thanks Dana,

    The wall area I have to insulate is about ~500 sq feet, so it seems like one of the spray foam kits might do the trick. I don't love the idea of the HFCs, and I'm a little leery of the spray foam kits generally. I may reach out to a local installer and see what the cost would be to have someone tackle that project for me. If I'm going to be spending $700 on the foam kit, I might be better off just paying a pro to ensure it's done right - only question is if I can get someone out here for such a small job.

    It sounds like I should plan on some sort of facing on the inside if I don't go with spray foam though, so thank you for that info as well.

  3. Jon_R | | #3

    With an inch of spray foam, you need a Class II interior side retarder - standard latex doesn't meet code.

    Make sure that the wall complies with the recommendations in Table 2 A or B here.

    Do focus on air sealing, preferably with some testing to verify.

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