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Vapor barrier on the inside against sheet rock or outside against sheathing or both sides of rock wool?

Ken Swartz | Posted in Energy Efficiency and Durability on

I’m doing a rehab remodel on a house I recently bought and have the exterior walls exposed on the house. I’m trying to upgrade the existing insulation since I had to replace quite a lot of the framing to repair dry rot. I’m trying to minimize moisture buildup in the wall spaces from both directions as well as effectively insulate the house. Plus, I’ve been getting clobbered on utilities.

I’m installing Roxul rock wool R23 into the 2×6 stud cavities. I’m going to be installing osb sheathing over top of that, because that is what the engineer is requiring to buildup the sheer strength. Over top of that, I’m going to be installing Rmax Thermasheath-3 2 inch R13.1 over the sheathing. Lastly, I’m installing waterproof paper for the stucco that I am cladding the house with.

I have been doing quite a lot of reading and really getting more confused. Should I install a vapor barrier against the sheet rock before placing the rock wool or after I place the rock wool? Should I put an additional wrap outside the sheathing to protect it?

Lastly, is there any other recommendations or tips, anyone can pass along.


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  1. D Dorsett | | #1

    Foil R-max Thermasheath-3 tests at less than 0.3 perms, which makes it a VERY tight class-II vapor retarder, almost a vapor barrier, in which case you don't want the interior side to also be low-perm, or it will have created a moisture trap. (Moisture that checks in takes forever to leave.)

    You don't say where you are located, and climate makes a huge difference on whether the 2" polyiso will be sufficient for dew point control at the OSB layer. If the average wintertime temp at the OSB is below the average dew point of the indoor air (about +4C, or 39F for cooler climates) you'll need some sort of vapor retarder on the interior, but not a true vapor barrier. Smart vapor retarders that become vapor open when the moisture levels in the wall cavities are high would work. (Certainteed MemBrain, Intello Plus, etc.) It would be placed beween the wallboard the rest of the assembly in a single air-tight layer.

    It's important to leave a vented air gap between stucco and the next layer, even with the vapor retardent foam. Stucco hangs onto rain & dew moisture, releasing it in intense bursts when warmed by the sun. Warm stucco that is open to the outdoors at both the top and bottom will convect that super-saturated air to the outdoors rather than cooking it into the wall, as long as it has a convection path.

    The waterproofing paper (usually #15 asphalted felt) is your drain-plane & weather resistant barrier- no need for a separate layer of housewrap. But it's extremely important that your window & door flashing extend out to the waterproofing layer, and is lapped correctly do drain bulk moisture to the exterior side of the #15 felt rather than directing it toward the foam layer.

  2. Ken Swartz | | #2

    Based on what you mentioned, I was considering B.O.'s Slicker MAX (yes this site advert got me thinking) or something like it. Do you recommend another product over this type of fiber-based rain barrier?
    I live in Roseville California.
    Thanks for the information!

  3. Expert Member
    Dana Dorsett | | #3

    Roseville CA is in US climate zone 3B. In that climate you don't need or want ANY type of vapor retarder on the interior side of your wall stackup with a vapor permeance lower than standard latex paint. (This means no vinyl or foil wallpaper, only latex paint.)

    Make the sheathing air-tight to the framing with caulking or can-foam in every stud bay, seal the seam between doubled-up top plates of the studwall with a bead of caulk (or caulk + tape), and where the bottom plate meets the subfloor, etc.

    You don't need a full mesh layer for the rainscreen gap. Affix the foam to the wall with 1x4 furring onto which the metal lath or whatever for the stucco hangs, through-screwed into the studs 1.5" with pancake-head (not pan-head) timber screws 24" o.c., and cut strips of the roll-mesh used for ridge venting on roofs at the top & bottom of each cavity to keep them from becoming critter-condos. It works fine, and provides a generous 3/4" rainscreen for the stucco to dry into. Don't skimp be generous with the ventilation entrance & exit area, both top & bottom.

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