Air leakage to the exterior does fairly little for drying capacity, but it can draw HUGE amounts of air-transported moisture into the stud cavity from a less-than-air-tight interior side. On the 1" iso it's 1000x better if it's foil side out, seams taped to make it as air-tight as possible. If you can mastic-seal the OSB and caulk/foam very edge and hole, as well as caulking the studs & plates to the inside of the OSB with acoustic sealant or 1-part foam, making it your primary air barrier that's good too. All sides of each stud bay need to be air-sealed, and using the foil facer of the iso as a secondary air barrier is worth it/ since the foil facer is still a vapor barrier you would have to establish a cavity vented to the outdoors between the iso and OSB to get appreciable drying around it, but a vented cavity would defeat it's insulating function.
If you don't put a better air-barrier than t & g planking on the interior side of the wall you're screwed, since there will be very substantial convection occurring between the interior air and the air in the cavity. A smart vapor-retarder such as MemBrain really works, but it's not super-cheap. Another approach would be to use a semi-permeable (not perforated) 1/4"-1/2" XPS fan-fold siding underlayment, taping all seams with housewrap tape and sealing all edges and electrical boxes (including the wiring side with can-foam. But read the specs- you need a version that is under 1.5 perms, preferably 0.8-1 perm. (The
Pactive 3/8" goods would work, but not the 1/4", the
half inch fan-fold Owens Corning stuff would work too.
To reduce convective transport of moisture into the cavities you need to go with something more air-retardent than low-density fiberglass- R19 batts are the 3-legged mangy dog of the wall-insulation world- it's a fluffed-out R13- exactly the same weight per square foot as an R13 batt, which makes it 5x as air permeable, it's as crummy as it gets for 2x6 framing insulation by almost any measure. The better cavity-fill alternatives (in descending order of max-air-retardency to least ) are:
Open cell spray foam (which would run ~$2.00-$2.25 per square foot, installed)
Dense-packed cellulose (similar, cost o.c.foam, if blown in mesh)
Dense packed fiberglass (1.8lbs density Optima, L77, or Spider- more expensive than o.c. foam, but higher-R)
Damp sprayed cellulose (usually cheaper than o.c. foam)
R23 rock wool (Roxul is available at
Lowes and
Home Depot these days, other vendors are usually from commercial distributors
R21 "cathedral ceiling" fiberglass batts (any vendor, but definitely NOT low-density R22 or R23s).
The air retardency factor slows both exfiltration transport of interior moisture into the cavity, as well as convection. With open cell foam it's essentially blocked. Closed cell foam works for the exfiltration part too, but going with 5" would be ungodly expensive at $5/foot, whereas a flash-inch (at a buck a square foot) + kraft-faced R19s compressed into the remaining 4.5" would be sufficiently protective, since an inch of foam puts a ~1 perm vapor retarder between the interior and OSB, and puts more R outside the fiber layer, dramatically reducing the number of hours that the exterior of the fiberglass is below the dew point of the conditioned space air. Moisture will still get in past the kraft-facer on the batt, but the kraft facer behaves somewhat like a smart vapor retarder- it's ~0.4 perms when dry, but can be as high as 2-3 perms when it's 70% RH on the cavity side (a moisture point when mold takes off like crazy), which allows it to dry more quickly.
At only 1" closed cell foam still has 20x the drying capacity of poly sheeting. At 2" it still has 10x, so unlike polyethene and foil, it's not a true vapor BARRIER, but a vapor retarder. The same is true of the XPS fan-fold stuff I recommended above. Latex paint on gypsum has about 5x the vapor permeance of 1" closed cell foam, but that's too vapor-open for your stackup and climate (but latex paint would work just fine with that stackup in the more temperate US climate zone 4.)
The reason there is mold either with or without the plastic I tried to explain in the previous post but I'll boil it down to two sentences:
1> In the no-plastic case, in YOUR climate, with only 1" of exterior iso the OSB sheathing stays below the dew point of the conditioned space air for way too many hours over the course of a winter, loading up with moisture to levels that can't dry quickly enough in spring to avoid mold growth.
2> Where there is plastic on the interior, the wintertime absorption rates are much slower, but the drying rates are now YEARS long, rather than months, so it gets moldy too- it's a moisture-trap.
Dealing with moisture issues in order of importance:
1> Getting the flashing on windows/doors lapped correctly to the housewrap is of absolute primary importance, 10x more important than any of the rest.
2> An order of magnitude below that, but still critical, air sealing all layers of the wall, all 6 sides of the stud bay, including the interior side, and making the insulation layer air-retardent, to limit air transported moisture from accumulating in cold OSB.
3>Also important, guaranteeing that the OSB has a path to drying via vapor diffusion, but not so vapor open as to load up excessively over the winter months. How vapor open you can keep the interior side without loading up the OSB in winter is a function of the local climate, and the ratio of exterior foam-R and cavity-fill R.
A lot of mis-placed focus in the 1980s was on limiting vapor diffusion of moisture rather than air-sealing, with rampant mis-use of polyethylene vapor barriers that made the very mold issues they were intended to prevent even worse. Air sealing the cavities from the interior side is FAR more important, but requires
a lot of detail work to get right, but it's do-able. A square-nch of air leak into cheapy-R19-insulated cavities moves more moisture to the sheathing than a whole wall's worth of latex-painted wallboard. Wallboard is much easier to air-seal than t & g planking, which is why you really need a smart-membrane type vapor retarder that is semi-permeable to make your wall work. (That's true even if you bite the bullet and go with open cell foam, since 5.5" of foam is still over 10 perms which would load the OSB in your climate, whereas MemBrain is ~0.8-1 perm in winter, and only becomes vapor open when the OSB releases it's moisture during warmer weather. It loads up slow, dries fast- fast enough that mold can't get going very easily.)