Hi, Working on a house thats started already, Didn't stumble across this site and wufi software till the other day. the climate zone is: zone8. winnipeg, manitoba the wall assembly from the outside in is: acrylic stucco, 2 coat portland scratch coat 1/2" thick, 1 layer of typar, 2 layers of 15lb building felt, 1.5" of fiberglass faced polyiso. 1 layer of typar 1/2" plywood 2x6 wall cavity filled with Roxul batts (r22) the flashing on the house is done with peel and stick and metal which i dont think will ever fail. kinda worried about the insulation balance after reading this forum. would using the membrane product on the inside solve the wall assembly not working in wufi ? thanks

OK the short state of the current condition is you have about...

...R9-R10 of iso on the outside of the sheathing with R22 center cavity on the interior...

...which is insufficient exterior-R for dew point control at the sheathing and...

...effectively zero potential for drying to the exterior.

The mean January temp in Winnepeg is about -15C/+5F (see: http://weatherspark.com/#!dashboard;ws=28187 ) Assuming an interior RH of 35% @ 20C you'd have a dew point of about 4C. As long as the temperature of the sheathing averaged over the winter months was 4C or greater you'd have no issue, but let's see how far off you really are. The full center-cavity R is the R9 foam + R22 rock wool for R31. The difference beween the January mean temp and the room temp is 20C - -15C= 35C, so in the crudest simplest model you're looking at about 35/31= 1.13C per R. So with the sheathing R22 away from the 20C interior would be (1.13C x R22=) ~25 cooler than the room temp or (20C - 25C=) -5C, pretty chilly, but not a total disaster. By the beginning of March the mean outdoor temp is about -8C, warm enough the moisture accumulation will be nearly stopped, and the drying can begin.

At -8C you have 28C delta on R31 for 0.9C per R, and the sheathing is averaging 20C - [R22 x 0.9]= 0C, which is still colder than the +4C dew point. But during the day it'll be above the 4C dew point on most days, so a variable-permeance vapor retarder like Certanteed MemBrain would start purging moisture, even then, becoming quite vapor-open during the warmer part of the day when the sheathing is releasing moisture into the cavity, raising the relative humidity (RH) in the cavity, then dropping back below 1 perm during the colder part of the day when the sheathing re-adsorbs the cavity moisture. By the first day of spring the mean oustdoor temps are running warm enough that the sheathing is averaging about +4C, but the drying rate & re-adsorption rates are not balanced- it's realeasing moisture faster than it's re-adsorbing. The ultimate drying rate will be limited by your choice of interior finishes- standard latex runs ~3-5 perms, which means during those transition days/weeks where the sheathing averages +4C, it can release about 4x as much moisture during the day as it takes back at night.

It's critical to make the MemBrain as air-tight as possible to the interior, since a square cm of air leakage around an electrical box can exceed the whole wall's worth of vapor diffused moisture through MemBrain during the <1 perm cold-weather conditions.

While this quick & dirty analysis isn't as precise as a WUFI simulation, it's good enough to tell that it will work. If you had but an inch of iso on the exterior you'd be pretty screwed, and would have to either flash'n'batt the cavities with an inch of closed cell spray foam on the interior of the sheathing (which has other limitations/caveats), or go with less cavity insulation to regain dew point control.

Things that can screw this up are chronic excessive interior moisture from insufficient ventilation, excessive unvented cooking/bathing moisture, etc. MemBrain doesn't really care which side has the high RH- it'll happily go vapor open if you keep it at 50% RH inside all winter with humidifiers, and you're sheathing would hit saturation levels, maybe even get frost-damaged before the thaw. Keeping it at 30-35% RH would be easy to do via ventilation rates in Winnepeg's climate, and a heat recovery ventilator (HRV) under dehumidistat control would be nearly ideal for keeping both humans & house happy.

a square cm of air leakage around an electrical box can exceed the whole wall's worth of vapor diffused moisture

Air movement is 98% of the problem and it takes two things for air to move outward. A hole AND positive pressure. So yes, try to prevent the holes, but you won't be completely successful. So also prevent the pressure. In this case, that means making sure that the interior pressure stays slightly below the exterior when it's below freezing. This will pull drying air through the wall.

What will code allow you to use on the interior side?

The national Canadian building code is kinda squishy, requiring vapor barriers "...unless it can be shown..." that they're not needed, with a lot left up to the local inspectors, who too often presume that 6mil poly is a requirement. Most reasonable inspectors (and yes, the DO exist) would accept IRC prescriptions for exterior insulation as dew point control for comparable climates, but that ship has already sailed here. The definition of "vapor barrier" per Canadian nat'l code is an aged permeance of < 60 ng/Pa-s-m2 (a hair over 1 US perm, which is ~57 ng/Pa-s-m2 ), measured using ASTM E96 dry cup method. Anything between 0.1 and 1.0 perms is t is the definition of a (US) Class-II vapor retarder and thus deemed "legal" in lieu of poly by most inspectors. MemBrain SHOULD apply, even though it varies between Class-II and Class-III vapor retardency depending on the RH of the proximate air masses, since it meets spec when it matters (when the sheathing is cold, and the air is dry).

See:

http://www.nrc-cnrc.gc.ca/ccmc/registry/pdf/07%2026%2010_e.pdf

http://www.certainteed.com/resources/30-28-080.pdf

http://www.certainteed.com/resources/MemBrain_TB__1_-_Water_Vapor_Permeance.pdf

See also:

http://www.cufca.ca/research/The_Need_for_Vapour_Retarders_in_Above-Grade_Residential_Walls.pdf

I have no insight into whether there are local code restrictions that would interfere with using MemBrain in Winnepeg.

inspectors are pretty clueless I doubt they would even mention the membrain as long as they see a plastic on the walls.

I will use normal vapour barrier on the ceiling areas and just the membrane on the walls