Hi,

It seems that the concept of an interior moisture venting is quite new and not well understood by lots, including my draftsman.

I am looking at building a house I will personally live in, and I intend to live in comfort, on reasonable energy efficiency, and of course durability of structure.

The design I am currently looking at would consist of; from outside to inside:

-Siding
-Properly screened air gap
- 2 x Layers of 2" Polyiso with joints sealed and staggered
- Tyvec Drainwrap (adhesive membrane flashing)
- 1/2" Plywood
-5.5" wet blown cellulose in 2 x 6 framing 24" c/c
-Gypsum board
-Class III vapor retardant paint.

Details would be as per the Remote Manual, by the CCRHC. Except on a thicker lumber stucture, which I assume is possible due to my climate being warmer the Alaska testing grounds.

The building would be located in northern New Brunswick, Canada. From climate data, the  would be 5900 HDD (In Celsius)
Would the be around 10500 HDD from the Fahrenheit system?

Data (1970 - 2000) 3 KM from proposed building

The daily minimum average for:
December -13 C
January -17.9 C
February -16.4
March -10.1

Daily Average:

December  -8.6 C
January    -12.8 C
February  - 11.1 C
March      - 5.1 C

My dew point for interior condition of 21°C (70F) and 35% RH @ exterior sheeting would be 5°C.
Which would be reached @ an exterior temperature of -15.4°C assuming 44 % of insulation would be to the inside.

Would anyone experienced in this type of wall be comfortable in building this? Is there enough safety factor to building such a wall?

I think you are missing the Dow Thermax - 1" on the exterior.

Indeed the Dow Thermax be a viable product on the exterior. I would however be installing it 4" thick in two layers.
Polyiso = Dow Thermax

From http://dow-styrofoam.custhelp.com/app/answers/detail/a_id/3310/~/what-is-the-water-vapor-permeance-(perm)-rating-of-various-thermax%E2%84%A2-products%3F
"THERMAX™ Sheathing, THERMAX™ Metal Building Board, THERMAX™ White Finish, THERMAX™ Light Duty, THERMAX™ Heavy Duty and THERMAX™ Heavy Duty Plus have a water vapor permeance of 0.03 perms based on ASTM E96. THERMAX™ (ci) Exterior has a water vapor permeance of <0.03 perms."

Therefore, if you used two sheets of Thermax on the outside, you would have two vapor impermeable layers. If you wanted some drying to the exterior, the glass-fiber faced polyiso has a permeance of a little less than 1 in any thickness, since the permeance is determined by the facing. So one or two sheets of the glass-fiber faced polyiso would give you a vapor semi-impermeable insulation layer that would allow some drying to the outside. Depends on what you are looking for.

Yes, 5900 HDD (in Celcius) = 10620 HDD (in Fahrenheit)

Make sure that the interior side is also tightly air sealed - gaskets, smart membrane, etc.

With 4" of iso and 5.5" of cellulose cavity fill you have ~R23 iso (derated for cold-temp) and ~ R19 cavity fill. Assuming a 99th percentile outside design temp of -25C and an interior temp of 20C that means the sheathing at design temp is right about 0C.

Unless things have changed, Canadian codes require that the sheathing will be above the dew point of the conditioned space air (4-5C) at design temp condition to be able to use only a type-III vapor retarder on the interior. At the average winter temps it has PLENTY of margin with that stackup, and it would be code-legal in any US location with a similar climate, (eg Caribou, Maine: 99th percentile design temp= -23C ). But Canadian code's more stringent hurdle can be tough to clear. While your stackup would not be at high risk for moisture problems, you may be on the hook for a type-II vapor retarder on the interior to be code-legal in New Brunswick. (Fight like hell to avoid interior poly!)

Vapor-barrier latex runs ~0.5 perms, qualifying as a class-II vapor retarder, but a class-III vapor (standard latex paint, 2-5 perms) would give you an order of magnitude better drying capacity, and would be preferred. If you can sell the code-enforcers on a variable-permeance smart-membrane (eg. Certainteed MemBrain) as jonr suggested, it would have far more drying capacity toward the interior should the cavity become damp, enough to more than make up for the relative vapor-tightness of the foil facers. At typical wintertime indoor relative humidities it's a class II vapor retarder (< 1-perm.)

Switching to fiber-faced iso would still be only ~ 0.3-0.4 perms of drying capacity, still an order of magnitude lower than standard latex on the interior, so there is little benefit to going that route unless they insist on 0.5 perm latex (or poly vapor barrier) on the interior.

Dana1-

A single 4" think polyiso layer should give a perm rating of about 0.8 perms or so, would it not? And that would allow more drying to the outside than the 0.015 perm rating for two sheets of the Thermax. From what you have said, it sounds like many of the Canadian code guys want a sheet of poly or some significant vapor barrier on the inside.

In a single 4" layer it'll be somewhere between 0.5-1 perm, at 2 layers (4 facers) it's about half-that. So yes, while a single- layer allows SOME minimal drying rate, but it's marginal if drying to the interior is disallowed. If latex-only is allowed on the interior that 0.8perm exterior means little. Going single-layer and fiber-faced reduces the ease & reliability of air-sealing the iso, as well as about R2 of "effective-R" performance of the foil-facer in a rainscreen gap.

Canadian building codes have some things dead-right (like the requirement for a 10mm rainscreen), but the requiring the sheathing to stay above the dew point of interior air at DESIGN temp to allow a relatively vapor-open interior has no science to back it up. It works, sure, but so does keeping it above the dew point at the winter average temp (or the winter average daily low temp, if one wanted more margin.)

Another thought: At typical framing fractions a 2x6 cellulose wall comes in at ~R14 for a whole wall R, to which R23 (derated) is added for about R37. If one dropped back to 2x4/R13-cellulose on the studwall that portion would deliver ~ R10. Adding 1/2" to the foil faced iso (4.5" total) for ~R26 you'd end up with ~R36 for whole-wall R and at -25C exterior/R20 interior temps and 2/3 of the R on outside the sheathing would be at +5C, making the assembly code compliant for a vapor-open interior finish. This would be nearly identical to the stackup being used on a local deep energy retrofit I've been advising on, where an outer layer of 1" foil faced iso is going over a single inner layer of 3.5" of reclaimed fiber-faced roofing iso. This is over a plank-sheathed balloon-framed full-dimension 2x4 studwall (circa 1905.) The estimated whole-wall R on this one is ~ R40 (not counting the reflective facer's benefit), which is somewhat higher due to lower than average framing fraction and full-dimension 1" pine plank sheathing and another half-inch of cellulose edging up the R. Interior finish will be standard latex. Design temp at that location is only -15C, so there's no question of running into wintertime moisture absorption at the sheathing on this project unless the occupants turn off the ERV and never open the doors or windows. The primary air-barrier in this stackup is lapped-caulked-and-taped Tyvek between the iso and plank sheathing, with 1-part foam sealing the bottom & top edges, but both layers of iso are also detailed as air barriers, since there are subsidy cash incentives for air tightness that ramp up at 3 successively tighter steps. (I'm not managing the project, and don't have full details on the air-tightness goals, but they're pretty aggressive, and I expect it will be tighter than R-2000 spec.)