Hello,

My fiance and I are planning on building a house, and we would like an energy efficient design. This house would be in Calgary Alberta, Canada (cold, mostly dry climate)

From what I have read, one of the leading methods is double stud walls with either dense pack cellulose or mineral wool batt (Roxul)

I was wondering if I could ask opinions on the following double stud arrangement (from inside to outside):

- 2x4 16" oc interior wall with poly vapor barrier behind drywall
- 4" gap
- 2x6 16" oc exterior wall (not staggered)
- plywood sheathing
- WRB (Tyvec housewrap?)
- 1.5" of Roxul comfortboard
- ventilated rainscreen gap
- WRB (Tyvec housewrap?)
- Lathe and stucco

From what I've read, a possible issue with the double stud wall is condensation on the cold sheathing, and that drying to the outside is preferable.

Do you think that this arrangement makes sense? Could I expect issues with rot on the exterior sheathing?

Thanks in advance!
Garrett.

What is the reason for going with 2x6 exterior studs if you are going with a double stud wall?

primarily two reasons:
1. code or structural issues
2. "we always do it this way"

Hello,

I guess partly #2. I was planning on the outer wall being the load bearing wall, and the inner being a non load bearing wall.

The house would be two stories, with stucco (kind of heavy) and 9' walls, so I was just thinking that the extra cost wouldn't be a big deal in the grand scheme of things, and that it would definitely pass inspection no questions asked. I guess the bigger issue (which isn't that big an issue to me) is the 2" of floor area you lose.

Other than the 2x6 exterior wall, do you see any problems with this arrangement with respect to condensation on the exterior sheathing?

Thanks in advance!
Garrett.

With a 13" wall cavity to fill with DP cellulose or something else good, you are already looking at upwards of an R40 whole wall level of insulation. I wonder about how useful that exterior 1.5" of Roxul is. As for the "cold sheathing" problem, I've never really understood that concern. With a well-insulated wall construction that has no significant level of insulation outside of the sheathing, that sheathing is going to be "close" to the exterior air temperature anyway, and going superinsulated vs code doesn't significantly change that temperature profile, only the overall rate of heat loss. If the interior is reasonably well isolated from the wall cavity, so that the cavity is not open to interior air, then moisture that can be adsorbed by the sheathing comes mainly from the outside air, and wood exposed to outside air temperature routinely adsorbs and releases moisture as temperatures cycle, with no problem. Plywood certainly is the sheathing of choice, vs OSB. It's permeability goes up with moisture content more so than with OSB, and that rainscreen gap provides a lot of exterior drying potential when outside air temperatures moderate. I wouldn't worry about sheathing moisture issues with what you have proposed.

That's great. Thanks very much for you input. So we will not install the exterior Roxul.

Thanks again!
Garrett.

Garrett - what do you feel the price of your stackup is not including drywall and stucco?

Using pricing from my local home depot in Calgary, I figure per foot for a 9' wall approximately $40. This assumes we do all the work ourselves, and insulate with Roxul comfortbatt through the entire thickness of the wall.