Western Montana cabin and home
My wife and I are planning to buy a property in Western Montana, climate zone 6B, in the next few years. Our plan is to build a small cabin to use as a vacation property and then after I retire from the military we will build a large home and live there full time. We originally were looking at log cabins and homes because of the rustic look, but then I realized just how inefficient they are. Efficiency and cost are the main concerns and we would be going for the Pretty Good house concept over trying to earn any certifications. I figure that costs would mostly be controlled by have functional, but not overly complicated floor plans (or intricate roofing lines) built on slab on grade.
With regard to efficiency, I have been reading a lot on this site trying to get a lot of the basics down. For that area R40 walls seem to be the recommendation. I see that there are a lot of ways to accomplish this; double stud wall and other variation of deep walls like trusses with dense cellulose, 2×6 with 4 inches of exterior foam, PERSIST with 2×4 walls and 8 inches of exterior XPS.
Of the various deep wall designs I think I like the double stud the most. Sheathed in zip system would make the air barrier on the outside which would then be covered by the rain screen and siding. Is there a greater concern for rot on the zip sheathing because it is on the cold side?
2×6 sheathed in zip system puts half the R value inside the walls and outside the walls. It seems like the concern with this system is that the outside foam needs to be thick enough to keep the sheathing from reaching the dew point. However, is that sill a valid concern if zip is used instead of traditional OSB or plywood?
PERSIST with no insulation inside a zip system sheathing would keep all the wood pretty warm. The biggest issue with this seems more to be associated with connecting cladding and 1×4 strips to the studs through 8 inches of foam.
Which would you do and why? What if you wanted to go with a timber frame, how would that change your position, if at all?
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Replies
Keith,
Q. "I like the double stud the most. Sheathed in Zip System would make the air barrier on the outside which would then be covered by the rain screen and siding. Is there a greater concern for rot on the Zip sheathing because it is on the cold side?"
A. Yes, the colder the sheathing, the more you have to worry about moisture and rot. For more information on this issue, see:
How Risky Is Cold OSB Wall Sheathing?
Monitoring Moisture Levels in Double-Stud Walls
Q. "2x6 sheathed in Zip System puts half the R-value inside the walls and outside the walls. It seems like the concern with this system is that the outside foam needs to be thick enough to keep the sheathing from reaching the dew point. However, is that still a valid concern if Zip is used instead of traditional OSB or plywood?"
A. Yes, moisture can certainly condense on the interior side of cold Zip sheathing. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thanks for the links, I had not seen those yet. I suppose that since Zip System is only treated and fully taped on the outside it doesn't limit the OSB concerns for being cold and collecting moisture on the inside surface of a double stud wall. Taped plywood for the air barrier and adding a WRB wrap might be better than Zip for a double stud wall.
With regard to the 2x6 walls with exterior foam it looks like 4 inches of XPS would meet the requirement for climate 6 (R11.25 requirement and 4 inches of XPS = R20) and still obtain a total of R40 for the wall.
"Taped plywood for the air barrier and adding a WRB wrap might be better than Zip for a double stud wall."
Yes. For even better moisture permeability, you can also use fiberboard sheathing, simple pine board sheathing (which can be cheap if you have a local source for rough sawn boards), or even skip the exterior sheathing and use diagonal bracing for the structural function and housewrap to hold the insulation in.
But in your climate, plywood would surely be fine because it would dry well in the spring even if it got a little damp in the winter. So the alternatives are just additional options, not solutions to a problem you would have.
The options with a lot of foam will have higher material cost, so I don't see a reason to prefer them.
I've learned the hard way this winter that adding a separate wrb creates an on going problem. Unless you can install it and immediately cover it with a rain screen and siding, you may need to deal with wind blowing it off, requiring reattachment, often poking holes in the sheathing in the process.
Although my gc taped the sheathing religiously, our primary air barrier is a membrane on the outside of the inner stud wall, where it is safe from most accidental penetration. Next week's blower door test will let us know how well it works.