Sheatless double stud detail?
Greetings,
This is my first question while I’ll been a reading of this blog for some years now. I live in the Ottawa, Ontario region which is in zone 6 and I’m in the process of designing a house with the pretty good house approach in mind. I prefer simple construction methods and details that would allow me to take on as much as I can while what ends up being contracted out is something easily buildable that any contractor should be familiar with.
This brings me to the good ol double stud wall. We here to this day still have a passionate discourse about everything from dew point, permeance, OSB sheathing getting wet and the importance of attention to detail for the integrity and performance of your dense-pack cellulose wall system. There is builder in Quebec called Rocket Construction who uses no structural sheathing much like the guys at 475 High Performance have shown in this project: https://foursevenfive.ca/blog/project-spotlight-three-tree-builds-high-performance-house-without-sheathing/
And here are the details from Rocket Contruction:
https://a0101aa3-8c16-4faf-8079-2dc1531fecc4.filesusr.com/ugd/cf4fee_cfbfd446af554da1b72711c9d6673488.pdf
What do you guys think of such simplified systems?
William
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Replies
Hi William,
I'm also from Ottawa, Canada and will be starting my house build this summer using a double stud wall with exterior rigid insulation. I have decided to use BP Excel Extreme Sheathing because it has been used by a local architect with success and it can also be a air barrier if tapes.
In the above linked wall assembly, they use SONOclimat ECO4 (similar to BP Excel Extreme Sheathing) which is also a structural sheathing but also has insulating properties of R-4 per inch.
As for 475 High Performance method, they forewent the sheathing and simply used diagonal bracing for the walls and roof.
SONOclimat ECO4 specs:
https://eco-building.ca/wp-content/uploads/2019/07/T12670-106_SONOclimat_ECO4_En_08-14636585404223102286.pdf
BP Excel Extreme Sheathing specs:
https://bpcan.com/wp-content/uploads/2018/04/tds-excel-18-04-2018.pdf
Arnold
William, Rocket Construction's assembly is excellent for cold climates--the interior Intello prevents most moisture from getting into the wall, and the Sonoclimate Eco4 is not only insulating, as Arnold mentions, but it's vapor-open at 26 perms.
I asked my friends at Rocket Construction their thoughts and they said that the BP Excel Extreme sheathing is potentially an option but it's only 3.5 perms and could result in moisture accumulation in late winter and early spring.
Michael,
I'm curious about the reason why a fiberboard like the Eco4 might not have a condensation problem like OSB or plywood. Is it because it is less dense? Water molecules can go through without accumulating on the fiber?
thanks
Thierry
Thierry,
The Eco4 has a lower risk of condensation because it is far more vapor permeable than OSB or plywood. Importantly, at 26 perms, it is also far more permeable than the interior Intello smart vapor retarder at 13 perms. The Intello acts as the control layer. If moisture passes through it more slowly than it can pass through the Eco4 and Mento, then there is little chance of condensation. This is an excellent wall design.
Peter, one correction--Intello is 13 perms when the relative humidity is high, but in normal, dry conditions it's only 0.13 perms, so it blocks most moisture from getting into the assembly.
I've known about Sonoclimate Eco4 sheathing for a while and spec double stud walls regularly; for some reason I've never thought of combining the two. Hmmm...
Michael,
Thanks for that clarification. I used the perm rating from the OP's detail and failed to point out the whole point of this material - that it is a variable vapor retarder. I do like the idea of relatively permeable wood fiber insulation/sheathing products. I wish more were available in the US market.
Hi William and Arnold
I'm also from Ottawa and planning to start building this year in the Gatineau hills. I'm familiar with the wall that Rocket Construction is doing. I'm also looking at a double-stud walls and would like to use the Eco4 panels though I'm not sure about its availability. Additional bracing like Simpson WB Strong Tie may be needed since the Eco4 is not as strong as plywood.
I'm planning to go with rockwool instead of cellulose since I'll be doing the insulation it myself. I'm still not entirely convinced that using the pricey Intello + membrane is necessary in my case. I might go with poly or even foil-faced foamboards as recommended by Quebec's Novoclimat 2.0 guidelines. That being said our climate is changing fast, winters are warmer and summers are hotter, so we might need to adopt building techniques of warmer climates faster than we would like. I might decide to bite the bullet and go with Intello+, to be on the safe side and to allow some drying to the inside during the warmer months.
Arnold, will you have an interior vapor barrier ? Using a low perm board outside might trap moisture inside your wall.
Thierry
Thierry, Eco4's structural ratings are comparable to OSB; unless you're in a very high-wind zone or a seismically active zone, I don't think you'll need additional bracing.
Thierry,
"Will you have an interior vapour barrier? "
Canadian building codes don't offer a choice. There is some debate as to whether variable perm vapour-retarders meet the requirements. On the face of the way the section is worded they don't.
Malcolm,
Yes good point. In that case Arnold is at risk of creating a moisture sandwich.... as people from Rocket Construction said
I'm not sure what's the lowest vapor permeance that is safe on the cold side in our climate when a vapor barrier or retarder is present on the warm side.
For my wall I'm choosing layers with an increasing vapor permeance, from inside to outside, pretty much like the system described above.
Thierry
Thierry,
As I recall from another discussion Arnold is proposing to use mineral wool as his exterior insulation. He should be good.
Your approach sounds smart to me.
Here is a blog for a double stud wall build in NY. Some nice detailing for the slab on grade, rafters, exterior sheathing and windows.
https://underthesunblog.weebly.com/