Wall assembly – build-ability vs performance
We’re in the final stages of design tweaks, etc before building (off grid) in SW Montana next spring (Climate Zone 6). After pouring through all of the GBA and Building Science articles I can get my hands on it seems that the idea of a Perfect Wall isn’t as concise as one might hope. In light of that, and the fact that in our area we don’t have a huge choice in builders, I want to make sure we choose a wall assembly that is heavy on “build-ability.” Here are my two final-ish candidates:
#1:
2X6 wall with 3/4 inch plywood sheathing (advanced framing)
Polywall 2300 WRB rolled or sprayed
6” of Rockwool Comfortboard 80 on the exterior (R24)
Rockwool Comfortbatts between stud bays (5.5” R23)
#2
Same as the first except open cell foam sprayed between stud bays (~R20-ish?)
For the roof I’m pretty much set on traditional vented attic with closed cell foam on the attic floor and then blown fiberglass or cellulose – hoping to hit R60+.
Please feel free to poke holes, suggest other approaches or materials, etc. At our elevation we have extreme snow loads (150psf) as well as wind loads (115mph).
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Replies
Hi Peter.
I can't see anything wrong with either of your proposed assemblies. So this is just food for thought.
If it were my house, I'd be considering dense pack cellulose for the wall cavities for a few reasons including its ability to buffers moisture and because it is one of the more environmentally benign insulators.
I'd also frame the roof in a way to avoid the closed cell spray foam, specifically with raised heel (or energy heel) trusses. And I'd blow cellulose into the attic floor as the insulator. Here, you'd need a ceiling air barrier. More and more high performance builders are turning to OSB or a smart vapor retarder for the ceiling air barrier. OSB is rigid and not easily damaged; they like it for its durability. The smart vapor retarder offers an air barrier and, well, smart vapor control.
What is the siding and ventilated air gap plan?
Siding will be steel siding or reclaimed wood (or a combination of both) over 1X4 strapping.
I'm not sure how easaly buildable either of your walls in an off grid area. 6" comfort board is esentially 24" of batts squished down, so you are paying for R96 of materials but getting only R25, so it definately won't be cheap.
Installing it is also not easy as it will deform from the compression of siding strapping, so you have tweak every screw to get your siding flat. It is also far from "standard" trade type install.
If you want to design foam free, I would say go with either double stud with batts or a Larsen truss type setup.
It is pretty straight forward to say to a framer "build a 24OC 2x6 wall, install the exterior OSB, then outside of that balloon frame another 2x6 wall that runs from foundation to the roof." About the only non standard thing is that you need slightly wider foundation or offseting the walls a bit so both can sit on a wide bottom plate.
A staggered double 2x6 wall with R23 batts + fiberboard exterior sheathing is about the same R value as your proposed assembly.
As Brian mentioned, there are cheaper way of sealing a ceiling than SPF. Air tight drywall with SPF over any top plates works just as well.
Are there any long-term tests of air-tight drywall?
I tend to think that drying lumber and shifting foundations mean that any air-tight drywall will be hopeless in the long term, as drywall pops, cracks etc. But maybe that's bias from experience with expansive clay soils :)
Or maybe the approach doesn't really rely on drywall seams not cracking because it's really relying on the gaskets on the top-plates?
I think air-tight drywall works best as a secondary air-barrier in conjunction with using the sheathing as a primary one. Or use a variable-perm membrane under the drywall. Poly would also work as you won't be air-conditioning.
I agree Malcolm.
It seems like using the sheathing as the primary air barrier and getting all of the important details--mudsills, windows, doors, penetrations, top plate--tight as you build is the best approach for energy efficiency and other reasons. Then to minimize the likelihood of wet wall, help insulation perform as it should, etc. a secondary interior air barrier is worthwhile. I'm in favor of air sealing all six sides of wall cavities.
In your ultimate wall would you add OSB on the interior as well to act as a smart vapor control?
Peter,
It's pretty time consuming and expensive way to get variable vapour-control. And detailing interior OSB as an air-barrier would be a nightmare.
I'm with Brian on some points, Akos on others. There is nothing green about extensive use of spray foam, and a double studwall is both greener and cheaper than rigid rock wool (or rock wool batts) at any performance point, and air sealing can be done reliably with other materials and methods for less money.
Double-studwalls where the structural studs are on the exterior have a pretty short learning curve- most framers can understand how to get it right. The interior course of studs can be 2x4 or even non-structural grades of finger-jointed 2x3s. That's far less complicated than hanging 6" of expensive & heavy rock wool on the exterior.
A double studwall full of cellulose has measurable thermal-mass benefits during the shoulder seasons compared to rock-wool or spray foam solutions too.
I part with Akos on the exterior fiberboard, unless you happen to be in the parts of Canada where MSL's 1.5 " thick SonoClimat ECO4 fiberboard sheathing is available. The typical 1/2" or 3/4" asphalted fiberboard products will be prone to bowing over time if cellulose is used in lieu of rock wool.
https://www.mslfibre.com/Upload/Documentation/T12670-106_SONOclimat_ECO4_En_08-14637050076751289383.pdf
If going with cellulose cavity fill it may be easier to align the studs rather than staggered studs, to be able to staple blowing mesh between stud bays, making it easier to consistently hit the target density.
From a verditude point of view, cellulose is far greener shade than open cell polyurethane or rock wool, since it comes with a NEGATIVE carbon footprint (it becomes sequestered carbon), which matters quite a bit when going for high-R assemblies:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png