Cold-Climate Wall Assemblies
If there is a thread out for this then I apologize, but I wanted to ask about “pretty good” wall assemblies in 2022.
I am a carpenter who is building a 900 square foot addition at his own home as we as helping customer make more energy focused choices while not going deep into the passive house level.
Here is what I am seeing folks talk about:
1. 2″ Zip-R sheathing with 2×6 insulated wall cavities. How does the 2′” of the foam on the inside of the sheathing cope with any condensation.
2. Exterior foam or Rockwool board with 2×4 or 2×6 interior with wall cavities insulated. This is a great option but in climate zone 6 and colder requires a fair bit of foam to keep the sheathing warm.
3. Double stud walls of varying thicknesses.
4. Flash and batt. 1″ of foam on the inside of the sheathing and then fill the wall cavities with dense pack or batts.
5. T-Studs?
What are some other options that both make these days?
Any input would be great!
Thanks
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There are so many assemblies these days, but I like either a standard stud wall with continuous exterior insulation, or a double stud wall. I have not worked with ZIP, but it seems like a sound system.
Either of the above options will be fairly easy to construct and with a rain screen detail added, be very high performing. Being detail oriented with air sealing will create a good end product, having a plan on how your air barrier will be continuous over the entire exterior of the build is key.
Design decisions that keep the building shape simple are often overlooked, but are very important for a good envelope.
I used 2 slightly different methods on an addition and then a renovation in zone 5. Both used 2 layers of 1-1/2" XPS foam board over sheathing with strapping and rain screen siding. The addition used regular OSB with a peel and stick membrane. The renovation used zip. I would go either way depending on materials costs. I might even use regular OSB and tape the seams. On the addition, I lined up the outside of sheathing with the foundation and carried the XPS down to footing. The house is very comfortable and efficient.
The last batch of XPS I bought claims it is much greener as it uses better blowing agents.
Climate zone 6 here. I’ve beat this topic to death for my project. When factoring in constructibility, cost, and performance, I’ve decided on Sheetrock, 2x6 frame w/ blown fiberglass or cellulose, zip, 2.5” unfaced EPS, rain screen. Zip R sheathing that thick is expensive and has lower shear ratings, low enough my engineer has a problem with it. Rock wool is expensive and hard to source. Polyiso has the same R value in the cold as EPS, and unfaced EPS is somewhat vapor permeable. 2.5” is R12.5 in cold temps which negates the use of vapor barrier on the inside. 2.5” with rain screen calls for 5-5.5” screws which are relatively inexpensive. Any longer than that and screws get pretty spendy.
What kind of screws did you use? And what material did you use for your rainscreen?
What zone are you in? Zone 6?
With any of these, there's a learning curve, and it gets easier to do efficiently and effectively with experience. My preference is double stud walls, because the cost can get reasonably low, competitive with the others, with an experienced crew. And because the embodied carbon is low or maybe even negative. Don't underestimate the need for an experienced insulation crew dedicated to a high-quality installation.
If you're talking about the Pretty Good House approach, that involves more than just performance--it includes the up-front carbon emissions of the materials we use, and foam above grade is almost never necessary or appropriate in a PGH.
My personal favorite wall is insulated with cellulose in the framing cavity and uses rigid wood fiber on the exterior. That is currently a bit more expensive than other options, so I usually go with my second-favorite, double-studs with cellulose.
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This sounds like exactly what I am hoping to build especially with TimberHP releasing their fiber board Q1 2024 (this year). This would be in climate Zone 5. From my discussions with others in this forum there seems to be a decent amount of push back on exterior insulation due to the added complexity but I honestly don't know if I see that. Sure, it is different but from what I've read and seen it's not that difficult.
I am building an addition for myself right now, and I am going with a double stud wall. Reasons: Flushing up the exterior finish from the old to the new will be easier without exterior insulation, and i got a good price on a lift of 8' 2x4, so I have lots of cheap lumber for studs.
An assembly I've used is 2x8 @ 24" O.C., which gets you a whole-wall R-value of around 25.
In climate zone 4, this gets you to "pretty good" according to p.86 of the Pretty Good House book (and the Building America/BSC study the table draws from).
So if you're on the zone 4 end of "cold" like I am, it may be worth comparing a 2x8 wall vs a 2x6+ci wall to see what makes the most sense for a project.
Around me code (zone 5 and 6) is 2x6 + R5 rigid with interior vapor retarder. This is a relatively easy wall to build, some siding can be directly nailed up over the foam or if you want a rains screen (which is a good idea for any painted siding), the strapping can be nailed up with a framing nailer. No need for long screws plus the labour to install it.
When you run the math, there really isn't an ROI on going much above that.
Brandon's suggestion of a 2x8 24" OC wall which has a similar assembly R value is also a great option.
I've been reading the DOE report from 2013 (https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/moisture_management_high_rwalls.pdf) and it looks like all the thick assemblies with fill only (or open cell spray foam) resulted in higher wall assembly moisture content in colder climates. It appears that the wall with exterior foam (no sheathing) or a layer of closed cell spray foam exterior to batt fill performed better. I would assume that the exterior foam model would perform similarly to Zip-R while allowing any moisture that gets into the sheathing to dry better (assuming a rain screen was installed). What I'm taking away from this report is that regardless of the wall assembly; a layer of closed cell foam or XPS exterior to any interior batt fill is important to mitigate moisture on the sheathing. I was considering a 2x6 wall with Rockwool, Zip-R6, and possibly a 2" layer of exterior Rockwool (due to its vapor open properties). I'm not sure if I'm interpreting this report correctly.
It also appears that if rigid foam is to be used exterior to any sheathing that 2/3 of the R value must be located to the exterior of the sheathing to be effective in preventing the sheathing from hitting the dew point (based on what I have read from the Cold Climate Research Center Reports).