Critique my plans! Zone 7, exterior EPS, smart vapour retarder
Hey,
See attached the plans I’ve been working on for our home addition. *They aren’t complete yet*. I’m a contractor, worked for others before, just starting out on my own. I really want to nail the details properly. Climate Zone 7. -40C in the winter, +20C in the summer, generally low humidity year round. House is not air conditioned because we don’t need it here. Addition will be in floor radiant heat.
My main questions are around the wall assembly and vapour retarder. Climate zone 7 so my air barrier is on the inside and also is my vapour retarder. I’m planning to use the Pro Clima Intello Plus varpour retarder, Rockwool insulation, OSB sheathing, 1.5″ EPS on the outside. Will the EPS allow enough drying potential from the inside out or will it trap water vapour in the cavity?
Is there a recommended WRB to use that would be vapour open? I would like to use ZIP or ZIP-R but up here it’s $80/sheet (not including tape) compared to $25/sheet for OSB. ZIP-R starts at $100/sheet. I can a roll of a good quality WRB such as Siga Majvest for $1000. As mentioned, my air barrier is on the inside so having that exterior air barrier doesn’t really help me much. It makes more sense in my mind to use OSB and spend the money on a really good mechanically fasted WRB or self adhered WRB.
Any critiques on my assembly or my way of thinking?
Thanks to all!
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Replies
Your wall design will work in climate zone 7, the EPS (depending on the manufacturer) will be about the same permeance as the OSB. The mechanically attached WRB will also be fine, I would think about taping the seams of the OSB and making that a secondary air barrier. Zip tape or 3M's 8067 All Weather tape would be my choice.
If it's available, I would look for fiberglass-faced polyiso instead of the EPS to get a better R-value per inch. If you can find reclaimed roofing polyiso from a commercial roof tear down job it is usually at a nice discount. I would also suggest trying to go much thicker if you can with the exterior insulation. Self-adhered WRB with a nice primer would be my vote if Zip is not an option.
I know people always talk about the cold weather performance but even in your climate the polyiso will perform better in terms of R-value per inch than EPS. Probably above R-4.5/inch which beats EPS at R-4/inch.
Source: https://buildingscience.com/documents/information-sheets/info-502-temperature-dependent-r-value
A very quick peek at your plans….
Consider a taller heel on the trusses, r50 loose fill cellulose isn’t much in your zone, and the stuff is inexpensive. Shoot for more.
You do not have nearly enough exterior insulation to keep your sheathing warm. Remember, foam insulation greatly restricts (or eliminates) outward drying potential. And you are contemplating OSB, which further complicates the deal.
Extend the rigid insulation to the roof baffles, looks like a weak point at the plates.
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-120-understanding-walls
The R value ratios change when you have an interior vapor retarder. This is pretty close to what the OP is proposing and works in Edmonton which is zone 7.
https://cwc.ca/design-tools/tool/effectiver/calculator/wall.php?id=4339
Use unfaced/permeable rigid insulation which allows for a bit of extra drying capacity to the exterior.
Appreciate all the feedback!
Unfortunately as I increase rigid insulation depth I reduce permeability. 7/16” OSB is about 44 perms. So ideally I want a perm higher than that. PolyIso has a much lower perm than XPS or EPS.
It’s also pretty easy and secure to nail vinyl siding through 1.5” of rigid insulation. Too much rigid and it becomes necessary to run strapping. Vinyl siding by code is also is its own rain screen which is nice. I’m trying to balance cost and performance a bit here.
Appreciate the feedback. Definitely gave me some things to read up on and consider
Nik,
I agree. You either want to use a small amount of permeable foam, or you want to go right up to the recommended thickness where permeability doesn't matter. As Akos said, If you go with the former you need an interior vapour-retarder
"7/16” OSB is about 44 perms." I think that is a bit off. OSB is about 2 perms, or 12 when damp.
Thin exterior insulation spooks me- regardless of vapor retarder. We can't build perfect buildings, moisture will migrate into the assemblies.
The plot is a little thicker here- you aren't going to run AC in the summer. AC will promote inward drying as the indoor air will have a lower moisture content. This system, too, is much happier with mechanical ventilation.
Why don't you consider a different envelope approach? 2x8 walls with 24in OC studs? Or Bonfiglioli walls? As a builder, you're going to have to develop price-point assemblies that'll let you sleep at night. After losing my shirt on a project, I avoid exterior insulation in most cases. It makes sense with some retrofits, otherwise other approaches keep things more wheel-house. That said, your addition is pretty simple- the busier the architecture, the more labor intensive exterior insulation becomes.