Is there a smart way around minimum exterior insulation thickness for Climate6B?
I am in need of your help. Contractors in my area are very shy about using enough exterior insulation to meet the recommended minimum of R11.25 and incorporating rainscreen. They all share the concern of the wall being too thick and not wanting to do something they are unfamiliar with. This has led us to being left with the option of a traditional cavity insulation only wall assembly. It would look like: drywall – vapor barrier (6 mill poly or smart membrane) – 2 by 6 studs with R23 or 24 cavity insulation (fiberglass)- sheathing- WRB/air infiltration barrier- 1 by vented rainscreen – wood shiplap siding. Although this beats our code requirement of R21 this is a disappointing wall for our new home. Which brings me to my question- Is there a safe and builder friendly way to add exterior insulation (Rigid foam board on outside of sheathing or Zip R) that is less than R11.25 which also performs well in Climate Zone 6B?
I have read They All Laughed …. by Joseph W. Lstiburek and Rethinking the Rules on Minimum Foam Thickness by Martin Holladay, but I am have been confused with all the conflicting opinions out there. I appreciate all of your insights.
Nick
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Replies
This type of wall has been built around me for a long time and works just fine (Montreal is in Zone 6).
https://effectiver.ca/calculator/wall.php?id=4029
https://effectiver.ca/calculator/wall.php?id=4199
For a more robust assembly go for faced batts or a smart vapor retarder.
It is also better to spec a somewhat permeable foam (ie unfaced EPS/GPS/XPS) and not tape the seams of the foam. This allows for a bit of extra drying to the outside which is always a good thing.
Thanks Akos! If the exterior foam is not taped what would the air barrier be?
In the list of components of these two assemblies I see an exterior air film and an interior air film component. What is that? I am not familiar with that terminology.
Also, they use an asphalt impregnated paper for their WRB. I assume a Tyvek or similar standard house wrap would be OK to substitute?
The air barrier should be taped sheathing. This puts the air barrier behind the foam and protected from the elements.
Sometimes people tape the seams of the foam as well as a backup water and air barrier but it is not needed and in this case it reduces drying capacity. A better use of your time is to detail the interior poly/membrane or drywall as air tight as the secondary air barrier.
The stagnant air near a surface acts as a small amount of insulation. The air film R value is a simple way of quantifying this effect and it is included whenever you do a full assembly R value calculation. When doing U factor based wall calculations, all codes use similar values for the air films.
House wrap or asphalt paper are pretty much the same thing. Since the WRB is behind the foam, the permeability of it doesn't matter much anyways. You can search their database for a wall with house wrap, I'm sure it is also in there but it won't change the characteristics of the wall:
https://effectiver.ca/calculator/search-go-to-the-calculator.php
Akos, if you didn't have the external foam and it was just a traditional wall with only cavity insulation should the taped sheathing still be the air barrier?
I am wondering if there is any safe assembly with only ZIP R6 for Climate zone 6b? It seems like Zip products are used widely around here in the smaller thicknesses.
Zip R6 is pretty much the same as the assembly I linked to earlier, just sheathing and rigid is swapped. You build it exactly the same way with a warm side vapor retarder.
With Zip you need to tape the sheathing but the polyiso they use has about the same permeability as unfaced XPS, so overall you get similar exterior drying potential.