Wall assembly and condensation with Zip vs Zip R
Deciding final wall assembly.
Climate zone 5A.
Aiming for an R35-38 wall.
2 x 6 studs with Rockwool Comfortbatts – R22/23
Option for exterior continuous insulation:
A. 1″7/16 Zip R6 + 2″ Comfortboard 80.
– R value would be 6.6 + 8.4 = 15.
– Total thickness would be 1″7/16 + 2″ + 1″ (furring strip) = 4″7/16. It would require a 6 inch nail/screws.
B. 7/16 Zip R + 3″ Comfortboard 80
– R Value would be 12.6
– Total thickness would be same as above, with same nailing
– Cost maybe higher due to higher cost of Comfortboard.
There is controversy that the polyiso of Zip R is on the wrong side. If I have a Zip R6, where I have R value of 6.6, plus additional R 8.4 Rockwool comfortboard 80, is there still a risk of condensation on the inside surface of the polyiso? I am getting conflicting info. It seems that if you use Zip R9, then the risk of condensation on the inside is minimal. In my assembly where there’s R15 outside of the innermost side of polyiso, should there be any risks?
I’m preferring Comfortboard 80 outside due to fire resistant properties. Could use a 2 inch Halo Exterra GPS board which provides R10 but it’s less fire resistant, and vapor permeability is low for drying outside.
Hoping to get some insight here.
Thanks in advance!!
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Replies
In your climate zone, the IRC model code allows a class 3 interior vapor retarder if you have at least 27% of the total wall R-value on the exterior. That is what we generally use to determine whether a wall is safe from condensation risk.
What almost everyone misunderstands about Zip-R is that the OSB is the least permeable part of the assembly when dry; polyiso is somewhat permeable and its fiber facing is vapor open. The exterior coating on Zip is also vapor open. If the OSB gets damp, its permeance opens up to somewhere around 2 perms to allow drying to the exterior. Higher permeance would be better/safer but it's not a vapor barrier like the foil on most polyiso used on walls.
That said, the risk of condensation on the interior side of the foam layer follows the same ratio rules as any other type of exterior insulation. The more highly you insulate the interior relative to the exterior, the higher the risk of condensation.
Thank you for your reply.
After I posted this question, I found this instagram post from Christine Williamson. The last 3 slides/photos also explain what you have mentioned.
https://www.instagram.com/p/CkA6yyCgw_U/?igsh=b3lnazhqOHkzM2ww
Overall, seems like using Zip R isnt an issue if you have additional exterior insulation over it, and the ratio of R values of exterior/interior/total insulations remains within suggested limits.
I will discuss with my builder. My cavity insulation would be R22-23 ComfortBatt. And exterior would be 12.6 or 15. Overall R value of 35.6 vs 38 won't make much difference.
I would not use zip r at all