Closed cell on inside of Zip-R: Vapor sandwich?
I’ve seen this a couple of times now, where a flash of closed-cell is sprayed on the interior of the zip-R foam.
How risky is this assembly? I understand that there’s no “real” vapor sandwich because the foam is directly on foam, but to me there seems like risk of moisture accumulating on the outer side of the studs with limited ability to dry due to foam on 3 sides of it.
A builder has recommended this for my own upcoming home. Initially I dismissed it, but then I saw experienced high performance designers spec’ing this (I believe Steve Baczek on Build show Boston).
What are your thoughts on this assembly- is there a big risk of moisture?
I’m in MA Zone 5. My builder suggested Zip-R6 (not 9 due to budget), a flash of closed cell, then dense packed cellulose.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Spray foam is something you design out in a new build. In case of something like ZIP assembly it doesn't add to air sealing (the taped ZIP is already a great air barrer) and doesn't change the assembly R value that much.
About the only thing it saves is you installing a warm side vapor retarder. If you are looking at cellulose, you'll already be installing something to to contain the insulation. The upcharge to a variable perm membrane (ie Intello plus) is noise and comes with none of the risk of a bad spray foam install.
2x6walls with R6CI and warm side vapor retarder are perfectly fine in zone 5/6.
Polyiso is slightly vapor open; in fact it's more vapor-open than the OSB (when the OSB is dry), so if the studs need to dry to the exterior, they can. If your foam-to-fluffy ratio is good, I don't see this as a risky assembly. Like Akos, I also don't see it as a resource-efficient assembly.
Great, thank you so much Akos and Mike!