Taping Polyiso sheets over zip?
Hello GBA,
Wrapping up detailing on a house build in climate zone 4C. Our wall assembly previously called out 2″ of cavity rock black R-8 for an open joint vapor open rainscreen but we’ve since moved to 1.5″ R-9 of polyiso installed in 2 layers for budget and more importantly supply reasons. The sheathing on this project will be zip over 2×6 @16 o.c. w/ r-21 batt.
Couple questions, first if I’m treating the zip as the primary air barrier do I need to / should I be taping the polyiso seams? I guess that will be my vapor barrier now and I will need to avoid anything impeding vapor from the inside.
Second I had the issue of seeing behind the siding sorted out with the cavity rock and now I’m needing to revisit that. Since I don’t need another wrb and its more for looks, would I be completely off base following a JLC article written by Steve Baczek https://www.jlconline.com/how-to/exteriors/installing-effective-rainscreens_o and just using felt paper over the poly?
Thanks for the read
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Replies
You don’t have to tape the seams in the polyiso but it wouldn’t hurt and it would act as a secondary WRB. There is nothing between the polyiso and WRB facer of the zip you have to worry about trapping moisture in.
You can cover the polyiso to make it appear black but keep in mind you’ll give up the radiant barrier benefits by doing that.
argylest,
A classic rain-screen has primary (the cladding and flashing), and secondary (the WRB and sealants) layers of protection against water intrusion. With open-cladding the primary layer of protection moves back to the WRB, so I'm not sure it's a good idea to take the usual advice around using foam for your WRB as applying to that situation. I would detail it much as you would a cladding, and include a drying path - something like a dimpled WRB - behind it.
There's a great video that just came out from the Unbuildit Podcast on Youtube about open joint claddings, featuring Architect Steve Baczek, Peter Yost and Jake Bruton.
https://www.youtube.com/watch?v=3zS5NTlzcRY&t=258s