Replies

1. Expert Member
   BILL WICHERS | Mar 28, 2023 09:40am | #1

   I don't really think it's necessary to have a drainage plane between the polyiso and the sheathing. There is absolutely no reason for a drainage plane on the front of the polyiso, since the rainscreen gap already serves that purpose.

   If you do want to put something behind the polyiso, I'd use something very minimal like Tyvek Drainwrap.

   Bill

2. Expert Member
   Akos | Mar 28, 2023 03:09pm | #2

   2" polyiso is close enough to the required amount of exterior insulation for condensation control for a 2x6 wall, I would insulate with full size batts.

   I've built pretty much this wall in zone 5 and have never included any drainage behind the iso. Taping the iso doesn't hurt but it is also not really needed if you already have the sheathing taped.

3. 
   steve41 | Mar 29, 2023 08:51am | #3

   Thanks Bill and Akos.

   It sounds like there is some consensus to nix the drainage plan behind the polyiso, which was my thinking as well.

   With that said, I still wonder if I'm missing something, since the Risinger video makes reference to the drainage plane detail coming from Dr. Lstiburek, buildingscience.com. I have not yet found the detail on buildingscience.com.

   To me it only makes sense as a path for condensation drainage. But that path would need to be open to air at the bottom to allow drainage, or would there be a permeable barrier at the base to enable condensation escape.

   Any other perspective on this?

1. Expert Member
   Akos | Mar 29, 2023 10:21am | #4

   I believe it is coming from Joe L's beer screen idea:

   https://buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed

   There it is about diffusion for assemblies with insufficient exterior rigid for condensation control (ie 2x6+R5). You have enough exterior rigid that this is not an issue.

   I remember reading a study that looked at water management for commercial exterior rigid walls with WRB under the rigid. Even under extreme wind driven rain, no water made it to the WRB layer.

1. 
   steve41 | Mar 30, 2023 08:56am | #6

   Thanks for the link Akos. Great article. I have a fair bit of Polyiso to install so I have already started to kick up my beer game :)

   Footnote 6 addresses my concern about the additional gap:
   "I am not allowed to tell you how I know this for reasons I can’t get into…but trust me on this….you need to keep the gap less than 1/8 inch to limit the loss in thermal performance of the exterior continuous insulation below 5 percent….which is less than the loss in thermal performance that occurs due to the thermal bridging associated with attaching the cladding and continuous insulation itself…."

2. Expert Member
   BILL WICHERS | Mar 29, 2023 11:19am | #5

   Lstiburek has sometimes conflicted with himself over time. I chalk that up to learning more as time goes on, since it's often the case with buildings that it takes years to see if an idea really works or not. I do not fault him for that, and I think he is honest about things when he finds out something he thought was a good idea wasn't so great after it had been tested over the years.

   Risinger I think overdoes things sometimes. This is sort of like belt, suspenders, overalls, double underwear, parka, coat, and poncho. Sure, it does give you extra layers of protection, but did you really gain enough to justify all the extra cost? I like that he tries to think through details that many miss, but it's also possible to go overboard with details that don't really gain you much for the costs they impose. Many redundant WRBs is an example of this. If you have one properly installed WRB and flash things properly, you're in pretty good shape.

   Bill

1. 
   steve41 | Mar 30, 2023 08:58am | #7

   Agreed Bill: "....I think overdoes things sometimes. This is sort of like belt, suspenders, overalls, double underwear, parka, coat, and poncho". :)