External wall layers comparison question
External wall layers comparison question: Zip R Insulated sheathing vs. 4″ rigid foam (atop and exterior to) OSB sheathing?
This is going to focus strictly on exterior, above grade wall layering without concern to climate zone or other factors for the moment.
From the beginning of our building project, we have had in mind to use Huber’s Zip- R Insulated Sheathing (comes in 1″ to 2 1/2″ insulated panels, each performing at various R-Values) and Zip tape. (Our project thickness is not yet determine, and I do not to be believe to be relevant to the upcoming questions)
If there is one concept that has been etched in my brain from the past several weeks of studying the topic of building wall layers (above grade) is the value (not just R-value) added when rigid foam is placed to the exterior of the sheathing to keep it warm, and that “Wood-framed walls with an adequate layer of exterior rigid foam do an excellent job of preventing problems associated with vapor drive. The exterior rigid foam keeps the wall sheathing warm and dry during the winter, so moisture can’t accumulate in the sheathing”. Also it was my understand that a WPB/house wrap would encapsulate the foam and OBS layers.
Zip-R insulated sheathing seems to be layered in an opposite manner than mentioned in the previous paragraph, as its rigid foam is located to the interior of the wood sheathing, and the WPB is layered external to that sheathing for a 3-in-1 product. Additionally, Huber’s online technical information lists a frequently asked question: Question: ” Can I install exterior foam insulation on top of R-Sheathing?” Answer: ” No. An additional layer of exterior foam on top of ZIP System R-Sheathing is currently not recommended.”
Q: How does Zip R insulated sheathing compare to OSB > Rigid Foam > WPB, of course assuming the rigid foam of each laying concept is of equal depth?
-or-
Q: Is this product so new we are all just sitting and waiting to see if it preforms as the manufacture expects it to?
I am probably missing something very elementary in understanding this, but it’s starting to keep me awake at night, so I need some insight.
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Replies
Nina,
First of all, I'm not familiar with the abbreviation "WPB." It sounds like you are talking about a water-resistive barrier (WRB).
You're right that Zip R sheathing puts the rigid foam between the OSB and the studs. That's convenient for siding attachment (especially for cedar shingles or vertical boards, neither of which plays nice with vertical furring strips). But it means that the OSB is cold during the winter.
If you are worried about the long-term effects of moisture cycling in OSB (related to the fact that cold OSB gets damp in late winter), then you shouldn't use Zip-R sheathing. Instead, install rigid foam on the exterior side of your wall sheathing.
I don't think that cold OSB is a significant problem, as long as your wall is well detailed, and as long as you remember to install a ventilated rainscreen gap on the exterior side of the OSB.
For more information, see these two articles:
How Risky Is Cold OSB Wall Sheathing?
All About Rainscreens
-- Martin Holladay
Martin, Yes, WPB was a typo; it was late, sorry. I will read those articles; thank you. Nina