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Placement of WRB when it is also your drainage plane

user-6765831 | Posted in Green Building Techniques on

Hi all,
This is Asaf from SF Bay Area Climate zone 3B, just over 20″ of rain yearly here.
After much back and forth with my contractor (who happens to be my father in law- ie I can’t push him beyond a certain point before I put familial relations in danger) we have agreed to use “alternative” approach to drainage plane.  He has never installed furring strips and doesn’t intend to now.  He is open to installing foam with posterior grooves or a housewrap such as Hydrogap to improve drainage of system (will likely combine with grade D paper).  Relevant details include conventional stucco finish, innie windows, 1.5″ of polyiso (though can substitute EPS- easier to find with grooves) that will be taped

Usually, with innie windows I know I should install WRB beneath foam, but given one layer of my WRB will function as a drainage plane, should I do something different?  I would think drainage plane should be just underneath stucco, but Benjamin Odyke gives option to put it behind foam too-seems like a waste if one tapes the foam.  I also wonder if stucco wouldn’t just bond to the housewrap and negate the drainage properties unless grade D paper installed over the Hydrogap.

I am attaching reference materials from Benjamin Odyke. 

Appreciate any assistance.

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  1. GBA Editor
    Martin Holladay | | #1

    You can trust details from Benjamin Obdyke. It is a reputable company.

    There is a very slight degradation in thermal performance when the drainage plane is between the rigid foam and the sheathing -- but the amount of degradation of thermal performance is too small to worry about.

    If you choose to install a drainage gap between the rigid foam and the sheathing, everything will work fine -- just get the details right! Integrate your window flashing with your WRB, and pay meticulous attention to the drainage details. Inspect, inspect, inspect (before the rigid foam is installed).

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