Second WRB with Zip and rigid foam?
I’m replacing the siding (wood clapboard) on my house and part of the process involved replacing the old sheathing (1x hemlock 125+ years old). I used Zip OSB for the new sheathing and will be adding 2 inches of rigid foam (XPS) and 1x strapping for a rain screen. My question is, should I place a second WRB, such as 15# felt, on the outside of the foam, under the strapping? I hadn’t planned on it until now, as I was relying on the Zip as the WRB but it seems like it would be nice to have a WRB to keep the foam dry from the outside. I know it’s not necessary to have the second WRB but is it a better approach? Are there downsides, such as making drying to the outside more difficult? Thanks for any thoughts.
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See Martin's article on "Where does the housewrap go."
I think it will help to clarify which approach is optimal for your situation.
You didn't mention your climate zone. I hope you have checked to make sure that your rigid foam layer is thick enough to keep your sheathing above the dew point during the winter. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
Every wall needs a WRB; the WRB is recommended by building scientists and is required by code. If a wall has two layers that are equivalent to WRBs -- an option which is possible -- you have to decide which layer is the "real" WRB, because it's essential that your window flashing and door flashing is integrated with the "real" WRB.
The bottom line: Go ahead and install asphalt felt on the exterior side of the rigid foam if you want. But before you do the work, you need to know whether you intend to integrate your window flashing with the Zip sheathing or the asphalt felt.
Thanks Steve and Martin for the responses.
Sorry, I forgot to mention I'm in Zone 6, southern Vermont, and am working with R-13 walls so I believe my exterior foam is thick enough. And I should have specified I'm using "outie" windows.
I see from Martin's article and your response the WRB can go outside or inside the foam so long as it's integrated to the flashing around openings. I guess since I have the Zip OSB in place, to save some work, I should use that as the WRB and flash the openings following the drawings included in the FH ProHome article:
Even though I'll proceed using the Zip as the WRB, theoretically, is there a downside to adding felt paper in this situation, as far as vapor movement/wall drying is concerned?
Thanks again for your prompt feedback.
Q. "Is there a downside to adding felt paper in this situation?"