Drain plane behind XPS rigid foam
Hello,
We are in climate zone 6 and plan to remove vinyl siding and install metal siding (something like McElroy Max Rib panel or Wave panel).
Our current WRB is 1/2” XPS foam board taped with Tyvek house wrap tape. Our 6” wall stackup is polyethylene sheeting, R19 fiberglass batts, OSB sheathing, ½” XPS foam and vinyl siding.
We’ve recently installed new windows and used liquid flashing (Prosoco R Guard system) for the rough openings. At this time I noticed most of the taped foam joints near the window locations had failed so I suspect this is representative for the rest of the house. I did have rotten OSB around many of the window rough openings.
I plan to remove the XPS foam, apply Prosoco liquid WRB (Spray Wrap MVP) to the OSB, reinstall the foam and seal the seams/broken pieces with my foam gun. I’m debating if I should install a drain plane (like Tyvek drainwrap) behind the foam and would like other opinions? Prosoco’s Spray Wrap MVP is vapor open at 25 perms so the ½” XPS foam would still be the limiting factor for drying the OSB to the outside.
I know it would be nice to add additional foam thickness at this time but all the additional expense and trouble doesn’t seem worth it base on my heat load calculations.
Thank you, David
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Replies
You should absolutely install a rain screen, unless you add a lot more foam. See this current discussion: https://www.greenbuildingadvisor.com/question/rain-screen-recommendation-under-vertical-metal-siding?cid=275226&discussion=response#comment-275226
Michael, Thanks for the reply. I just want to clarify a few things. The drain/drying plane I'm talking about is between the OSB and the 1/2" XPS rigid foam. The new metal siding options we are considering will have a "built-in" air gap/rain screen similar to the vinyl siding we have now. I am considering adding the Tyvek drain wrap between the OSB and 1/2" XPS for improved drying (drying no longer limited by the 1/2" XPS permeability). You are saying this is definitely worth doing? Thanks again, David
Ah, sorry, sometimes I read too quickly. I don't think you need a separate rain screen directly behind your cladding. 1/2" XPS is somewhere around 2 perms which is somewhat vapor-open so you would probably be ok without a drainage plane between your sheathing and foam, but including one would be good insurance. This article has relevant info: https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh.
Thanks for your assistance.