Sealing exterior rigid foam over top of housewrap
I am planning the following wall for new construction, from inside out:
– 2×6 framing
– osb sheathing
– tyvek drain wrap as both wrb and primary air barrier
– rigid foam as both thermal break and secondary air barrier
– 3/4″ furring for rainscreen
– hardie cladding
My question is about sealing details for the rigid foam. Does it make sense for the foam to be used as a secondary air barrier with seams taped and edges caulked? If so, then I wonder how to treat the bottom edge of the foam. Intuitively I would think it would need to be left un-sealed in order for the house wrap to be able to drain from behind the foam, but that seems to defeat the purpose of the foam being an air barrier.
Also for innie windows that will be flashed to the house wrap, is it necessary to add a second flashing at the sill to direct the bulk of moisture that makes it past the cladding to the outside of the foam, or is it sufficient to just let it drain behind the foam?
Any advice is greatly appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Green,
Once you have chosen your WRB location, all window flashing should be directed to that WRB. Very little liquid water will reach this WRB, and any water that does will be removed by evaporation rather than drainage.
For more information, see "Where Does the Housewrap Go?"
You should protect the bottom of the rigid foam; the usual method is with metal flashing. Here is a link to a video that shows the detail: "How to Install Rigid Foam Insulation Outside a House."
Thanks Martin, that answers my question about flashing. I'm still wondering about sealing the foam though. Am I correct in thinking that I should not caulk the bottom edge of the foam to the metal flashing in order to allow the WRB to drain? And is it even worth taping the seams of the foam if it is not my WRB, or can it still provide substantial air sealing?
Hi Green -
Since you have a free-draining space to the exterior of the rigid foam, you have a secondary drainage plane as well as a secondary air barrier with the rigid foam. Your cladding stops most liquid water; that which makes it past your cladding is likely to drain down the face of your exterior rigid foam, and then your final "stop" is your WRB connected to your flashing.
So, very little water will ever reach your WRB but if/when it does, yes, you need the bottom of your WRB to drain out and so you can't tape closed the bottom of your rigid foam.