Advice on proposed zone 6 wall assembly
We are building a cape in zone 6, Waldoboro Maine. I am considering the following wall assembly and have several questions regarding the components. Forgive the length of this, but I don’t know how to make this any briefer.
The assembly is Prefinished FiberCement smooth lap siding, 1″ of XPS rigid foam covered with Rain Drop by Greenguard (grooves for draining), 7/16″ OSB sheathing covered with Crinkled building paper by Dupont, and 24″ on center 2×6″ studs with cavities filled with dense packed blown-in cellulose insulation. While not ideal from an R-value standpoint, I will take special care to prevent the majority of potential air leakage.
I chose FiberCement siding because its 15 year warranted finish will preclude frequent painting and with its trim looks wood-like. Does the Rain Drop by Greenguard provide sufficient channeling for air drying behind the siding? The siding manufacturer specifies 1″ max. of rigid foam unless 3/4″ lath strips are applied over the studs for nailing. I want to avoid the extra depth of the wall beyond the OSB so that the fiberglass casement windows don’t require additional framing to fur-out the opening for support.
For the rigid foam in Zone 6 I realize that a minimum of 1 1/2″ is recommended to keep the OSB sheathing warm enough, so that dew point is not reached on the face of the OSB. Does, however, a high enough perm rating on the foam allow for enough drying to the exterior to prevent moisture damage to the interior of the wall? I was surprised to find that Dow’s 1″ XPS rigid foam has a perm rating of 3.5 (ASTM E96). This is vapor semi-permeable not semi-impermeable as many people suggest for 1″ of rigid foam. Does this dry enough to the exterior, so that I don’t have to instal a vapor retarder to the inside of the building envelope? Does the sheathing product Zip System help in preventing moisture damage within my proposed assembly?
I must admit a low-tech 12″ wall is looking more appealing, but I am not ready to deal with the different challenges such a wall presents.
Thanks for your help!
Update: the rigid foam I had initially suggested, 1″ XPS shiplap by Dow with perm of 3.5 is not available in the U.S. I don’t know the perm value for their regular tongue and groove panels , but I suspect its more in line with Owen’s perm. I guess the higher perm available to Canadians is for the extra cold and potential dew point problems.
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