Product compatibility of wall assembly components for Climate Zone 4a
I am in the planning phases of a new home for my family outside of St. Louis, MO. I’m having a hard time pulling together all the compatibility issues that worry me when detailing my wall assembly. I am striving for a tight house that is well insulated, but not necessarily trying to achieve any recognized benchmark. I would hope that with a future PV array I could be close to net-zero with my goethermal HVAC system planned for. Here is what I am have at least planned for at this point: (from inside to out)
– 5/8″ Drywall
– Certainteed Membrain smart vapor retarder (even though I probably do not need it)
– 2×6 studs on 24″ centers leaning towards Johns Manville Spider Spray-in Fiberglass Insulation
– 7/16″ OSB continous wall sheathing
– WRB to be Tremco Enviro-Dri sprayed applied product (lower than I like permeability but air-tightness is why I like it)
– 1″ Owens Corning Formular 150 XPS continous insulation (not taped because already have a WRB)
– Then either brick w/ 1″ air space behind or Vinyl Siding (I can’t afford the brick on 4 sides)
My windows will be ‘bucked out’ the 1″ and detailed around so that my WRB follows the OSB/wood plane and my window flanges can be taped with a product compatible to the Enviro-Dri
My biggest concern is am I creating a vapor barrier with my spray-applied WRB that I should be concerned about moisture being trapped either between the OSB and foam or on the inside of the wall cavity?
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With the exterior XPS, the OSB will tend to dry to the interior. So the permeability of the WRB doesn't really matter.
You might want to look for reclaimed foam, which would be cheaper and more environmentally friendly than new XPS. Or eliminate the exterior foam all together and opt for a Bonfiglioli wall instead.
Q. "My biggest concern is am I creating a vapor barrier with my spray-applied WRB."
A. According to the manufacturer, Tremco Enviro-Dri has a permeance of 12 perms. In other words, the product is vapor-permeable. So I'm not sure why you think it is a vapor barrier. It isn't.
That said, Steve Knapp is right: Walls with exterior rigid foam don't depend on outward drying. They are generally designed to dry to the interior. For more information on this concept, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Your wall will work.
Thank you for the responses, I missed that perm rating in the Tremco documentation completely. As for the foam, I have begun to look around at local resources for reclaimed foam. If I can find it I am most certainly interested in going that route.
Note also that you don't have to use XPS. EPS or polyiso will also work.
And as I found from these boards that polyiso is better for the environment and in my area about the same price as XPS and provides other benefits such as better fire ratings.