Spokane, WA zone 5B rigid foam and drying
I’m a long time subscriber to Fine Homebuilding and follower of the website, and finally I’m able to put all of the knowledge to work building my family’s new house this summer.
In effort to achieve the best bang-for-buck performance I’ve been working with the builder and the insulator to determine the right solution to remove thermal bridges and up the R value. The place we’ve landed is with 8″ top and bottom plates, staggered 2×4 studs 12/24″ OC, and Blown in Blanket System (BIBS) insulation behind 7/16″ sheathing. All penetrations will be spray foamed. This achieves R31 nominal and should still be quite high in practice. We are also evaluating 1″ continuous rigid foam over the sheathing to bump to R36 and break the thermal bridges at the plates and joists/trusses.
The problem I’m trying to solve: my builder says in our climate we just need to wrap the sheathing in Tyvek, put up the rigid foam, then side it with a Hardi product. The insulator says we need the same thing but with the addition of a 4 mil vapor barrier behind the drywall.
We are in the cooler, drier southwest corner of Spokane, WA (zone 5B, just a a couple dozen miles from 6B), and just about everything I read about climates, interior-exterior insulation ratios, rain screens, warm/cold sheathing, etc tells me that I need to confirm the design will keep my sheathing from rotting. Given my data sources I can imagine that most of this advice comes from wetter coastal regions, so I am looking for input on how my region relates and how I go about designing the proper wall system for years of dry happy walls.
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