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Spokane, WA zone 5B rigid foam and drying

Andy Huska | Posted in Green Building Techniques on


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.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    Installing a continuous layer of exterior rigid foam is a great idea, as long as it is thick enough to avoid moisture problems. If you have an 8-inch-thick wall filled with R-31 insulation, the minimum R-value for your exterior rigid foam layer (in your climate zone) is about R-12. (The R-value of the rigid foam layer must represent at least 27% of the total R-value of the wall assembly to keep your wall safe.)

    All of this is explained in this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    So, if you are using EPS for the rigid foam layer, you'll need at least 3 inches of exterior rigid foam, not 1 inch.

    This information may discourage you, and may lead you to conclude that you'll build the wall without any rigid foam.

    I think that a smart vapor retarder like MemBrain makes more sense than interior polyethylene (because MemBrain allows some interior drying). If you end up installing exterior rigid foam, it's essential that the wall have NO interior polyethylene.

    For more information, see How to Design a Wall.

    -- Martin Holladay

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