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Substitute for rigid foam on unconditioned attic wall?

I have a simple single story gable roof house with an unconditioned attic. I plan on adding some exterior rigid foam to the walls, but I don't need the foam's insulation value, or any air sealing details, on the gable walls of the attic. I figure I could put tyvek on the sheathing, lap it over the top of the rigid foam of the lower story, and then install some wood spacer blocks on the gable wall that are the same depth as the foam below, and screw the headloks through them when I attach furring strips. This would save me a couple hundred bucks on materials.

Are there any examples of this being done? I can't find anything, but I have a hard time imagining it is a new idea. I'm a little worried that the foam will compress a little bit, but the wood blocks won't, and there could be a visible alignment issue. But I wouldn't think that foam would get compressed enough for that to be a problem. Also, I'm not sure how large the blocks should be to maintain the rigidity needed to support the siding -- or if that is even a realistic concern. The foam would be 2-4" thick.

Asked by Nick Welch
Posted Mon, 03/31/2014 - 14:57
Edited Mon, 03/31/2014 - 16:18

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Nick,
Your plan should work. Ideally, you will install vertical furring strips on the exterior side of your wall foam, to create a rainscreen gap between your siding and the rigid foam. If you have furring strips, they can run continuously up to your rake, and you can shim the furring strips a little with shim shingles if necessary (if you are worried that your blocking isn't exactly the same thickness as the rigid foam), using the shims to make everything coplanar.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 03/31/2014 - 16:19

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