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Community and Q&A

Vaulted ceiling insulation

Seth001 | Posted in General Questions on

Hello, I’m building a home in northern Iowa climate zone 6. My roof is vaulted with a ridge beam and 18” deep roof truss. I used zip sheathing on the walls and the roof. Now I want to thermal break my entire envelope. I will put 2” r-10 halo exterra on the walls and I’m wanting to put 2”-3” halo exterra on the roof deck. I’ve been trying to research and see what my best option is and what would be the best way to insulate under the roof deck but I’m having a hard time coming up with somthing. My thoughts are: 2”-3” halo exterra above zip roof sheathing,  another layer of sheathing then roof cladding. Spray foam 2-3” on underside then fill remaining 15”-16” with blown in cellulose. Any thoughts would be greatly appreciated. Thanks a lot!

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  1. Expert Member
    BILL WICHERS | | #1

    You want at least about 40% of your total assembly R value on the exterior if you use exterior rigid foam. 50% on the exterior is better/safer. If your target is R49 for the assembly, that means ideally about R25 on the exterior, which is 5" of the GPS you're planning on using. If you insulate that way on the exterior, you can use whatever you want on the interior -- although I think batts might be easiest to install here.

    Note that exterior rigid foam is a much better way to go than spray foam when you have the option. The only times I'd go with spray foam here would be if you either can't put up exterior rigid foam at all (such as you just put on a new roof), or if you can't put up enough exterior rigid foam (if it would complicate architectural details). If you're limited on how much exterior rigid foam you can use, you can use a layer of closed cell spray foam on the underside of the roof sheathing to make up the rest of the R value for the "exterior" side, then finish off with open cell, batts, or other materials.


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