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High-end remodel

alcoprop | Posted in General Questions on

I am doing a higher end remodel in climate zone 5A. We found out a lot of mice have gotten in the walls and ate up the old foam board under the siding, destroyed the insulation and ate wiring. So with this find we decided to rip all the exterior drywall down to the studs, remove all the siding and foam board to the studs. I plan on putting up 7/16 zip, 1.5″ comfortboard 80, 3/4″ furring strip and then siding on the exterior. Interior I planned for 2″ of closed cell, and then R15 comfortbatt since we have 2×6 walls. We are doing all Marvin casement windows also. Is this the right plan for a great performing wall? 

Attic space: My spray foam guy wants to do an unvented conditioned attic with open cell. He only estimated for 5.5″ of open cell. Two things on that is I don’t believe that is enough for proper Rvalue and I have now read in climate zones 5 and up that open cell on the decking is not a good idea. I have zero duct work in the attic. All of it is in the crawl where we will be doing proper vapor barrier and closed cell on the walls to condition it. Thoughts on all this please. 

Thanks for everything, I am a new member and look forward to learning a lot!

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  1. walta100 | | #1

    The only reason to do something as expensive as spray foam between the rafters is that someone was stupid enough to put the HVAC ducts and equipment in the attic.

    50% more surface area at 1/3 the R value costing 3 times as much. If your only tool is a hammer everything looks like a nail. I think the correct answer is NO thank you.


  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Shaun.

    I agree with Walter. I can't see the reason for a flash coat of spray foam in your walls. Detail the ZIP Sheathing as an air barrier, skip the foam, and in my opinion, consider dense pack cellulose for the cavities.

    You are right that open-cell foam is not a recommended choice for an unvented cathedral ceiling. If you are not planning on using the attic space and there are no mechanicals up there, I'd recommend air sealing and insulating the attic floor instead.

    Here are some articles I would recommend for you:

    Is Using Closed-Cell Spray Foam Worth the Trade-Offs?
    Open Cell Spray Foam and Damp Roof Sheathing
    How to Build an Insulated Cathedral Ceiling
    How to Insulate an attic floor

  3. Expert Member
    BILL WICHERS | | #3

    I don’t see a reason to use spray foam here. As Walter mentioned, you have no mechanicals in your attic, so you don’t need to make it conditioned space. I’d air seal the attic floor and then use blown cellulose to insulate it. If you want to use spray foam for something to keep that contractor busy, flash the floor for air sealing (a thin layer only), but there are more economical ways to do this too, especially if the floor is open and free of any old insulation.

    Spray foam in walls rarely makes sense. Thermal bridging of the studs kills the performance. Critters can chew up spray foam just as easily as they chewed up rigid foam, so don’t believe anyone tell you that “spray foam is mouse proof”. I’d use mineral wool batts in the walls, which is a LITTLE more critter resistant, and replace the rigid foam. Make sure everything is tightly fitted, and use some metal protection on the lower edge to keep critters from chewing their way in. There are purpose made perforated metal strips for this application, or you can use things like hardware cloth — but you want stainless hardware cloth in this location if you are going to use it for this purpose.


    1. alcoprop | | #4

      Bill thanks for the reply. I was looking at the benefits of the Rvalue with the closed cell in the walls. Having 2x6 walls spraying 2" of closed cell, 3.5" of R15 comfortbatt, and then 1.5" R6 comfortboard 80 on the exterior would get my R value around R35. My spray foam guy is for sure doing the crawlspace so he will be there already and gave me a good price for doing the 2" on the walls. But if there is more of economical way of getting a high Rvalue I am all for it. Thanks

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