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SPF thickness under roof deck

gojoe6869 | Posted in General Questions on

Hello All,
We are building a pole barn house. I’m located in Southeastern Ohio. We have the shell completed. It was built with 1/2 rigid foam under the metal. It was built with a completely sealed house in mind and to use an HRV/ERV for fresh air as well as a ducted/non ducted minisplit combo for heat and AC . My question is how many inches of Closed Cell SPF should I put under the roof deck and in the walls? I’m thinking 4″ under the roof deck and 1.5 in walls then finish the wall cavity with rockwool. The walls are very thick. Part of my reasoning is we have a 30×40 pole barn that has approximately 2.5″ of SPF under the roof deck and 4″ in the walls, the building is in direct sunlight most of the day. A 12k btu Mitsubishi Mini Split keeps it 70 degrees even in 90+ temps. Am I thinking right?
Thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to follow the flash-and-batt method, you need to follow the ratio rules for combining foam insulation with fluffy insulation. For more information on the ratio rules, see this article: "Combining Exterior Rigid Foam With Fluffy Insulation."

    (Although the article talks about exterior rigid foam, the same ratio rules apply to flash-and-batt jobs. Note that the ratio rules for roofs differ from the ratio rules for walls.)

  2. charlie_sullivan | | #2

    Note that conventional closed cell spray foam has an outrageously high global warming impact. Fortunately, you can now specify "HFO" blown foam and avoid that impact completely.

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