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Rigid insulation thickness formula for unvented roof the same as walls?

rodrob15 | Posted in General Questions on

Do roofs follow the same formula as walls for rigid insulation thickness? As an example, if I’m in a zone 5 climate, 27% of the R-value is required for rigid insulation for 2×6 walls.

Long-story short, I’m building a small cabin with 2×6 rafters. The entire cabin will have a cathedral ceiling and I’m planning on an un-vented assembly, so currently I’m thinking R19 Fiberglass or R23 Mineral wool batts, with a layer of foam on the roof. By the formula above, I’d need roughly R8 in exterior insulation. I likely will do 4” of EPS at R16, but just want to make sure I’m looking at this correctly.

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Rodrob15,

    You need more exterior insulation on a roof assembly than on the walls--in Zone 5, about 41% on the roof as opposed to 27% on the walls. I think you will find these articles helpful: How to Build an Insulated Cathedral Ceiling, How to Install Rigid Foam on Top of Roof Sheathing, and Combining Exterior Rigid Foam With Fluffy Insulation.

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