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Community and Q&A

Cathedral insulation

user-6259794 | Posted in Energy Efficiency and Durability on

I am building a new house in zone 5. We have part of the house under a cathedral roof (5/12 inside pitch). I don’t have exterior rigid insulation on the exterior of the roof, just the walls. My insulation guy is recommending 2″ of closed cell foam, and then 5″ of open cell foam but that only gets us to R32 and I really don’t want all that foam. Would I be able to have 2″ closed cell and then install denim batt insulation in between the trusses? I’m not sure this if this would be a concern but that would leave an air space between the denim and cc foam. We don’t have anyone locally that does dense pack cellulose. I guess I could use insulation supports to hold it tight to the cc foam. Any suggestions would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that discusses a hybrid roof assembly that combines foam insulation with fluffy insulation: Combining Exterior Rigid Foam With Fluffy Insulation.

    Although the article discusses rigid foam rather than closed-cell spray foam, the same ratios discussed in the article for rigid foam apply to closed-cell spray foam when combined with fluffy insulation.

    Briefly, in Climate Zone 5, you are aiming for a minimum of R-49. At least R-20 of that R-49 needs to come from the closed-cell spray foam (about 3.5 inches of closed-cell spray foam). The remaining R-29 can come from fluffy insulation.

    For an overview of all of your options, see this article: How to Build an Insulated Cathedral Ceiling.

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