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Insulating a flat ceiling in conjunction with a hot roof?

Todd Loiacono | Posted in Energy Efficiency and Durability on

Hey there… My 950 sq ft. house has a hot roof insulated with 4.5″ of spray foam. The roof system is 2×8 rafters with 2×6 collar ties. The attic space is basically a crawl space. I’m planning by adding insulation to the flat ceiling using 5.5″ of rock wool. I currently have zero moisture issues in the house and heat with wood, generally keeping the house humidity pretty low. Am I doing anything bad by adding the insulation to the flat ceiling?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Am I doing anything bad by adding the insulation to the flat ceiling?"

    A. No.

    That said, I hope you used closed-cell spray foam rather than open-cell spray foam. For information on problems in homes with open-cell spray foam installed on the underside of the roof sheathing, see these two articles:

    High Humidity in Unvented Conditioned Attics

    Open-Cell Spray Foam and Damp Roof Sheathing

  2. Todd Loiacono | | #2

    Thanks for the quick reply Martin. Closed-cell foam was used. I'll check out the 2 articles for my own edification.

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