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How to insulate a gambrel roof?

darwyn | Posted in General Questions on

I have a house with a gambrel roof and every year I have major ice dam issues as well as interior wall damage on the lower pitch on the second floor. I have attached a few pictures of the problem as well as photos of the original plan sheets.
I am in need of re-roofing the house this year and I would like to fix the problem.
Is there a fix? This rafters on this house are 2X4’s.

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  1. GBA Editor
    Martin Holladay | | #1

    The roof slopes need to be well insulated. In climates that suffer ice dam problems, most building codes require a minimum of R-49 insulation in roofs. While your lower gambrel roofs are nearly vertical, they are still roofs, not walls (as evidenced by your ice dam problems).

    You can't get R-49 insulation in a space that is only 3.5 inches deep. (That's how much room you have between the 2x4 rafters.)

    The solutions involve either: (a) adding more framing depth on the interior of the rafters to install R-49 insulation, or (b) adding enough rigid foam on the exterior side of the roof sheathing to bring the assembly up to R-49.

    Solution (b) requires a second layer of roof sheathing on the exterior side of the rigid foam, as well as new roofing.

    Here are links to two articles that you should read:

    How to Build an Insulated Cathedral Ceiling

    Prevent Ice Dams With Air Sealing and Insulation

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