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How do I insulate an existing cathedral ceiling in Maine?

Kevin Barker | Posted in Energy Efficiency and Durability on

The ceiling I want to insulate was built with 6 x 12 rafters 4 feet on center. Sheathing is 1 1/4 thick tongue and groove pine then asphalt shingles. So the boards are exposed. Also no vents at all.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kevin,
    Assuming that you want to be able to continue to see the existing boards (the boards which now form your ceiling), the best way to insulate this type of roof assembly is to install one or more thick layers of rigid foam above the existing roof sheathing, followed by a layer of plywood or OSB roof sheathing and new roofing.

    Here are links to two articles that will explain everything you need to know:

    How to Build an Insulated Cathedral Ceiling

    How to Install Rigid Foam On Top of Roof Sheathing

  2. Kevin Barker | | #2

    Thank you Martin
    I would like to cover up the boards on the inside as there is a fairly new roof.
    Ridged foam and drywall?

  3. Bob Irving | | #3

    Insulating between the beams will leave you with huge thermal bridging, and it will be difficult, although not impossible to air seal the bays (air getting into the bays can introduce moisture which can condense and cause rot and mold; depending on how you finish the interior). Insulating this roof from the exterior is the right and best way to do the job, but it is not a cheap/quick solution.

  4. GBA Editor
    Martin Holladay | | #4

    Kevin,
    The problem with insulating this roof assembly from the interior is thermal bridging through the 6x12 rafters.

    If you have to insulate on the interior, I advise you to install a thick layer (or layers) of rigid foam on the underside of the rafters. Because the rafters on 4 feet on center, you would probably need to strap the ceiling first with 1x4s or 2x4s. Ideally, you would install at least two layers of rigid foam with staggered seams. All seams should be taped with a high-quality tape to limit air leaks. Once the insulation is installed, you would need to install a new drywall ceiling. The usual method in New England calls for 1x4 furring strips between the rigid foam and the drywall.

    There will be less risk of moisture accumulation in the roof sheathing if you don't install any fluffy insulation above the rigid foam. However, if you want to add fluffy insulation, you could -- but only if you could figure out a good way to include ventilation channels between the top of the fluffy insulation and the underside of the roof sheathing.

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