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Regarding insulating a cathedral ceiling

Alex03281 | Posted in Energy Efficiency and Durability on

I am insulating material, cathedral ceiling, using R-30 unfaced batting between the rafters, and would like to use a rigid foam board to increase the R-value. Will I create a problem if I use 6-mil poly between the rigid board and the sheetrock?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, if you are installing fiberglass batts between the rafters, you need to install ventilation baffles to create an air space between the top of the insulation and the underside of the roof sheathing. You also need soffit vents and a ridge vent.

    Assuming that the ventilation baffles and vents are properly installed, you can install fiberglass batts and rigid foam as you describe. I would skip the polyethylene if I were you -- rigid foam is already a vapor retarder, so any code requirement for a vapor retarder is already satisfied by the rigid foam. If you live in a cold climate (zone 5 or colder), the polyethylene will probably do no harm. But it is unnecessary.

    For a full explanation of the various methods used to insulate this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.

  2. Alex03281 | | #2

    Thanks ... And yes, I used a plastic rafter vent called Pro-vent and ran from the soffit vents to the ridge vent. When purchasing materials, I was advised that the rigid foam did not need the poly, as you noted due to it being a vapor retardant. The seller suggested though that if I did use the poly, it could be between the unfaced batts and the rigid foam.
    The reason they suggested that was because the rigid foam, though interlocking, requires a specific tape at the joints, but they said the tape is sold in Canada, but not in US...or at least not sold at Home Depot.
    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    You definitely want to tape the joints of the rigid foam. What type of rigid foam do you plan to use?

  4. Alex03281 | | #4

    Planning to use Foamular Extruded Polystyrene (XPS). It calls for a tape called Owens Corning JointSealR foam joint tape.
    The seller of the rigid foam doesn't carry the tape and couldn't find a distributor for me.

  5. GBA Editor
    Martin Holladay | | #5

    Two points:

    1. JointSealR tape can be purchased online. Here is one source:

    2. You might consider using 3M All Weather Flashing, Siga Sicrall, or Siga Wigluv tape. For more information, see Return to the Backyard Tape Test.

  6. Alex03281 | | #6

    I just bought the joint sealing tape from the link you provided.
    Thank you so much!

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