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Insulating a Cathedral Ceiling

Amika | Posted in General Questions on

Hi. I am looking for help deciding if my plan for insulating a cathedral ceiling will be OK and not be prone to moisture issues.
Background info:
I live in northern Wisconsin, I need to reach r49 in insulation in the rafter bays to reach code.  Our area is subject to both cold and hot conditions depending on the season.
The plan:
Have a vented roof system with a variety of insulation types. Rafters will be 2×12’s
-Nearest roof deck will be a vent. Baffle vent will be 1″.
-1″ eps foam board next to baffle. Cut and cobble approach and sealed around edges to stop airflow with spray foam. (R4)
-r38c owens corning unfaced fiberglass batts next to eps to fill remaining rafter bay.  (Yields r35 compressed to 9 1/4″) according to their compression chart.
-2″ xps foam board fastened to underside of rafters and taped to qualify as vapor barrier. (R10)

Would there be and moisture issues because of the eps and xps on either side of the rafter bay?
My understanding right now is that eps would be breathable and allow vapor through to exit to the vent.

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    It would work but I'm not sure if the EPS baffles are worth the effort. You are better off selecting a high density batt (FG or MW) which doesn't suffer from wind washing. If you install the batts with care and don't push it in too far to block the vent channels, you can even skip any baffles.

    I would check your local code, most allow for lower R values for compact roofs, this might let you get away with much less rigid bellow the rafters.

    XPS is not the greatest insulation as it has a pretty high environmental footprint. GPS or foil faced polyiso is much better for similar costs.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Amika,

      I'm with Akos. The EPS cut & cobble buys you very little, and I'd do as he and Striker9 suggest and use baffles only where necessary at the eaves. If you do go ahead, you are fine with the foam on both sides. The roof will still have some drying to the exterior through the rafters. It's no different than using pre-made foam baffles.

      A couple of things I'm not clear on:

      - Are you suggesting both baffles and 1" EPS underneath? It makes more sense to use the rigid foam as your baffles.

      - "Nearest roof deck will be a vent" ?

      1. Amika | | #4

        Malcolm Taylor,
        Thanks for the reply. What I meant by the "baffle" would be a vent created myself by ripping 1" eps foam board to 1" and tacking that in place along the rafters for the eps foam board to rest on.

        "Nearest roof deck", I was attempting to describe my plan and the order of layers of insulation. By this I simply mean a 1" deep vent under the roof deck.

        1. Expert Member
          MALCOLM TAYLOR | | #7

          Amika,

          Ah got it. Sometimes I'm just not very imaginative.

    2. Amika | | #5

      Akos Toth,
      Thanks for replying. I will try and find out if locally I can have a compact roof less than r49. I am not opposed to polyiso also in place of the xps. I have not heard of GPS, I will research that as well. Thank you

  2. Striker9 | | #2

    I live in a similar climate and was considering doing the same

    1" air gap
    3/4" ̶X̶P̶S̶ EPS foam board
    Mineral wool
    2" foil-faced polyiso under the rafters (Less vapor permeable than XPS and greener)
    drywall

    I've seen others recommend using house-wrap like Tyvek, that is vapor-permeable, to create the baffles.
    However, creating the site-built baffles using XPS or Tyvek does seem like a lot of work. What I may end up doing is creating 2-foot baffles from the soffit vents up, to avoid immediate wind washing, and then not bother with them the rest of the way. I'd then install the mineral wool as Akos mentioned.

    1. Amika | | #6

      I would be hesitant to use both xps and foil faced polyiso on opposing sides of the rafters as I think both are a vapor barrier. By using eps Nearest the vent I was hoping it would be permeable enough to allow moisture to escape to the vent

      1. Expert Member
        MALCOLM TAYLOR | | #9

        Amika,

        You may find the discussion about impermeable baffles in this article useful:
        https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs

      2. Striker9 | | #10

        You're right, I had meant to type EPS which is more vapor-permeable than XPS.

  3. johnwtaylor | | #8

    I glued (caulking) 2" EPS blocks to the trusses (not the sheathing as it may get stripped off one day) and used 3/4" Mylar faced EPS for baffles. The 3/4 EPS had enough flexibility so the pcs could be cut exact, pressed in and minimal canned foam used.

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