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Impermeable insulation and vented cathedral ceiling (Zone 6)

Bob Manninen | Posted in General Questions on

I have a question which is driven by the 2009 IRC code requirement for unvented cathedral ceilings; specifically the requirement that at least 1/2 of the total R-value be made up a an impermeable insulation layer, next to the sheathing. Ostensibly, this is required to mitigate condensation on the boundary between the impermeable insulation and the permeable insulation. My basic question is: why isn’t this required for a vented cathedral ceiling assembly? If a “baffle”/ventilation channel is employed in a cathedral ceiling assembly (assume 1 1/2 or 2 inch space) between the roof sheathing and the “permeable” insulation (e.g., fiberglass, rock wool, cellulose) why isn’t that “baffle”/ventilation channel boundary also required to be made up of an impermeable layer at least 1/2 the total insulation value of the roof assembly? Won’t the same condensation issues arise between the baffle interface and the permeable insulation? Or, is this boundary assumed to be leaky enough that any condensation will be taken care of by the vented air stream?

I’m specifically asking because I want to use a 1″ polyiso material for the baffle/ventilation channel, in a vented cathedral ceiling assembly, making sure that there is an effective air seal between the vent channel and the permeable insulation.

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Replies

  1. Kevin Dickson, MSME | | #1

    Yes, the condensation is "taken care of" by the vented air stream. That doesn't mean that there is zero condensation, but that the condensation evaporates within a few hours. That's not enough time for rot and mold to develop, and it's the whole idea behind that most common and most robust assembly, the vented attic.

  2. Bob Manninen | | #2

    I'm really talking about the other side of the vent channel. (between the vent channel material and the permeable insulation. It isn't in the vented air stream and, theoretically, could be a cold enough surface for water to condense.

  3. GBA Editor
    Martin Holladay | | #3

    Robert,
    The answer to your question is that most ventilation baffles are either leaky (not really desirable, but this explains why there is no moisture problem) or vapor-permeable (the better solution -- any moisture buildup is handled by diffusion).

  4. Bob Manninen | | #4

    Ah, I was beginning to suspect that conclusion; so, using 3/4" XPS would probably be a better choice for the ventilation channel material. I would like to air-seal at the ventilation channel boundary; I guess I'll also need some kind of vapor retardar in the finish/ceiling boundary. I'm using V-match pine for the finished interior instead of a plaster/sheetrock/painted treatment..... Would the pine provide an adequate vapor retardar?

  5. GBA Editor
    Martin Holladay | | #5

    Robert,
    Pine boards are neither an air barrier nor a vapor barrier. If you want a board ceiling, be sure to first install a layer of drywall (taped) before installing your boards.

  6. Bob Manninen | | #6

    So, even though I've provided an air-barrier at the ventilation channel boundary, you would still advocate an additional air-barrier at the paneling boundary? Or, are you dubious (probably justified) that it's almost impossible to make a boundary at the ventilation channel boundary?

    I guess I understand that pine boards are not a vapor barrier; I was under the assumption that you're really trying to "reduce" the possibility of any vapor drive from the living space into the insulation...

  7. GBA Editor
    Martin Holladay | | #7

    Robert,
    Q. "So, even though I've provided an air-barrier at the ventilation channel boundary, you would still advocate an additional air-barrier at the paneling boundary?"

    A. Yes.

    Q. "Are you dubious (probably justified) that it's almost impossible to make a boundary at the ventilation channel boundary?"

    A. Yes.

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