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Cathedral ceiling: vented or unvented?

thomase00 | Posted in General Questions on

The previous owner of my home built a cathedral ceiling with skylights in the attached family room. I recently had a chimney removed from this room and it became apparent that the space between the ceiling and the roof sheathing is totally un-vented. The ceiling is finished with stained pine and I’d rather not have to take it apart (I believe that there is drywall underneath the wood).Also, when it is really hot in the summertime, the room smells like an attic, which is probably explained by the lack of venting.

I was do for a new roof before the chimney removal and definitely need one after. Initially, I was thinking that I would have the roofer remove the sheathing (it’s probably not in good shape anyway due to lack of venting), and lay down some fiberglass between the rafter bays, cover with rafter chutes, re-sheath, and install a ridge and soffit vents. However, the contractor who worked on repairs after the chimney demolition noted that my rafters are only 2x6s and that there probably won’t be enough room for insulation AND venting. On the other hand, the pitch of the ceiling looks like it’s not as high as the roof pitch, so there may be plenty of space near the ridge, just not so much near the soffits.

The contractor suggested sticking with an unvented roof, but that I should instead have the top side of the ceiling spray-foamed from the outside before re-sheathing. Obviously, this would only work if there is drywall behind the tongue-and-groove wood ceiling. It kind of makes sense to me since it blocks water vapor from making contact with the roof sheathing AND seals the interior space off from the air in the void between the ceiling and the roof. However, this is not how un-vented cathedral ceilings are usually built. Everything I have read about this says that the roof deck itself should be foamed, but in my case, that would require taking the ceiling apart.

Comments?

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Replies

  1. thomase00 | | #1

    Also, I currently have some fiberglass insulation in the unvented ceiling. Condition is unknown.

  2. GBA Editor
    Martin Holladay | | #2

    Thomas,
    Your contractor is correct. It's possible to install spray foam insulation from above, when the roof sheathing is opened up during a re-roofing job.

    Is it the right solution in your case? It's hard to say. You might prefer to install an adequately thick layer of rigid foam above the existing roof sheathing.

    For more information on your options, see these articles:

    How to Install Rigid Foam On Top of Roof Sheathing

    How to Build an Insulated Cathedral Ceiling

  3. thomase00 | | #3

    Rigid foam above the sheathing gets complicated because I have skylights, not to mention that fact that it would look architecturally different than the roof and the main part of the house.. Also, the roofing contractors I have spoken to don't seem to be too experienced with that kind of roof installation.

    If I foam it from above, can the void between the foam and the sheathing remain unvented? There won't be much if any room for soffit venting intake between my 2x6 rafters because closed-cell foam needs to be at least 5.5" to hit R-38 (I'm in Massachusetts). Also, assuming it is unvented, can I cover the entire roof with ice and water shield, or should I only do the bottom few feet to allow better breathability through the sheathing?

    I've talked to a local spray foam contractor and they claim to have done this exact thing before, leaving it unvented.

  4. thomase00 | | #4

    Massachusetts.

  5. GBA Editor
    Martin Holladay | | #5

    Thomas,
    Q. "If I foam it from above, can the void between the foam and the sheathing remain unvented?"

    A. Yes.

  6. Steve Knapp CZ 3A Georgia | | #6

    Thomas,

    Where are you located?

    Edit: Sorry. Missed that detail in your earlier response.

  7. Expert Member
    Dana Dorsett | | #7

    It takes fully 7" of HFO blown closed cell foam to hit a code-min R49. You have 5.5", and wouldn't be able to install more than 5.25" (trimming closed cell foam is labor intensive), for about R35, and much higher thermal bridging than R49 fluff on an attic floor, due to the much shallower depth of the 2x6 framing fraction.

    But if you filled the cavities with open cell foam (~R20) or R23 rock wool batts and 4" of roofing polyiso (R22-R23) or a 4.5" nailbase panel above the roof deck it would meet code on a U-factor basis.

    The skylights are going to have to be completely re-installed (or replaced) anyway, if you're stripping the roof deck. In snowy MA skylights are a major ice-dam starter, and you might consider getting rid of them, or reducing their size for ice-dam mitigation purposes.

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