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Community and Q&A

Insulating unvented cathedral ceiling

Robert Jones | Posted in Energy Efficiency and Durability on

I bought a house with a vaulted ceiling. Drywall is nailed to 2×6 roof rafters. It is unvented for the most part. I have no idea what kind of insulation is up there. I want to insulate it but don’t want to tear off the roof (which is relatively new), or the ceiling. There are big overhanging eaves that can be opened to look up into the cavities. I’m wondering about my options here. Is it possible to spray closed-cell foam up from the eaves and fill the space? Any suggestions?

Robert

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Robert,
    Where are you located? Your 5 1/2-inch-deep space doesn't leave much room for insulation.

    You're probably going to need to open up the rafter bays, or at least make some access holes to allow for insulating. Drywall is easy to patch.

  2. James Morshead | | #2

    Don’t bother with energy wasting venting. Just have a contractor install spray foam insulation and you will be much happier with an efficient insulation that actually works and controls moisture. 5-1/2 inches can be filled or almost filled with 1/2 pound, open cell foam or you can go with 3 to 4 inches of 2 pound closed cell foam. Unvented roof assemblies are outlined in the International Residential building code under section R806.4. This type of installation works extremely well. The only issues you may encounter are genuine lack of knowledge and sometimes crazy ideas about physics from some builders or building officials.

  3. Robert Riversong | | #3

    James,

    You sound like a PR man for the foam industry.

    Robert asked about insulating without opening either roof or ceiling and spray foam can't be applied from the soffits, particulary if there's already insulation in those cavities.

    Second, there is nothing "energy-wasting" about a vented roof and several prominent building scientists still prefer them to unvented roofs, particularly in cold climates (I teach building science and hygro-thermal engineering and will not install an unvented roof in a cold climate). The poster didn't indicate what climate he lives in, and if it's a cold or very cold climate zone (IEEC zones 6 or 7), then 3 or 4 inches of closed cell may not meet code minimum requirements.

    Third, neither closed nor open-cell spray foam "controls moisture". Closed cell foam is a vapor barrier but also can trap moisture leaking in from above. Open-cell foam is vapor-permeable and can hold significant amounts of liquid water. The vapor control layer should be interior of the thermal barrier, and relies mostly on a continuous air barrier. The water control layer should be exterior to the roof sheathing. If the thermal layer can also serve provide safe storage and release of water vapor, then it serves as part of the moisture-control assembly by buffering indoor relative humidity. Cellulose does this very well, while plastic foams do not.

    Depending on Robert's climate, I would agree with Martin that 5½" is not adeqate space for insulating, let alone proper roof ventilation. The ceiling should be dropped to investigate and probably remove existing insulation, deepen the ceiling cavity (perhaps with cross-hatched framing or a dropped ceiling), re-insulate and install new drywall.

    And, by the way, there's nothing "crazy" about building science and the elementary physics upon which it's based. What's crazy is ignoring it to promote a personal bias.

  4. Robert Jones | | #4

    Thanks for the replies.
    To clarify: I live in Seattle, so not a very cold (or hot) climate (which makes me thinks an unvented roof might be OK).
    I've read that closed-cell foam requires really good acces and I am really trying to avoid tearing my roof off, or dropping the ceiling.
    Robert

  5. Gina | | #5

    Robert, I've just finished gutting/insulating/drywalling a whole house, and frankly, removing and replacing drywall is the cheapest thing you can do to a house. I have two cathedral ceilings, and both were insulated with closed cell foam. One is unvented, the other has a dropped ceiling. I have documented the whole process on my website, at http://www.my-green-home-project.com/spray-foam-insulation.html if you'd care to have a look.

    Good luck to you!

    Gina

  6. John Vickery | | #6

    I've been researching this subject for a few years now as a concerned designer/builder of custom homes. However I recently took on a a-frame house built in 1977 that has never been remodeled. When I opened up the ceiling/roof I found fiberglass batts against the roof decking and nothing else. It's a t&g roof with shigles, fiberglass batts, t&g interior pine ceilings. That's the entire assembly. There is zero mold or signs of moisture damage anywhere in the assembly. However the bays between the two floors which are 2x12 and are empty have mold growth in a few spots. So for all my reading I can not explain why this assembly has no problems after all these years of no venting and no "proper" assembly.

    House located in hot springs, ar.
    Shaded lot.
    Slab floor
    Dark and very moist interior with very little windows.
    Small stem wall on all side that are rock.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    John,
    Every house is unique. The factors that may have saved the roof sheathing of the house you describe could include:
    (a) It's not in a cold climate, so the roof sheathing never gets very cold.
    (b) The roof is steep, so snow doesn't sit on the roofing for long.
    (c) It's possible that the indoor relative humidity is relatively low -- you never know until you measure it.
    (d) There may be enough air leakage at the "soffit" and ridge to provide air flow that acts the way a ventilation gap ordinarily would.

    Of course, I'm speculating. The proof is in the pudding -- the roof sheathing isn't rotten. That's good.

    Trust me -- many builders in Vermont have opened up roofs like that and found mold and rot.

  8. User avatar
    Dana Dorsett | | #8

    R19 batts are very air permeable, and on long unobstructed rafter bays you have quite a stack effect. It doesn't take a huge amount of air leakage to provide some drying there.

    Also, if the batts are unfaced or kraft-faced, the drying toward the interior would be substantial. And a t & g plank roof deck is far more tolerant of wintertime moisture drives than OSB, not to mention that those moisture drive in Arkansas climate zone 3 aren't nearly what it is in cold climates.

    If one viewed an A-frame roof as a tilted wall (which a steep roof kind of is, in locations where it doesn't hold snow), you'll note that in US climate zone 3 a 2x6 wall has no prescriptive ventilation or vapor retarder requirements under the IRC, and even roofs would only need R5 on the exterior to have sufficient R for dew point control for R25 cavity fill. An R19 batt performs at R18 when compressed to 5.5" in a rafter bay, which means R3 of exterior R would get you there, even for a low-slope unvented roof. The roof deck itself is about R1, and with air leakage and rapid draining due to slope it's not all that risky, even if it's not great.

    If the batts have kraft facers on the interior side that's actually a pretty decent if crude "smart" vapor retarder for dealing with wintertime moisture drives. If the gypsum is air tight, kraft facers would be sufficiently protective without ventilation for a zone-3 location, since it's a class-II vapor retarder during the drier-air winter months, band becomes more vapor open when the warmer springtime outdoor (or rooftop) temperatures drive moisture out of the roof decking, raising the humidity inside the cavity, which in turn raises the permeance of the kraft facer.

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