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Cathedral ceiling with chimneys and skylights – Is an unvented roof best?

Vanessa | Posted in General Questions on

We’re renovating a 60-year old house on the coast in northern Calif. (climate zone 4C), and need help deciding whether a vented or unvented roof is best. The house is 1 1/2 story and will have cathedral ceilings. The living room/dining area will be vaulted to the cathedral ceiling, and an open loft will run along the other side of the house. We gutted the interior of the house already, because of mold due to a leaky roof (the house was abandoned for years before we bought it), and fortunately the old-growth redwood and Douglas fir framing & sheathing are still in good shape. We’d like to do our best to avoid future moisture issues in this house.

I’ve read all the GBA articles and blogs (and many other reports)about insulating a cathedral ceiling, and understand that roof structures that block roof ventilation channels are problematic for a vented roof. That’s certainly the case here, as there’s a wide central chimney with 3 flues (for 2 fireplaces and an indoor BBQ), plus another small chimney and 2 large skylights, that together block the airflow in 11 rafter bays (out of 56 bays total).

We had to replace the roof ASAP, as it was leaking badly, so the house has a new asphalt shingle roof with ridge vents and soffit vents. Both the roofer and our builder highly recommended a vented roof in our climate, which is mild (usual temps. range from 40s-60s F) but wet (about 60 inches of winter rain), and often foggy as the house is on an ocean-front bluff. (We couldn’t find anyone with experience installing external roof insulation, so that wasn’t an option).

The roofer & builder said the roofing materials will get moldy if the roof isn’t vented, and the shingle warranty would be voided in a “hot roof.” They also told us that the lack of ridge vents in some rafter bays isn’t a problem, just soffit vents are enough. But I wonder if ventilation channels that are blocked at the ridge would lead to moist air being trapped in the channel, and do more harm than good.

The rafters are only 2x6s, and there’s not enough head room in the loft to make the rafters deeper. So if we go with the vented roof we plan a 1″ deep ventilation channel made with site-built 1″ polyiso foam board baffles, then 3 1/2″ rockwool batts covered with sheetrock to meet the code-minimum roof insulation requirements for a remodel (which is R-19).

Would it make better sense to seal up the roof ridge vents and go with an unvented roof? In that case we’re considering using 2″ of closed-cell spray foam under the roof sheathing, with 3 1/2″ rockwool batts underneath, covered with sheetrock. That would increase the roof’s R-value, but would an unvented roof also increase the risk of the roof sheathing (new CDX plywood laid on top of existing 1×4 shiplap cedar boards) getting moldy? I also worry about indoor air quality issues with spray foam if we go with the unvented roof.

I would appreciate any advice. Thanks!

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Replies

  1. Deleted | | #1

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  2. GBA Editor
    Martin Holladay | | #2

    Vanessa,
    It would have been best to settle this question before you installed new roofing.

    If you want to vent a roof like the one on your house, there is a solution: Above the roof sheathing, you install two layers of 2x4 furring strips. The first layer is parallel to the rafters, with one 2x4 (on the flat) above each rafter. The second layer of furring strips (purlins) is perpendicular to the first layer, installed 24 inches on center (parallel to the ridge).

    Then you install a second layer of sheathing, followed by roofing underlayment and new shingles.

    This approach allows air to travel from the soffit vents to the ridge vent.

    But you didn't do that, and now it's too late. So I advise you to create an unvented roof assembly by installing closed-cell spray foam from the interior. If you want to install site-built ventilation baffles to separate the top of the spray foam from the underside of the roof sheathing (to make this a "half-ventilated" roof) you can still do that -- but I would recommend that this "half-ventilated" roof be insulated with closed-cell spray foam, not fiberglass batts, to avoid the possibility of moisture accumulation.

    For more information on ventilation baffles, see "Site-Built Ventilation Baffles for Roofs."

  3. Vanessa | | #3

    Thanks for the suggestions, Martin.

    I did try but was unsuccessful in finding a roofer or builder who would install a roof with external rigid foam board insulation, furring strips, and a second layer of sheathing to form an external ventilation channel. I gave the roofing companies and our builder copies of relevant articles (including yours) on building an insulated cathedral ceiling. But that's not how they build roofs around here, and there was a lot of resistance to trying something new. So we had to settle for a conventional roof design, as the roof was leaking badly and we couldn't wait any longer to get it re-roofed.

    If we install 2" of closed-cell spray foam from the interior, without a ventilation chute, that will meet the minimum R-value for condensation control in climate zone 4C (which is R-10). Would it then be safe to install rockwool batts in the rest of the rafter bay below the spray foam? Or would that increase the possibility of moisture accumulation compared to all spray foam?

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