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Insulated cathedral ceiling question

user-6900833 | Posted in General Questions on

I reviewed your article on this topic (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling) but am hoping to get your thoughts about my specific variation in mind.

I’m constructing a small (12×24) guest cabin with a “modern” post and beam frame. The plan for the (cathedral ceiling) roof assembly is v-groove over the rafters then 2 layers of rigid polyiso sheets staggered and taped, house wrap then Advantech screwed to the rafters followed by asphalt or fiberglass shingles over felt paper or whatever material is appropriate for the product.

Our larger cabin was constructed with metal roofing with, from the bottom, v-groove, foam, house wrap, horizontal 1/2″ strapping screwed to the rafters (so an air space) then metal screwed to the strapping. This has performed well thermally for 2 years including 2 NH winters and once the structure dried out there have been no apparent moisture issues.

The specific question – is there any reason the air gap used with the metal construction is needed with the asphalt or FG shingle construction? Any critique of either is welcome. Thanks, MF

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    MF,
    An air gap is not necessary (although a vent channel between the top of the rigid foam and the upper layer of roof sheathing will reduce the chance of ice dams).

    Remember, the rigid foam has to be very thick to meet the minimum code requirement of R-49.

    Housewrap is for walls. You need roofing underlayment, not housewrap.

    Remember to detail the lowest layer of rigid foam for airtightness. Even better: include a durable air barrier between the tongue-and-groove boards and the lowest layer of rigid foam.

    More information here: How to Install Rigid Foam On Top of Roof Sheathing.

  2. Jon R | | #2

    Without a vent channel, air-tightness is significantly more critical. Multiple air barriers will add air-tightness.

  3. user-6900833 | | #3

    Martin and Jon - thanks for the quick response, and the link to the how to install rigid foam article, lots of good info there, I will ponder. Our large cabin only has 3" polyiso on the roof boards, then horizontal strapping screwed through to the rafters then metal roofing screwed to that. There is a layer of house wrap in there too, can't remember if it was over the boards or the foam now. Despite the far less than code r-value the house performs well in cold mid-NH winters (2 so far). Last heating year we used 400 gal LP and 2 cords of wood for heat and DHW, the footprint is just under 1000 sq ft, full 2 levels are heated, basement level has radiant (Bosch combi boiler), space heat (Rinnai and soapstone wood stove) on ground floor. Thanks again for your advice and info - Martin Frank

  4. GBA Editor
    Martin Holladay | | #4

    Martin Frank,
    It's a shame to invest in a new timber-frame building in New Hampshire, and then to install below-code levels of insulation. Three inches of polyiso will give you cold-weather performance in the range of R-15 to R-18, which isn't much at all. (For more information on this issue, see Cold-Weather Performance of Polyisocyanurate.)

    I urge you to consider installing much thicker rigid foam.

  5. Expert Member
    Dana Dorsett | | #5

    For the existing cabin, in most of NH (US climate zone 6) the 3" of polyiso is good enough to allow an additional ~R18 below the roof deck without much moisture risk. In southern NH (zone 5) it's enough for R25-R27 under the roof deck. If it's possible to retrofit that, it's "worth it", but how much is prudent depends on your location.

    On the new cabin, two layers of 3" or 3.25" polyiso would be sufficient to hit code-min performance on a U-factor basis. This can be pretty cheap to do if using reclaimed foam (there are several regular vendors in New England), typically at less than a third the price of virgin-stock foam. But 4" of polyiso (2 layers of 2") above the roof deck with R20-R23 batts in 2x6 rafters under the roof deck also works, and may be easier to assemble.

    While the ventilation gap between the foam & Advantech isn't absolutely necessary with a shingle lay-up, it's still a good idea.

  6. user-6900833 | | #6

    As actual construction nears I'm back hoping for a bit more advice/clarification. Starting from the rafters (2x6 24" OC): 1x8 v-groove pine, a durable air barrier (would GCP Tri-Flex XT work?), multiple layers of foam taped at seams, 2x4 on flat over each rafter perpendicular to ridge, 5/8 Advantech Zip, asphalt shingles. Does this sound right?

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