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insulation strategy for an unvented cathedral ceiling

Abram Pearson | Posted in Energy Efficiency and Durability on

I recently bought an old summer home in Freedom, NH, climate zone 6a. It is about 100 years old and currently has no insulation. The rafters are 2″x7″ (mostly, they vary a bit in dimension) 24″ on center. I would like to create an unvented cathedral ceiling in the attic space. My goal is around R50 without building down the ceiling too much. The plan I have come up with so far is this:
3″ of XPS (in 2 layers) against the underside of the roof sheathing, edges sealed with foam
4″ of closed cell SPF to fill out the rest of the bay
2″ of XPS over the inside face of the rafters
then strapping and drywall
This approach seams like it should work to me but just wanted to put it out there and see if anyone sees anything wrong with it. I have been reading a bunch on the subject of unvented cathedral ceilings and think I understand it but basically I’m just looking for a little confidence in my strategy.
Any thoughts or suggestions would be greatly appreciated,
Thanks,
Abram Pearson

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Abram,

    That's a lot of closed cell foam. Have you read this article on insulating cathedral ceilings? https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

  2. Abram Pearson | | #2

    Steve,
    I did read that article. One of the methods described in it calls for 4" of closed cell spray foam against the underside of the roof sheathing followed by 5 1/2" fiberglass batts. The strategy I am suggesting moves the spray foam more toward the middle of the assembly which I guess is why I am unsure of it. Is it essential the spray foam is directly against the sheathing?
    The reason for the XPS first and then the spray foam is because the roof is sheathed with boards and there are big gaps between them. I am pretty sure the spray foam would expand out those gaps and push the roofing up.

  3. User avatar
    Stephen Sheehy | | #3

    If the house has a typical asphalt shingle roof, you might consider reroofing, putting foam sheets on top of the sheathing. I'd do the math and compare using reclaimed xps or eps on top and fluffy stuff between rafters, with your existing plan. Closed cell foam is expensive and cut and cobble is very labor intensive, especially in an old house with irregular rafter spacing.

  4. Abram Pearson | | #4

    I am actually in the process of reroofing now. I plan on installing a standing seam metal roof. I thought about adding rigid foam on top of the sheathing but decided against it because of the added time it would take. Winter is rapidly approaching and I feel I need to get the new roof on as quickly as possible. Also adding foam on top of the sheathing means I would have to trim out a new soffit and fascia all the way around to cover the built up foam which would add a considerable amount of time and cost.

  5. Steve Knapp CZ 3A Georgia | | #5

    Abram,

    First, let me clarify that I am homeowner with an interest in building science. (We all have weird hobbies.) Perhaps Dana, Martin, or another expert will weigh in and check my answer.

    The foam has to be applied directly to sheathing to keep it warm. This is a code requirement as well. It is my understanding that working on the inside out to the eaves can be difficult in some situations.

    At the same time, proper installation is important--especially if you have board sheathing that would be nearly impossible to seal from the inside with anything but spray foam. You should also take this opportunity to air seal the attic.

    Since you are on a green site, I assume you will want to use a closed cell foam with low global warming potential. A product such as Demilec would be appropriate.

    In climate zone 6, at least R-25 of the roof's total insulation value (R-49) has to be delivered by the foam. If we assume the closed cell foam is R-6 per inch, which might be conservative, you would need about 4 inches of spray foam. (There is a thermal bridging issue, but may Dana will address how that affects the assembly.) The remaining air permeable insulation has to be installed tight against the closed cell foam.

    See this article for more information: https://www.greenbuildingadvisor.com/articles/dept/musings/flash-and-batt-insulation

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

    I was typing my response and did not see your post. If you are re-roofing, you should consider install some or all your insulation above the sheathing. This would give you the opportunity to use reclaimed rigid foam that is one-third to one-half the cost of new material.

    See this article for more information: https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

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