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Best use of 11 7/8″ space in a cathedral ceiling

John Tillinghast | Posted in Energy Efficiency and Durability on

 

I am building a Cathedral ceiling great room using 11 7/8′ engineered I beams in Zone 4A.  A vented ceiling is required.  No exterior insulation will be possible on the roof.  I’d like to know, given the constraints of the 11 7/8′ space, what the optimal configuration of insulation would be?  The roof pitch is 10:12 and the exterior material will be metal.

 

I saw somewhere where a builder used foam sheet to create the air channel.  Could that be done, then an inch of closed cell [to create an air barrier] and then the remainder some less expensive blown insulation such as spider insulation?  Is there a better channel?  A better combination of materials?

Thanks

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Replies

  1. Expert Member
    Peter Yost | | #1

    HI John -

    Interesting that you say venting is required; is this your requirement or someone else? Code official? Just curious why this option is off the table.

    A great GBA resource is Martin Holladay's "How To Do Everything" - https://www.greenbuildingadvisor.com/article/how-to-do-everything . And within that collection this one on cathedral ceiling/roof assemblies: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling. I do like the approach of putting in stops to form the vent channel and then air sealing in the rough-cut rigid.

    Keep in mind that while venting provides drying potential, the best approach is to keep the assembly dry to begin with. That means a continuous water control layer (flashings connected to the WRB) and a continuous air control layer, the latter confirmed with blower door testing.

  2. John Tillinghast | | #2

    It is only required because all 3 builders and the building supply guy told me the same thing: they won't build a house without one. They also won't use many of the best techniques presented here in GBA. It is rural MO and unless I want to personally do the work [which I cannot] I have to work within the constraints of the tradesmen.

    Thanks for your references.

    1. Expert Member
      Dana Dorsett | | #4

      If going unvented, in zone 4A it would take 3" of closed cell foam(R18 minimum) to have adequate dew point control on 8-7/8" of 1.8lb Spider (~R37.5).

      That would end up at R55.5 total, with ~32% of the total being the closed cell foam on the exterior side of the fiber. The minimum safe fraction for keeping the fiber dry enough in winter would be ~30%.

      If insulating the cavity with a full 11.875" of a lower 1.5 density Spider would come in about R47.5, and would need R21 of insulation above the roof deck , either 5" of EPS, or 4" of roofing polyiso secured in place with an OSB or plywood nailer deck through-screwed to the structural roof deck. Reclaimed roofing polyiso is often available and is dirt-cheap compared to 3" of spray polyurethane, as long as the roof likes are pretty simple, keeping the installation labor low.

  3. Expert Member
    Michael Maines | | #3

    John, I prefer to vent roofs when possible, as it minimizes the use of foam (and the environmentally destructive emissions from the blowing agents used in most foam insulation). When builders have not yet entered the 21st century with their building techniques, at least venting is also forgiving of less-than-best practices. A benefit of using I-joists is that the top flange makes a great spacer for site-built baffles. I like to use 1/4" plywood for baffles but rigid foam is another option.

    Zone 4A roofs require R-49, according to the IRC. Even if you don't have to follow the IRC (or any building codes), that's still a good baseline for a minimum amount of insulation. But if you use the top flanges for baffles, that leaves you less than 10.5" for insulation. With dense-packed cellulose, my first choice in most cases, that leaves you only R-36 or so. You could add 3" of EPS at the interior side of the rafters to reduce thermal bridging and get the center-of-cavity value up to R-49, while still allowing a bit of drying to the interior, and the blowing agents in EPS are not as bad as in other foam products. There are other, more environmentally-friendly materials that could replace EPS, such as rigid mineral wool or wood fiber insulation, but I doubt your builders would use them.

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