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*Another* unvented cathedral ceiling insulation question – Zone 5B

Jack_zone5B | Posted in General Questions on

Hello ladies and gentlemen, happy new year.  I’ve scoured this site and others for the past 3 months looking for the proper way to make our finished attic more livable, and there are numerous articles (How to build an Insulated Cathedral Ceiling – https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling;  Over-roofing – Don’t do stupid things https://buildingscience.com/documents/insights/bsi-063-over-roofing; There’s Rot in the Ceiling  – https://www.greenbuildingadvisor.com/article/theres-rot-in-the-roof; Cut-and-Cobble).  I know what is theoretically correct but the old construction of our house presents enough quirks I felt like I needed to post my own questions.
Our house is in zone 5B, a few hours southeast of Spokane, WA, and was built in 1895.  It has a “finished” attic with sloped ceilings and wood floors.  Everything is 2×4 construction with 9″ shiplap on the interior and 5″ wood siding, tar paper and cedar shingles on the exterior of the sidewalls.  The roof is skip sheathing, tar paper, and D-rib metal.  I believe the pitch is 10/12 or similar.  There is no duct work as the only heat comes from a small woodstove and a small propane stove downstairs (we are hoping to install ductless heat pumps in the future).  There is no insulation in the entire house and drafts abound.
So, if I have read the documents correctly, an unvented cathedral ceiling in my zone needs R-20 of impermeable insulation to prevent condensation, which in my 2×4 rafters I could get with spray foam alone if it wasn’t so expensive.  So then I was considering the “peanut brittle” method of cobbling in reused rigid foam and spraying the last 2″ with spray foam.  However, should I be worried that the rigid foam won’t ever fit evenly against the roof because of the skip sheathing?  Aka, should I spray 2″ of foam first so that it gets in between the skip sheathing and then put the rigid foam on the interior, or would that lead to condensation between the spray foam layer and the rigid foam?
I should probably note at this time that I realize that 4″ of foam isn’t going to meet code for total insulation of an attic/cathedral ceiling, but at this point the roof is fine and I don’t really want to rip it off to put on 6″ of rigid foam.  Maybe that’s a future project?  Anyway, if I can just stop the drafts with the spray foam it will make a huge difference.
Thank you for the help ahead of time.  Perhaps someone could even comment on if they know of any rigid foam scrap dealers in the eastern Washington/northern Idaho area.

Jack

p.s. – uploading pictures failed

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    I don’t know of any reclaimed insulation guys in your area, but one of my used equipment dealers, critical power, is in Spokane. They recover industrial equipment for reuse, and they might know of some insulation reclaimers in your area since they’re in a similar industry. I’d give them a call and ask, they’re friendly people.

    Bill

  2. Jon_R | | #2

    IMO, the gap that the skip sheathing creates is beneficial and should be preserved.

    Consider polyiso (strips or continuous) below the rafters. More insulation, reduced thermal bridging and another potential air barrier.

  3. Expert Member
    Dana Dorsett | | #3

    With the skip sheathing and interior side ship-lap just 5" of continuous polyiso on the underside of the rafter edges and NO CAVITY INSULATION would nearly meet IRC 2018 code min on a U-factor basis.

    4" of continous polyiso would meet IRC 2009 code min on a U-factor basis.

    Air sealing the ship-lap could be achieved with vapor permeable housewrap detailed as an air barrier prior to installing the foam.

  4. Jack_zone5B | | #4

    Thanks for the reply everyone - I will check into that equipment dealer.

    Dana and Jon - unfortunately I didn't mention how short the attic ceiling is (about 80" at the peak, and the door frames we just took out were just tall enough for a 5'9" person to walk under). I might be able to squeeze in 1" thick foam...

    Does anyone have concerns of the ordering - whether rigid foam on the interior of the spray foam poses a problem?

    Jack

  5. GBA Editor
    Martin Holladay | | #5

    Jack,
    It's impossible to meet all of your desired goals -- to have insulation that is cheaper than spray foam, without removing the roofing, and without lowering the ceiling. You have to face facts.

    You can keep your ceiling height by installing closed-cell spray foam.

    You can keep your ceiling height by removing your roofing and installing exterior rigid foam.

    You can install recycled interior rigid foam, but that's going to lower your ceiling height.

    Facts are facts.

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