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Zone 7-8 cathedral ceiling insulation

MedicineMan4040 | Posted in General Questions on

Building in Homer Alaska/Zone 7.
Cathedral ceiling.
Money is not an object.
I want the warmest ceiling possible.

What is wrong with, from the outside in, metal roof, wood deck, 10 inches closed cell foam within 10″ rafters.
Then 1 inch foam board at 90 degrees to rafters.
Then channel rustic cedar or something similar.

If I can omit the wood deck and spray directly to the metal even better.

And a thank you for this site. I’m abs. clueless on construction of even a dog house. The body of knowledge here is a bit overwhelming to say the least!

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Replies

  1. Expert Member
    Rick Evans | | #1

    Medicine man,

    This ceiling would have an effective R value of about R-56. In such a cold climate, I would want a warmer roof.

    Also, spray foam packs a big environmental punch unless you use the foams with the newer blowing agents.

    I would definitely not spray directly to the metal roofing. If you do, it will be extremely costly to ever remove it or repair it as 2#foam is essentially glue.

    Consider a flat but high ceiling with raised heel trusses. Blow in 30" of cellulose and get R100 for a fraction of the price of the spray foam cathedral ceiling.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Medicine Man,

    I will agree with everything Rick had to offer. If you do go with an insulated roof line, there are plenty of option for minimizing the spray foam. Have you read this? How to Build an Insulated Cathedral Ceiling

  3. MedicineMan4040 | | #3

    Rick, Brian thanks for the input.
    Please give me the outside to inside layers (or inside to outside) so I can mentally picture this.
    Rick, you mentioned in that environment wanting a warmer roof---wamer/higher R than R-56?

    The linked article. Yes, have read it twice and (remember non builder, unschooled in building science), and will read it several more times for sure.

    At this point I've not reached a consensus on a sealed 'hot' roof versus one that breathes (ventilated roof).

    In a cabin we have up in the mountains there were large exposed redwood beams forming rafters and between them I installed 6 inches of expanded polystyrene/foam board. Running 90 and attached to the large rafters is cedar. The foam ends 2-2.5 inches from the roofing deck. The eaves have ventilation grills near the top of the outside walls. and at the ridge line of the roof is a vent running the entire length of the roof.
    Even here in the south eastern U.S. we've seen -32F there (elevation) yet the propane delivery man says I use less propane than anyone else on the mountain. The sides of the cabin are also 6 in foam board in a 'dryvit' system. Both the ceiling insulation and roofing insulation have been in place for 15+ years and never a problem. Maybe I should just emulate what we have here when we build the retirement home in Alaska (except I want radiant floor heat next time :)

    Thanks again for the input. So much to study!

    Robert

  4. Expert Member
    Dana Dorsett | | #4

    >"What is wrong with, from the outside in, metal roof, wood deck, 10 inches closed cell foam within 10″ rafters."

    The 10" of closed cell foam would be one of the primary things wrong with it. Closed cell foam (even HFO blown foam) is one of the least-green insulation materials in common use today, and it's one of the most expensive $/R methods going.

    For green high-R assemblies cellulose is a better fit, and even an R100 cellulose roof would cost quite a bit less than the R60-R70-ish closed cell foam.

    The warmest ceiling possible would be an R75+ cellulose in ~20"-22" deep open web or I-truss-rafter if insulated at hte roof assembly, or R90-R100 at the attic floor with vented attic (preferred). Beyond those levels the comfort aspects aren't much improved, and the cost isn't necessarily financially rational. See Table 2 p10:

    https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

    With a 22" truss-rafter the top chords can be utilized as a spacer for a vented roof deck, using half inch MDF or CDX as the exterior side air barrier for the cellulose. In zone 7+ it's worth using a 6 mil polyethylene vapor barrier detailed as an air barrier between the ceiling gypsum and insulation even with a vented cathedralized ceiling, and keeping the penetrations of that layer to an absolute minimum. It wouldn't be crazy to use OSB to sheathe the underside of the rafters, and install 2x4s perpendicular to the rafters under the OSB as a utility chase for the electrical, hanging the ceiling gypsum on the 2x4s.

    If vented attic, soffit-to-ridge venting with 60% of the total vent area at the soffits, with an interior side vapor barrier under your 28-32" of cellulose works. Be sure to engineer the joists for the dead-loading of the cellulose, which adds about 5lbs per square foot @ R100. The service chase approach to the electrical works there as well.

    Any vented roof deck approach not only keeps the roof deck drier, it also keeps the roof deck cooler, mitigating against ice damming issues (not that it ever SNOWS in significant amounts in Homer AK :-) )

  5. MedicineMan4040 | | #5

    Power packed reply and yet more to take in and study!!
    Thanks so much for all the information.
    It does snow in Homer.....nothing compared to Whittier or Valdez though.
    Ironically it is, on a day to day basis in January/February warmer in Homer than
    my home area of Roan Mountain TN....Home of course has ocean to buffer the temps
    and Roan does not (we're actually 2000 feet higher than the village of Roan Mountain).

    Oh, post reading the reply I had to google truss-rafter :)

  6. Expert Member
    Dana Dorsett | | #6

    >"...I had to google truss-rafter :)"

    I just googled it too, and it didn't immediately come up with the right image. I was talking about open-web trusses (as are often used for large span floor joists), which can be engineered to handle truly massive snow loads if necessary:

    https://www.tra.org.uk/wp-content/uploads/5790f49ba436d.jpg

    Or alternatively, I-beam type trusses used in similar floor applications, also commonly used in higher-R vented cathedralized ceilings:

    https://s3.amazonaws.com/finehomebuilding.s3.tauntoncloud.com/app/uploads/2016/09/19115917/Rafter3-700x695.jpg

    >"It does snow in Homer.....nothing compared to Whittier or Valdez though.
    Ironically it is, on a day to day basis in January/February warmer in Homer than
    my home area of Roan Mountain TN"

    I know what you mean about the "Alaskan Riviera" climate being not very cold. A friend of mine lived in Unalaska during most of the 1980s & 1990s, then got his nose frostbitten when he moved to Duluth MN.

    Proximity to the warm Japan Current often brings in massive snow storms, but I wasn't sure how often those made it into Homer. In the Aleutians it can come as either rain or snow- the first winter my friend was there he and a friend got snowed in for days taking shelter in an abandoned WW-II quonset hut, only to have feral horses trying to kick the doors in a day or two before the snow had sagged to a level that made it possible to open the door.

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