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What is the best flash-and-batt approach for a low-pitch unvented cathedral assembly?

rawlinson | Posted in Energy Efficiency and Durability on

New 2600 sf house is a low-pitch 1 1/2 : 12 shed standing seam metal over plywood sheathing on 1 3/4×14 LVL’s 24″ o.c., drywall ceiling. Zone 3 SF bay area; site is a north facing downslope just into the hot-dry 3B Zone, but sharing much of the climate of San Francisco’s 3C. I have in mind 2″ closed cell foam, either rigid panels sealed in place to joists with spray foam, then filling remaining cavity with 12″ net thickness R38 batts; or 2″ nominal spray foam, with 12″ of R-51.6 blown in spider fiberglass secured by netting. The foam, in whatever form, would seal the cavity and prevent condensation in the fiberglass.
1. I think the 14″ cavity with all foam would be costly and too thick a build up of foam, with multiple passes required, and not easy to justify to owner. But bringing in the special spray installation equipment just for 2″ would also command a premium price.
2. Using a layer of sealed rigid foam would make for a smoother interface than sprayed foam, if 12″ batts are used to fill the rest of the cavity.
3. Because there will be residential sprinkler piping as well as wiring in the cavity, the blown in fiber will make a more consistent fill, and gets a higher R value than batts.
4. Because the roof rafter assembly is already thick to meet structural needs, and potentially provides plenty insulation, I prefer to not add rigid foam above the roof deck– although it would be a thermal break it would make it harder to hold to my height limit and would complicate fastening the metal roof pans.
5. The metal roof is not sensitive to heat build-up, so I think unvented is okay and better than an air chute. An airspace underlayment above the deck would allow moisture and some air to boil-off from the bottom of metal interface.
Thanks for advice on any problems I’ll encounter and what you think is best approach.
Brian R

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  1. GBA Editor
    Martin Holladay | | #1

    One of the methods you propose — namely, "rigid panels sealed in place to joists with spray foam, then filling remaining cavity with 12 inches net thickness R-38 batts" — is known as "cut and cobble." I don't recommend this method for cathedral ceilings or roof assemblies. To learn why, I suggest that you read Cut-and-Cobble Insulation.

    The best way to insulate a low-slope roof, by far, is to put some or most of the insulation above the roof sheathing. To learn why, I suggest that you read Insulating Low-Slope Residential Roofs.

    You don't really have enough room in this roof assembly for the type of low "attic" that would be required for a vented roof assembly, so you need to concentrate on unvented approaches. Of all of the approaches you list, the best is flash-and-fill (a layer of closed-cell spray foam on the underside of the roof sheathing, with the remainder of the rafter bays filled with blown-in fiberglass).

  2. rawlinson | | #2

    Thanks Martin. Flash and fill would give me about R-60 before reduction for joists. I'm thinking of 2 inches polyiso closed cell as about R12, which exceeds the R-5 minimum cited for Zone 3 to protect against condensation. Can I count on that thicker-than-necessary spray application to seal against air leak better than a thinner coat, because of a larger interface? The rafters should have very little shrinkage because they are engineered LVL's so start out very dry.
    In reference to the relatively poorer performance (vs EPS) of polyiso during winter, although this site requires more heating than cooling, it doesn't get extremely cold, and I hope the good performance during hot weather will reduce need for cooling to a bare minimum if at all. The foam is there primarily to address the condensation-in-unvented attic issue, not so much to enhance the overall R value.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Can I count on that thicker-than-necessary spray application to seal against air leak better than a thinner coat?"

    A. Probably. But the most important factor determining whether you end up with an airtight job is the skill of the installer.

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