What is the best flash-and-batt approach for a low-pitch unvented cathedral assembly?
New 2600 sf house is a low-pitch 1 1/2 : 12 shed standing seam metal over plywood sheathing on 1 3/4x14 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.
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Posted Thu, 02/27/2014 - 19:28
Edited Fri, 02/28/2014 - 05:42
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