Unvented attic details, yet again
Hello all, long time reader here, first time poster and I want to thank you all for the valuable insights throughout the years.
And sorry for the lengthy post!
I’m building a residence in the Verde Valley of Arizona and framing has just started. I’ve been reading and studying all the “best practice” details for an unvented attic but I haven’t found a satisfactory, concrete solution. The house is in zone 4b but these are by county, and my location is at the lowest elevation, to which I argue it’s bordering zone 3b. (Camp Verde is at 3100’ while Prescott is more than 2000’ higher). Anyway I feel that moisture just isn’t as much of a concern here, allowing much more grace with building science and thus less attention in general is focused on this region.
General build info: insulated slab w/radiant heat plus forced air heat pump. ducts are in the attic. Double 2×4 walls with a 2” gap (9” total) and permeable SA v.b. Monopoly framing with mostly no overhangs . Trusses with <2/12. My current roof detail is as follows: Trusses 24” o.c. 1/2” decking. Henry blueskin pe200ht sa (or similar). Rainscreen. Standing seam metal (22ga with clips). Insulation flashed with ccsf under the roof deck, then built up opencell to reach a min r50.
A conditioned attic is a must for me. Exterior rigid insulation on the roof is possible IF I’m really convinced, but labor is greatly increased on my part.
My questions are:
What are thoughts on a roof membrane product like Vapor shield slopeshield SA that is permeable? Would this not allow drying and reduce the need for so much moisture concern?
In this case would only opencell work under the roof deck allowing drying both ways?
Does a hot dry climate affect these decisions?
My thoughts on foam are: its more costly, its more flammable, and I’m a little concerned about sound transmission of rain/thermal expansion of the metal roof (longest panels will be 38’)…I’ve had creative thoughts on getting alternative insulation between the truss bays.
Insulation contractors around here say “we just spray everything with opencell” or their eyes glaze over when I start talking “building science”. Maybe a lot of Arizona gets away with winging it. Maybe I’m overthinking it…
Your input is greatly appreciated! -Zac
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Replies
Zac,
Conventional gable trusses limit the options to insulate a conditioned attic, as they are really designed to accommodate the insulation and the air-barrier on the ceiling. Anything but spray foam is problematic because the truss webs make a warm-side air barrier/ vapour-retarder difficult. Exterior foam on its own works, but hybrid assemblies with batts below can be a problem too.
I'm going to leave it to others more familiar with your climate to weigh in on the risks of open-cell foam of that depth, except to say that if you want to rely on drying to the outside to reduce the risk, a permeable roof underlayment on its own doesn't help much. You need a vent channel, either below or above the sheathing, with inlets and outlets to provide a mechanism to remove the moisture.