Designing an Unvented Roof
My “unvented” head has too much info to configure this roof!
I’m going down the rabbit hole and/or getting too far into the weeds about the unvented roof I’m trying to plan. There’s so much information in the Q&A section; some old, some new, some science, some “best practices” and I think I’ve gotten too much information swimming in my head I can’t sort. Can someone please point me in a better direction because my architect needs some better guidance than me giving him 9 different “working” examples I’ve found. I think I need help with some details after a hundred hours of reading…
· Climate 4a (Middle TN). Desiring high performance roof.
· Single ridge, 6:12 roof with standing seam metal roofing. Think large simple design like a rectangular ranch/bardominium style house. Simple design and engineering, but large roof sq footage.
· Roughly 2/3rd of the house (rooms split equally between either ends of the rectangle) is cathedral ceiling with the remaining (center section) 1/3 a large, conditioned attic that’ll contain the HVAC and ERV.
My latest idea:
· Entire roof unvented. Overhangs to meet some* passive standards. R- 54 total. From the conditioned side going outwards:
o Flash and batt (2 inches of closed cell spray foam under decking) and as much fiberglass as the rafter bay will allow. I’m thinking R-19 batt, R-14 spray foam (R-33 total interior)
o Traditional plywood (or OSB if Hunter will allow it their panel to be screwed to it) deck nailed to the top of the trusses and/or rafters.
o 4” of exterior polyiso insulation on top of the decking in the form of a Hunter panel (H-Shield NB). R-21 is what they report. Taped seams. Their H-Shield NB panel is OSB topped.
o Synthetic underlayment
o Standing seam metal roof attached directly on top of the synthetic underlayment screwed into the OSB Hunter panel.
My questions I’m turned around trying to solve:
· Is the first layer of decking (the one directly on the trusses) going to have issues being sandwiched between closed cell foams (spray and polyiso)? Should I be concerned with which way the roof dries, which admittedly is confusing? Doesn’t seem like the truss sheathing will dry any direction. Should I abandon the flash and batt here? Is this what ICC R806.5 is talking about with the class 1 vapor retarder?
· Will the polyiso have so much thermal drift I’ll lose the roughly 31% delta recommended exterior R value that Martin Holladay wrote in his articles? Also, ICC R-15 min for CZ4. Thermal drift has been a vague research topic, and aggressively negative if you manufacture EPS…
· Do I need underlayment on the first layer of sheathing, or just the layer directly under the metal roof?
I’m thinking of the roof this way to try to somewhat limit complexity for the labor, who seem to be throwing up cheap houses to take advantage of the hot real estate market. Faster and cheaper seems to be the builder mantra in Mid TN and my confidence is waning in finding a builder with A+ crews. I think most crews could accomplish this idea, especially a decent roofing crew who would finish everything after the first layer of sheathing. I thought about doing the entire roof with SIPs as there is a manufacturer locally, then I saw the cost… I can essentially fabricate the SIP on site using conventional tools, lumber, and labor with Hunter H-Shield NB panels for about half the cost (not as much R-value, but still). Please correct me if I’m wandering off the rails here. Thanks in advance.
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