"#1 DO NOT use foam insulation with out using sheet rock unless you like dangerous
house fires with deadly smoke and quickly spreading flames."
hmmm... would really matter? I mean, by the time fire burned thru an inch of cedar, the place would be filled with plenty of deadly smoke and quickly spreading flames. I wouldn't use foam for other reasons, but smoke toxicity is not one of them. ...because I can't think of a
good house-fire smoke.
Span sag: Every span sags. Even steel floored buildings have camber built in such they become closer to flat when they sag with people and their stuff.
On SAT's house... I expect that if anyone asked the Q of "New construction roof, 2x8 flat, on 6' centers of 4x6 beams -okay?" people would say NO - too flexy/saggy" --Is it?
What is the span of those 4x6 main beams? ...will adding the weight of 5/8 sheet-rock (+ extra hangers/nailing-strips) be too much? -- a falling structure can kill faster than fire. ...since we are being extra safe here)
Maybe you should ditch the idea of F-Glass insulation with cedar covering and go to Aerogel insulation with Nomex honeycomb panels. ...don't forget to instal redundant fire suppression systems.
Vapor and venting: Just what is the climate there? Most venting that I see strikes me as complete over-kill. But then again my POV comes from handling the water vapor that drives to the outer skin of a jet - you know, bags of moisture breathers in an aluminum tube flying in -100+ degree air. (big moisture gradient, as it freezes to the inside of a non-permeable surface)
Anyway, that's my 2¢ worth, but you know I'm unconventional, so you may just want to knock it all down and build something to today's code. --
else it's all wrong.