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Thank you. This is the most comprehensive easy to understand tutorial on flat roof construction I’ve seen.

"Polyiso: At a mean temperature of 75°F, the polyiso sample also exhibits a level of thermal conductivity which correlates to its published R-value of R6. 0/inch. However, as the mean temperature drops to 15°F, the R-value decreases to approximately R2. 0, representing a significant 66% loss in R-value." It also costs more than EPS or XPS. I would consider burying the the polyiso under a blanket of XPS so it will not be subjected to the most extreme cold, and thus, retain its marketed R-value. Note, Polyiso's R-value begins its plummet around 55F. So, IMO, Polyiso is a warm climate insulation.

I am doing warm roof on 6/12 pitch. Exposed rafters and roof board inside. I like mineral wool but it does cost. With post and beam timber frames we try to keep moisture going out of structure. I am used to seeing thick roof assemblies with purlin.

If I would like to have a decking surface on top of the flat roof it seems strategy 4 is the only real way I could do that. Would you agree or are there other options that could be used? What do you think the best way to insulate it would be? Thank you.

I'm assuming all 5 strategies listed would be for an unvented roof design, correct? Is there a reason why in Strategy 2 and 3 you cannot run ductwork in the rafters/trusses/roof framing area? As long as the insulation is touching the underside of the roof deck in an unvented design, this should be an acceptable practice, correct?