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We would propose you use a rigid board insulation for a flat roof, and there are several to choose from. Probably one of the more common types of insulation used for flat roofs is rigid polyiso boards, but that has not turned out to be as excellent as originally thought. It turns out that the performance actually gets worse the colder it gets. That's an odd thing to say about insulation, but there you go. You can read more about it here -
The difference between Polyiso, EPS & XPS Foam Insulation & Styrofoam
It's still a great insulation, just not quite up to the R6.5 + levels that were originally claimed. So the more 'energy savvy' builders we've heard of have begun including a final layer of rockwool rigid insulation board on top of the polyiso to keep it warmer, that way it actually performs more like the manufacturers say it does. So for figuring what amount of insulation actually makes sense, you'd need to do energy modelling, but if you're just going to wing it, in our experience doubling building code R values generally matches up with our lifecycle analysis.