Roof Insulation – GreenBuildingAdvisor

I am trying to determine the the best way to insulate the roof. 

This is in Canada. I have a flat roof with with a 2 ply membrane over rigid insulation slopes and sheathing. 

 My first thought is 5″ of closed cell polyurethane which gets me to R 31.  This would leave nowhere for any water under the top membrane to go though. 

I could also to a 2″ of closed cell and another 6″ (or more, if people recommend) of fiberglass.  This would allow any water that gets under the membrane to evaporate downwards through the roof over time. I worry that it would also allow moisture to move the other way, up through the roof though, as per what I would expect from the default vapor drive.