My barrel sauna is made of Nordic Finnish pine instead of cedar and is not very air tight.  Winter alpine temps in the -20s F , heavy winds, snow.  And I’m certain my heater is low powered, but don’t want to dig up and replaced the supply wire size to allow upgrade. 
      I’m thinking of insulating my barrel sauna roof via layer
1.), a roll of SmartSHIELD -10mm 48″ x 25ft Reflective Insulation roll, Foam Core Radiant Barrier, Thermal Insulation Shield – Engineered Foil ( will this double as a vapor barrier or do I need sheet plastic under it?)  Then 2.) I  was going to score and bend purple rigid foam insulation boards taped together over that. I have a spare supply I need to use or donate. And
3.) And screwed on aluminum metal roof panels over top.
***Any parts of this roof  insulation plan needing revamp to succeed?!?