Insulating a carport closet

I have a 10' X 20' carport closet that I am converting to a small workshop. I want to insulate it properly. The carport has a flat roof that doesn't leak. I plan on putting batten insulation in the walls r13 for 2x4s. and r30 between the 2x10 support beams in the ceiling. Do I need to worry about a vapor barrier creating a problem in the roof since there will be no space between the insulation and the roof decking? I live in southern oklahoma where we have very few days below freezing and it is very hot in summer so I don't even know how much of a problem condensation is in general. Any advice would be appreciated.
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