Insulation, Radiant and Vapor Barriers - Pole Barn Insulation Advice/Question

I have a 32' x 48' x 12' that I am insulating, i have the perlins running horizontally 2' on center. I bought 1-1/2" thick polysterene 4' x 8' sheets of insulation, the R value is 7.5. Is this a good enough r value to keep it warm in the winter, i live in SE lower Michigan, temps are usually in the 20's. Should I cut the sheets to fit in between the perlins or should I leave the sheets whole and place them on top of the perlins? If i leave the sheets on top of the perlins would it be a problem to have the air gap of 1.5" from the top/bottom of the perlins to the steel or is this good? I am placing 1/2" OSB on the whole inside. I have also tossed around the idea of running 2x6 on the perlins to make all the OSB on the inside wall flush, this would still leave a rather large gap of air or I could just return the polystyrene sheets and then get batt insulation that is thicker, i think r-13 which is 3.5" thick??

Will there be livestock in the barn during the winter? Or is the barn for some other purpose?

No livestock, barn will be used as a workshop. I am just concerned that the R-7.5 will not be enough insulation and I am unsure on which way to insulate to get the best results considering I have already purchased the polystyrene but I can take it back and buy batt insulation which would save me money that I could then spend no the 2x6's to get the flush look of the whole inside wall.