I am putting a 5" slab in my new shop and want to insulate it. What type of insulation is proper for this application? Thanks

Where are you located? What kind of loads do you intend to put on this slab? Are we talking regular passenger vehicles or commercial duty vehicles and equipment?

I am in kansas and i will ber putting suprduty pickups on the slab , mostly 5250/f350 which weigh anout 7,500 pounds. however there will be the occasional F450/F550 which weigh around 10,000 pounds.

XPS or Type IX (2# density) EPS would be my choice. Both have a compressive strength of ~25 psi or 3600 pounds per square foot. My experience is that Type IX EPS is 30% to 40% less costly than the same thickness of XPS. Long term R value of both products would be comparable for the same thickness used. You would probably want to use ~2" thick material or a double layer of 1" with joints off-set.