Shop insulating advice

I would start by figuring out what insulating value (R value) you want for your climate, desired comfort level and energy usage . 2 inch foam is around R12. If you sheet the roof you could add at least R20 fiberglass insulation on top (my garage is R20 walls / R40 ceiling but it gets a lot colder here). Typically there is a plastic vapor barrier between the insulation and interior of the building as mentioned. You're going to need a lot of insulation.