Yeah, I would like to do the walls at a minimum, then the roof. The two 10’x10’ garage doors I don’t think I can insulate, since they roll up into themselves.
Those are often multiple layers and are a little better than a single skinned door, so I wouldnt sweat it too much. If you add any kind of radiant heat source, point them at the door(s).
You'll want to do ceiling and walls, even if it means adding an "attic" somehow, otherwise the cold roof will still take the heat away and can drip down in your walls.
Polyiso is cheap and light. With the plastic washers on screws used to secure it, it should be easy to put two layers on the ceiling assuming the metal frame can take it (it most likely can, especially if you get little to no snow in the winters). Since polyiso is also flame retardant and it's not a living space, you shouldn't need much if any sheathing. I'd probably furr over the final sheets and then put up a layer of 10-12mil white plastic as a vapor barrier and staple it down nice and tight to give a nice finished surface that bugs won't make nests on. Plus it will keep bugs from getting between the foam sheets as easy. But bare foam would probably also be fine for many years. Polyiso is already white, so you'll get the same light reflecting benefits either way (amazing what white ceiling and walls can do for illumination!).
