Garage ceiling insulation

On the other hand, proper air barriers are important to all forms of insulation. A proper ceiling should have well taped and sealed drywall, and fiberglass batts (if used) laid up tightly against the drywall. Even a small air leak is a much worse loss of energy than a section that is under-insulated but well sealed. And moist air through leaks will transport much more humidity into a wall or ceiling cavity than diffusion ever will. Fiberglass does not come up to its nominal "R value" if air can circulate around one or both sides of it.

Styrofoam is pretty much impervious to water vapor. You certainly don't need any more vapor barrier than that. Especially not if the attic space is ventilated (no vapor barrier at all is required by code below a ventilated attic). The kraft paper on your batts would be considered a "double vapor barrier" in this application and code (and good practice) would normally call for the paper to be slit with a knife or removed entirely.

I think the main issue with your ceiling is how well the styrofoam is fit together and kept tight and air sealed. Ideally you should use tape to seal the joints between each piece of foam. It also isn't clear how it is fit to the interior ceiling. Foam block products are not safe to leave uncovered; especially not in a welding/plasma cutter type of environment. It isn't all that flammable (not like kraft paper which is very unsafe to leave exposed) but foams of all sorts develop a very dense, nasty, and toxic smoke in the presence of open flame.