Ceiling – sheetrock over styrofoam on a garage ceiling

I have a few questions and observations that will effect what you should do. First, I see that the peak (ridge) seems to be over the existing framed and drywalled wall. What is on the other side of this wall? Is this garage attached to the house or free standing? Do you have or intend to put any heat or heat dump register in this space?

I also noticed that the roof framing is standard trusses, not attic trusses. Where is the attic space going to be located? If the attic space is going to be over the bay you pictured, then you are limited in the amount of insulation you can install and still have a floor for storage. If the collar ties are 2X6, then the most blanket type insulation you can install is R19. 2X4= R11. Fiberglass blanket would definitely be the most cost effective in a small space such as you have. I would use unfaced F/G, and a 4 or 6 mil plastic vapor barrier installed before you drywall. As for installing drywall, you really should install strapping at a max of 16" on center to mount your 5/8" fire rated rock to. 24" inch on center is too large for hanging rock overhead, especially 5/8" rock. If garage is attached, 5/8" drywall is minimum code in most areas.

I also noticed that you have a metal roof. Metal roofs are great, but are notorious for condensation. This makes it absolutely necessary to have a good vapor barrier and good venting to avoid potential interior "rain storms" lol.

My other consideration would be insulating the exterior walls. Since there seems to be only one long wall and very little framed cavity wall on the gable ends, it would be advantageous to go ahead and finish the job by insulating and drywalling these walls. If you only do the ceiling, you will not have a good barrier to air, heat and humidity leaking by the ends of the ceiling at the wall plates and entering the attic area. This kinda defeats the whole idea of what you are doing.

Definitely close off the ceiling rather than plugging the vents, IMHO.

The "fanfold" insulation [typically 1/4" X 4ft X 50 ft - R1 possibly some thicker versions available, on a quick look around - thicker would of course be higher R-value, as well] intended for use under siding (and sometimes roofing) might be another option, in larger sheets (less joints to seal.) More than just vapor barrier...and it might support more serious insulation (just not too much) above it.

I presume that you recognize and accept that until you do drywall over it, this (or vapor barrier, or XPS sheets, etc...even exposed fiberglass, to an extent) is a fire hazard. In a similar situation, I'm putting my effort towards getting the drywall on the ceiling so I can insulate the heck out of the ceiling - a 5000W heater does not go very far, otherwise. Specifically, 5KW * 3413 btu/kWh = 17065 btu/hr (input) - .vs. 720 sqft (30x24) of R1 (1 Hr x sq-ft x degree F/BTU) ceiling would only net you 23.7 degrees (F) (17065/720) above outside ambient if that were the ONLY place heat was lost (floor, walls, doors all infinitely insulating, and zero air exchange - obviously an incorrect assumption - just illustrating the point.)

If the walls are 8 ft high, amounting to 864 sq ft, plus the 720 sq ft of ceiling, and we assume that all are insulated to R11 (optimistic for many garage doors and all regular doors & windows - but low for most modern walls and ceilings) the same 5KW could heat to 118 F above outside ambient (but I'm still ignoring the floor and air exchange...)