I have a 30x40 shop 12.5' ceilings.
I have r24 in the walls, a combination of 1" foam board, and R19 fiberglass Batts. No windows. A single man door and 2 12'x12' garage doors. Also r49 in the attic
Currently I have a 1500 w heater out there. It has kept it above freezing. But any colder...