I live in Ohio and have created a golf stimulator in my attached garage. The garage typically stays around 10 degrees warmer than the outside without wall insulation except the interior wall.
I can raise the temperature with a 4000 watt electric heater another 20 degrees. The ceiling and walls are all finished with drywall.
What will provide better heat retention? Insulating the walls would be a big project; one that I want to avoid. I weather stripped the exterior garage door and that helped quite a bit. I suspect the priority should be the ceiling, then the door.