I've got a free standing 440 sqft garage on slab. The walls are 2x4 insulated with batts, vapour barrier and 3/8 spruce sheeting inside. The roof trusses have vapour barrier and 6+ inches of batts up there. Between each truss on the soffits there is a moore vent. The garage door is about 1 3/4 foam insulation. No windows, only a man door. I have a Cozy brand gas, direct vent wall heater, costs me about $100 per winter(dec to april) and I have the thermostat set at +7.