Garage heat

When I first built my garage (20'x28') I put plastic vapor barrier on all walls and the ceiling. I used 2 of the " sunflower" top of the tank models( propane) . As I insulated each wall , I just tore the plastic down and replaced it. Now insulated , I use a small construction box heater (220 v) and once it gets to 65° , it barely kicks on.
( I used a regular household box fan behind the sunflower heater to blow the air further, It helped)