My garage is attached, not insulated, 2 car with a single 17' insulated door. Previously, I had no heat, when I cleaned up the garage, I installed a rubber floor from rubberfloorinc.com, that winter, I noticed that my garage that was normally 10-15 degrees colder than outside temps, was now 10-20 degrees warmer.
Still cold, but not the bitterness of prior years, I have lived here for close to 25 years, the following year l installed a natural gas vented heater, I leave it set on 60 degrees.
The rubber floor did the trick for me, maybe it will for you? It is expensive stuff though, either rolls or tiles, I would recommend rolls for water runoff reasons.