an alternative would be to use a liquid other than water. For instance Glycerol freezes at 17C. If you had a big tub of that, you could hold the room a little warmer.
But its also a two edged sword, all this thermal mass makes it difficult to ever warm your garage too.
I had been trying to think of a different substance. Doesnât NEED to be liquid. But I wasnât able to think of anything else with the phenomenal heat capacity of water off the top of my head. Glycerol has a higher heat capacity, but 17C is quite a ways away from freezing, but nicely near room temperature. So if you happen to have a bunch laying around it doesnât hurt much of anything for certain.
its true, but latent heat of fusion is such a great place to dump energy. 
Another alternative could be to just put a small heat source in the gf when you leave. A small lightbulb or some other big resistor would do the trick.