If you have racks, when it gets cold, you can just put cardboard covers over top of each bin(on top of the wire, not inside the bin) and the rat body heat keeps it pretty toasty. When I did it during a hard freeze, I had to only cover 2/3 of each bin or the rats got overheated, with no additional heat in the room. So the room got kind of close to freezing but the rats weren't even staying all the way towards the back where it was more covered. When I put a hand in, you could feel the warmth.

So if it gets really super below freezing cold, I wouldn't try to heat it TOO much, if you use the covers. Here, it's heat you have to watch and I use a window AC which hates dust but I clean the filter a lot. When I've needed heat, I use a oil filled radiant heater.