You have a few options. What I did here in SoCal for mine while I had to house them in the garage, was I had a 250 watt red bulb just pointed in the corner towards the tubs. It didn't significantly heat a particular cage, as it was a good 2 or 3 feet away from all of them, but it DID heat up that corner of the garage to about the low to mid 70s, which was much better than the 50s and 60s that was the case for the rest of the garage.

In addition, you can also cover the rack system to insulate it. Tape together some black trash bags in an extra large sheet, and then hang that around the tubs. This will probably be more energy efficient than the lightbulb idea, and much less of a potential fire hazard.

-Jen