I put my fermenter in a cooler, then add a water bath. In the water, I place a very small (I think 8 watt? but I"m not sure) aquarium heater. It takes some adjusting at first, but it works great to keep the beer at 64 degrees in a 55 degree room without any worries.
Broken Leg Brewery
Giving beer a leg to stand on since 2006