I use Kane Heat Mats. You can buy them on any number of links on this site. You can get a relatively inexpensive thermostat, and keep the temperature around eighty degrees. This usually heats the air to around sixty five on a cool day in South Texas, which, like Houston, rarely falls below 32 F. I use infrared heat lamps during the daylight hours, and ofcourse the animals should be given access to the outdoors on sunny days, even if the air temperature is in the fifties.

T.G.