I'm a mechanical insulator by profession, here's what I'd recommend. Temperature loss occurs when two areas of different temperature come into contact with one another. These areas could be the air, or a pain of glass that separates two different climate zones. First look at your windows. Are they single pane, double, or triple? The more layers they have, the better job they will do of preventing heat loss. I live in northern California and my snake room has single pane windows. I proceeded to cover my windows completely with a single layer of reflective bubble wrap insulation (you can find this at home depot), and then I applied a second layer of fiberglass insulation (courtesy of my frozen rodent supplier) directly to the bubble wrap. This has prevented heat loss at the windows. Next, you'll want to look at your a/c/heating vents in the room. Remove the metal cover and make sure there aren't any gaps between the drywall and the vent opening, if there are fill them with mastic (found at home depot on the tile isle), then manually close the vent using the lever provided -unless you are planning on running the house heater in that room. Next, look at your doors. When they are closed are there any gaps around the frame of the door, is there a gap at the bottom of the door? If so, get some rubber weatherproofing strips (again found at home depot) and line the frame of the door with them. For the gap at the bottom of the door, get either a door sweep or place a towel in the open area. Now, consider how much "open-air" space there is in the room. This space should be filled with empty boxes stacked floor to ceiling. The less open-air space there is inside the room, the less heat exchange you'll have every time you open the door. Finally, if your room is only housing tropical reptiles, add a humidifier. Humid air holds heat better than cold air. This is what I did before I had my room stacked wall to wall with racks and cages.
-Anthony