Burrows and retreats are relatively humid and thus cooler than the surrounding surface, plus the substrate itself helps insulate the spider from extremes of temperatures. In nature the tarantula’s burrow usually is situated in a shaded or partially shaded area where vegetation or rocks help divert drying winds, increasing humidity and decreasing temperature. This means that all common terrestrial tarantulas can be kept without additional heating in most homes. Tarantulas also can survive quite well when the retreat temperature drops to as little as60°F(15.5°C), though they become sluggish and won’t feed.
Remember, this is the temperature in the retreat, not the ambient cage temperature.
Use a thermometer to measure the temperature both in the center of the terrarium (ambient) and the retreat (specific). Don’t just guess; few people can accurately judge temperatures, especially in the high relative humidity in a retreat.
If extra heat is needed (indicated by a tarantula becoming sluggish and curling up in the warmest area of the terrarium), you can try using a small undertank heating pad under a third of the cage floor away from the retreat. Never place heating pads under the entire floor, as this could force a burrowing tarantula to try to make its retreat in an area of excessive heat, leading to stress and death. Be sure all heaters (pads, strips) are correctly installed and not subject to dangerous shorts that could lead to fires.
Keep tarantula terraria away from air conditioners and heating vents; I’m not aware of tarantulas being able to catch respiratory viruses (“colds”), but certainly they are not adapted to moving warm or cold air.
If you need just a little bit of extra heat for a terrarium during the winter, try using a small (15 watts maximum) incandescent red light-bulb over the cage. A bulb larger than this can lead to excessive heat and dehydration. The red color will allow you to watch your pet as it moves around during the night. Never place a bulb within the cage, as it could easily cause bums when the tarantula crawls over it and covers it with webbing. Be very careful whenever offering extra heat to a tarantula—make sure your thermometer is working well and is placed so you know the specific temperature in the retreat, which must not exceed86°F(30°C).