In nature, tarantulas live in stable environments that they rarely leave voluntarily except at night. A tarantula’s burrow serves to conserve moisture, and the placement of the burrow helps the tarantula regulate its temperature. By digging into the soil or utilizing a preexisting rodent burrow (many common tarantulas lack the strong spines on the chelicerae and the legs that would allow them to dig into hard soils), a tarantula enters a world that is cooler and more humid than at the surface.
Think of desert tarantulas; they live in colonies that take advantage of the porous nature of local soils and may dig down several feet until the proper humidity and temperature are reached. Once a burrow is established by a spiderling, the spider keeps modifying its retreat as the spider grows.
Eventually, there may be several side tunnels and resting chambers beyond the primary opening where the tarantula usually lurks, waiting for food to stroll by. When tarantulas leave their burrows, it generally is on relatively humid nights, and females seldom wander more than a few feet away from their retreats.
One reason males live short lives in nature is that, when they become roamers in search of females, they must utilize temporary hiding places during the day, and they almost certainly are subject to dehydration as well as more predation. Species known to come from very humid tropical habitats, such as the curly-hair (Brachypelma albopilosum), need higher humidity in the retreat, so give them a chance to look for a relative humidity of nearly 80 percent.
Remember that 65 percent is the suggested humidity of the retreat, not necessarily the entire terrarium. In fact, it probably is best to give the tarantula a humidity gradient, much as we give reptiles and amphibians a temperature gradient to allow the animals to thermoregulate.
The burrow area should be most humid, with perhaps an area at the far side of the cage at only 50 percent humidity. Keep an eye on your pet and notice its movements. If the tarantula spends much of its time in a relatively dry region, then the humidity in the area you’ve selected for the retreat may be too high, so allow it to dry somewhat. If the spider spends all its time on and near the water dish, the retreat and perhaps the entire terrarium maybe too dry, so you should change substrate type, or add more water to the substrate at least near the retreat. Excessive humidity can be just as disabling as dry conditions.