Why do I need a humidistat with my power attic ventilator?

The humidistat allows the power vent to run in the winter, which will keep the moisture from building up.

The thermostat control on a power attic ventilator monitors the attic solely for temperature or heat buildup. Thus, the power vent will only run in the hot summer months. During the winter, moisture can build up causing a reduction in the effectiveness of your insulation. The humidistat allows the power vent to run in the winter, which will keep the moisture from building up.