Jen, Honeywell or Johnson Controls sell a variety of humidistats that can be used with your attic fan but there's a number of other issues that would have to be addressed such as line voltage or low voltage wiring to the electrical code, control location such as a wall or in the fan airstream, etc... The Johnson P/N W42AA-1C is a wall mount control that handles the desire for humidification or de-humidification and line or low voltage but expect to pay close to $200 or more for this type of control...
And be mindful when working with certain humidity controls; they're designed to operate equipment to achieve desired relative humidity levels regardless of temp...