To discourage mold growth while promoting energy efficiency, it is recommended to keep indoor relative humidity below 70% (ideally between 45% and 60%), and indoor temperature no higher than 68°F in the winter and no lower than 78°F in the summer. When natural ventilation is needed, you can use indoor and outdoor humidity and temperature monitors to judge if it is the right time to do so.

If the difference between indoor and outdoor levels is extreme, you may consider another time. Also, keep monitoring indoor relative humidity and temperature levels during natural ventilation and stop when they go over the levels recommended. During times of high humidity, especially during rainstorms, the air conditioner often does not run long enough to adequately control the indoor relative humidity.

However, even if the relative humidity goes higher than 70%, this doesn't present a problem as long as the relative humidity doesn't stay that way for an extended period of time.

Source: Energy Efficient Homes: Indoor Air Quality and Energy1Hyun-Jeong Lee, Kathleen Ruppert, Wendell Porter, UF IFAS FCS3275, 2008

Additional Resources

EnergyStar®  - http://www.energystar.gov/

Progress Energy  Home Energy Check-  http://progress-energy.com/

UF IFAS Energy Efficient Homes: Indoor Air Quality and Energy http://edis.ifas.ufl.edu/FY1044