A method for controlling a vehicle HVAC system to automatically defog a windshield glass and to prevent fogging or condensation of the windshield glass. In-cabin air temperature and relative humidity are measured and used to determine a dewpoint. Further, ambient or outside temperature is measured. If windshield wipers are being used or if the cabin air temperature is less than a predetermined temperature, the ambient air temperature value is used as a glass temperature, otherwise the glass temperature is determined from the ambient air temperature and vehicle speed. A fog margin, which is a measure of the likelihood of fog formation, is based upon the difference between dewpoint and the windshield glass temperature. The fog margin is used to control the HVAC system to anticipate and respond to potential fogging conditions.