Taking a page from the residential HVAC design guide, consider that many businesses, schools, and industrial facilities have attic space. As in homes, this area is usually ventilated via gravity vents, louvers, or a forced-air fan. This keeps the air from becoming very hot in the summer, thus increasing the cooling load of the occupied space below.
An improvement on this is to control the attic fan with a thermostat, running it only when the attic temperature is 5 or 10° above the outside temperature. Even though the average temperature of the attic air may be a little higher through the course of a day, the heat transfer will not be much different because air moving across a surface has a greater heat-transfer rate than a stagnant volume anyway. A variable-speed fan takes advantage of this principle by spinning the fan as slowly as necessary for the temperature measured. This saves fan horsepower as well as cooling energy.
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Excerpt from Energy Conservation Projects.