However, two independent experts with whom we spoke disagree on both counts. First, a ventilation fan that’s in the attic starts to work only when the thermostat detects a rise in temperature during the daytime, so it does little to contribute to heating the moist air that’s left at night. Second, these experts tell us that, because a ventilation fan that’s in the attic turns on during the daylight hours, it has to run faster and run all day long, which means that it consumes more electricity. In fact, its mere operation is counterproductive, according to the experts