Attic ventilation fans move huge amounts of air and create a negative pressure ‘bubble’ right over the conditioned space of the home,” says Brett Dillon, who owns IBS Advisors, which is an energy-efficiency and building-sustainability consulting company. “This sucks conditioned air into the attic, typically when outside conditions are really hot, through all the holes in the ceiling and wall top plates. Although this cools the attic down, it also causes outside air to get sucked into the house, which then makes the air conditioner run even more