plainly outside air, I rely on the temps available, the only days the room hit 90+ was when it was 98 outside (which is why it feeds from an adjacent room, with open windows); just lots and lots of fresh air. the room is 186 cu/ft, my exhaust is 400 cfm, it's slow but with the air being pumped into the room with my described intakes, I get circulation twice a mintue easily.