Came with the house when I bought it. Attic is open to outside and has insulation between it and upper living floor. Attic this time of year is probably 140 degrees. I can turn on attic fan and open a window and/or door downstairs and a huge volume of air goes through the fan, into attic, and out through vents. But it can only lower attic temp to ambient temperature, and the interior air, which you've paid to cool, goes outside. So you gotta cool off interior all over again.