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Attic fans are great for cooling at night, but I've found they make a difference only at cooler latitudes like Michigan where night temps fall 15 or 20 degrees

Attic fans are great for cooling at night, but I've found they make a difference only at cooler latitudes like Michigan where night temps fall 15 or 20 degrees below daytime highs of ~85F, reducing the house to 65 or 70 after sunset. In the mid-Atlantic where I live now, night temps don't drop more than maybe 10 degrees below daytime highs of ~90F. So bringing in the evening air via an attic fan puts the house at no less than a toasty 80 (with 70% humidity) -- still uncomfortable.

artimaeis 7y ago
Attic fans are great anytime the air temperature difference between the attic and the outside is great. In the hotter regions (such as the southeast US) they're great for venting hot air out rather than pulling cool air in. Attics will often get 20 degrees hotter than the outside air, so forcing that out helps to insulate the cooler indoor spaces. Of course in older, not-well-sealed housing the venting poses the risk of drawing cooler air from the house to the attic, negating much (or all) of the benefit. So their efficacy is very situational.
- randcraw 7y ago
  Attic fans aren't for cooling attics. Ridge vents, and passive and powered roof vents, and turbines mounted in the roof surface can cool attics far more efficiently than attic fans which consume over 100 watts and must run continuously on summer days -- a very costly way to air out an attic. Though old houses often have attic fans, they weren't there to cool attics. That was the purpose of widowed attic gables or breathing slats under wood shingled roofs. Attic fans are there to cool the house interior, especially the hottest rooms on uppermost floors. That's why the intake of an attic fan is the highest point inside the house and not the lowest point inside the attic. Even then attic fans aren't intended to run for more than perhaps 30 minutes a day after sundown -- just long enough to replace the hot air inside the home with cool air from outside. To vent an attic, the ideal air flow is to intake cool dry outside air from along the attic soffet, pass it along the underside of the hot roof sheathing, and have it exit high where attic heat is at its peak. And do this passively without a powered fan if at all possible.

artimaeis 7y ago

Attic fans are great anytime the air temperature difference between the attic and the outside is great. In the hotter regions (such as the southeast US) they're great for venting hot air out rather than pulling cool air in. Attics will often get 20 degrees hotter than the outside air, so forcing that out helps to insulate the cooler indoor spaces. Of course in older, not-well-sealed housing the venting poses the risk of drawing cooler air from the house to the attic, negating much (or all) of the benefit. So their efficacy is very situational.

randcraw 7y ago
Attic fans aren't for cooling attics. Ridge vents, and passive and powered roof vents, and turbines mounted in the roof surface can cool attics far more efficiently than attic fans which consume over 100 watts and must run continuously on summer days -- a very costly way to air out an attic. Though old houses often have attic fans, they weren't there to cool attics. That was the purpose of widowed attic gables or breathing slats under wood shingled roofs. Attic fans are there to cool the house interior, especially the hottest rooms on uppermost floors. That's why the intake of an attic fan is the highest point inside the house and not the lowest point inside the attic. Even then attic fans aren't intended to run for more than perhaps 30 minutes a day after sundown -- just long enough to replace the hot air inside the home with cool air from outside. To vent an attic, the ideal air flow is to intake cool dry outside air from along the attic soffet, pass it along the underside of the hot roof sheathing, and have it exit high where attic heat is at its peak. And do this passively without a powered fan if at all possible.