Is an Attic Fan Recommended or Not Recommended?

Attic Venting

• Any attic should have sufficient venting to prevent moisture and heat buildup. The Uniform Building Code specifies that the amount of space occupied by vents in an attic should be 1/150 of the total area inside the attic, and they should be evenly distributed between the soffits and the ridge or gables of the roof. Air moves between the vents either as a result of the temperature gradient from the roof of the attic to the floor or because of the difference in air pressure between one side of the roof and the other.

Benefits of Venting

• The primary purposes of attic venting are to control moisture and temperature. In the absence of venting, condensation forms on the framing and in the insulation. Not only does this promote the growth of mold, it also clumps the insulation and rots the wood. During the winter months, this condensation can even turn to ice. In the summer, the temperature of the attic can exceed 150 degrees Fahrenheit on hot days in some climates. If the ceiling is poorly insulated, air conditioners have to work harder to keep the house cool, running up the electric bill.

Power Venting

• An attic fan can increase the circulation in the attic either by blowing air in through the soffits from the ceiling below or sucking it out through the gables or ridges. Whether it blows it in or sucks it out, there must be enough inlets and outlets to allow the air to circulate freely for the fan to be ineffective. Not all attics need a fan, as a well-designed passive vent system usually provides adequate circulation. Some roof designs, however, like flat roofs or tile roofs, don't allow for proper placement of the vents and require power ventilation.

Considerations

• Increasing the air circulation in the attic with an attic fan is never a bad idea in the summer, but in the winter, it can draw in cold air that will lower the temperature of the house. An attic fan should be properly sized and should have enough power to change the air in the attic 10 times per hour. While a solar attic fan may be adequate for a small house, it is unlikely to have enough power provide this amount of circulation for a house with an attic space in excess of 1,000 square feet.