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Attic Fans – A panacea for summer and winter woes

Does your house seem to "lose its cool" in summer?  Does excessive humidity in winter cause mold in the attic? An attic fan may be just what you need.

In summer, heat builds up in an unventilated or poorly ventilated attic where temperatures can easily exceed 150 degrees F.  This attic heat finds its way into the interior spaces by either inadequate insulation or physical air leaks around pipes, cables, ducts and gaps in the structure.  This creates an undue burden on the air-conditioning system.  

In winter, heated moisture-laden air from everyday activities (cooking, showering) finds its way into the attic where it condenses on the cold underside of the roof structure.  In some cases, heat trapped in the attic may melt snow on the roof, and cause ice dams at the cold eaves. 

What is the role of the attic fan?

An attic fan augments the passive air circulation in the attic provided there are sufficient openings for air to enter the attic.  With proper placement, an attic fan enhances the air circulation and brings the attic temperature to within 10 degrees of the outside temperature.  There are three important steps to ensure the effectiveness of attic fans:

  1. Seal air leaks between the living spaces and the attic to prevent air movement between the spaces during summer and winter.
  2. Uniformly distribute insulation on the attic floor (minimum 12 inches, R-38).  Seal all seam joints at the ducts in the attic space and insulate the ducts to minimum R-8.
  3. Provide sufficient air intake vents in the attic for the roof-mounted attic fan to be effective. Homes built since the 1980's usually have a standard soffit-ridge vent system.  However, older homes only have end gable louvers which provide minimal ventilation; screens at these louvers are often obstructed by twigs and bird debris further restricting ventilation.  

How is an attic fan controlled?

The ideal way to control the operation of an attic fan is with a thermostat and a humidistat.  In summer, the thermostat operates the fan when the attic temperature reaches a preset level (usually 90 to 95 degrees F).  In winter, the humidistat operates the attic fan when the relative humidity exceeds a preset level (usually 40 to 50%).  This reduces attic moisture.  Eliminating mold and mildew on the roof structure is the ultimate goal.



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