How do you determine what size fan to put in an attic to cool it properly?


How do you determine what size fan to put in an attic to cool it properly?

Asked by Rose

Our house is 2000 sq ft with cathedral ceilings in the living areas and 8 ft in the bedrooms. The company that did the house energy efficiency test said there were no govenment guidelines to size an attic fan and the websites that sell attic fans have calculators but since they sell attic fans I don't know how reliable they are. Are there any studies/edu tests that have studied this? How can we tell if the company that installs the attic fan/s has properly sized and sealed the units? Thank you

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Joshua Lloyd's picture

I recommend that you DO NOT install a power attic fan as studies have shown that the fans actually use more energy than they save by cooling the attic.

These power attic fans can actually create more problems than they solve.

  • Some of these fans can pull as much as 1500cfm of air, and if there is not enough soffit, gable or ridge vents, the fan will pull the air from your home because air takes the path of least resistance. So you can becooling your attic with the air inside your home that you just paid to cool, plus you could also be pulling in excessive moisture.
  • I have even tested homes that enough bypasses in the attic that the fans actually back draft the water heater in the basement pulling combustion gases into the home.
  • Plus attic ventilation has never been about cooling the attic temperature; it has always been about drying the attic if moisture was present.

The first defense against the excessive moisture getting into your attic as well as the hot air getting into your home is air sealing.

  • You want to make the ceiling plane is as air tight as possible.
  • And then add insulation if your attic does not have the recommended levels.

You can check the recommended levels of insulation for your region on the Energy Star website here.

You can also view my blog post about attic air sealing here, as well as a great article from a well respected professional in the home performance industry about power attic ventilators here.

For more information:

Read "How effective are the different attic ventilation methods? I read mixed reviews, and many have a commercial bias." a Q&A answered by Danny Kelly.