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How to Size Attic Exhaust Vent Fans for Your HomeBy: Danny Lipford
I want to install attic vent fans in my roof. Is there a formula on how many you need and the size? -Lin
Attic exhaust vent fans can reduce your air conditioning energy costs in the summer as well as prevent condensation and ice dams from forming during the winter. While most are wired into your home electrical system, solar powered vent fans are available that require no electrical hookup. Attic vent fans are designed either to fit over a hole cut in your roof or to mount to the inside of a gable vent.
To determine what size power vent fan(s) you need for your attic, you first need to know the size of your attic in square feet.
To determine the size of your attic, multiply the width by the length of the attic floor in feet. For a single story house, this is usually the same as the square footage of the house itself, plus any attached garage area.
- Example (20’ wide by 50’ long attic): 20’ x 50’ = 1,000 sq. ft. attic space
Vent Fan Size
Next, multiply the square feet of attic space by 0.7 to get the minimum number of cubic feet of air per minute that the fan should be rated to move.
- Example: 1,000 sq. ft. attic x 0.7 = 700 CFM minimum fan rating
Add an additional 20% (CFM x 1.20) if you have a steep roof, and 15% (CFM x 1.15) for a dark roof. Attic vent fans are commonly rated from 800 to 1,600 CFM, making one fans suitable for attics of up to around 2,200 square feet.
Vent Fan Location
Locate roof mounted fans on the back of the roof below the ridge (but not so high as to be visible from the front of the house) in the middle of the main part of the attic. Install gable mounted fans on the gable vent at end of the house faces away from the prevailing winds.
Intake Air Vents
It’s also important to have plenty of soffit or gable vents for the fan to draw air into the attic. To find out if you have enough vent space, divide the cubic feet of air per minute that the fan is rated for by 300 to come up with the minimum number of square feet of intake vent space needed for that size fan.
- Example: 700 CFM ÷ 300 = 2.33 sq. ft. intake vent area
If you prefer the answer in square inches rather than square feet, multiply the answer by 144 and round to the nearest inch (2.33 x 144 = 336 sq. in. vent area).
Set the thermostat on your attic vent fan so that it cuts on between 100° and 110° F. Humidity sensors are also available that cut the fan on if moisture in the attic becomes too high.
It’s not a good idea to use a power vent fan if your house has a ridge vent, since the fan will tend to draw in air from the ridge. This works against the natural function of the ridge vent to expel hot air out of the attic as it rises. Also, it’s possible the fan could create enough draft to draw rain in through the ridge vent.
Good luck with your project,
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