How to Size Attic Exhaust Vent Fans for Your Home
Ask Danny, Green Home, Heating and Cooling, Roofing by 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
Hi 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.
Attic Size
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).
Fan Thermostat
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.
Ridge Vents
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,
Further Information
2 Comments on “How to Size Attic Exhaust Vent Fans for Your Home”
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June 26th, 2009 at 7:48 am
Solar attic fans are not only the least expensive (in terms of total cost of ownership, which is purchase price + operating costs), they are also the most environmentally friendly way to ventilate an attic. Unlike conventional electric attic fans, solar attic fans don’t use any electricity from the power grid at all! I have really been trying to get the word out about this method of attic ventilation at Solar Attic Fan Info because I think not enough people are aware of the benefits – please check it out!
November 13th, 2009 at 12:05 am
Danny,
I have 1921 sq./ft. of attic space. I need to find out how many holes needed in blocking between 22 1/2″ center to center roof rafters for attic ventilation. The structual engineer on the remodel wants 2×6 blocks that will be 22 1/2″ long with 2″ diameter holes nailed to wall and to upper roof. I took the 1921 sq./ft. divided by 150 sq./ft. of area which gave me 12.8 or 13 sq./ft. of ventilation area. I’m in California so the code equires 1 sq./ft. of ventilation area per 150 sq./ft. of area. Take the 13 sq./ft and divide by 2 which gives you 6.5 sq./ft. of soffit vent area. I have to find the area of the 2″ diameter circle which is [3.14(1 squared) = 3.14 divided by 144 = .0218055 sq./ft. per hole]. Then take 6.5 sq./ft. divide by .0218055 = 297 holes required. I can only fit (5) 2″ diameter holes onto the 22 1/2 long blocking. Take 297 and divide by 5 = 59 blocks needed. Here’s the spot that confuses me. The attic area only has space for only 47 blocks. I would like to install a roof fan, but would the 47 blocks be enough attic ventilation in the soffit area. I took 1921 sq./ft. x .7 = 1345 x 1.2 = 1614 CFM required for fan. Would a 1600 CFM fan be suffcient that I can reduce the number of 2″ diameter holes in blocking? Do I need to install a gable vent?
Thanks, PaulZ