How to Size Attic Exhaust Vent Fans for Your Home

Solar powered attic exhaust vent fan mounted on roof.
Solar powered attic exhaust vent fan mounted on roof.

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,

Danny

Further Information

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41 COMMENTS

  1. 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!

  2. 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

  3. I just had my house re-roofed and notice when I went into the attic that there the sheet metal pipes that vented to box vents in the roof were not sealed to the box vents. Is this normal, or should these pipes(?) be be sealed to the box vents. I can see light coming through the underside of the box vents.

  4. We are getting ready to have attic ventilator fans installed in our 11/12″ hip roof that has ridge vents. To minimize the problems (ridge vent draw air/rain) and increase the benefits for the HVAC system, how far down from the ridge vent should the fans be installed? The roof height is 15′.

    • Hi Laura,
      As you noted, it’s not a good idea to combine a ridge vent with power exhaust vent fan. This is particularly true if the ridge vent is your only means of venting the attic. If you have other vents (such as soffit or gable vents) to act as intake vents, it would lessen the amount of air pulled in the ridge vent by the power vent fan. If you decide to install the power vent fan anyway, it should be located a couple of feet down from the peak of the attic to remove as much hot air as possible. Good luck with your project!

  5. I am going to install an attic fan in the gable. I have soffit and ridge vents. To minimize the problems should I reduce the size of the fan? Is the distance below the peak important? The area is 1250 sq. ft, dark roof, and a shallow roof. Thank you.

    • Hi Jody,
      If you have soffit and ridge vents that are working properly by natural air flow, you probably don’t need a power vent in your gable. If you do put one in, it should be as near the peak as possible.

  6. I have a one story steel frame house, the rafters are also steel so it becomes an oven in the attic. I have no soffits, but there is a gable vent at each end of the house and four ridge vents. I was wondering if I could install a couple of fans in the ridge vents facing outward to exhaust the heat?

    • Hi Bryon,
      If you can’t add soffit vents, your best option is to add a few roof vents down low on the roof on each side to let cool air in and create a natural air flow through your attic as the hot air rises. Other options include adding a thermostat controlled power vent fan in the roof near the peak or a vent fan in your attic blowing out of one of the gable vents. However, there can be problems associated with using a power vent in conjunction with a ridge vent. See our article Combining and Attic Vent Fan with a Roof Ridge Vent to find out more. Good luck with your project!

  7. I have an attice without any soffit vents or a ridge vent, I just havbe two very small gable vents, one on each side at the top. I was told I should have 4 box vents installed acrossed the back of my roof and then two sets of gable vents installed on each side-down low to allow for the proper venting. Is there any problem with this solution?

  8. I am replacing a 2700 sq ft hip roof with new shingles and the roofer wants to install a ridge vent and eliminate my existing 5 turtle vents and one power vent. I’m concerned about the ridge vent providing adequate air flow. I have soffit vents now but suspect that they may not all be clear due to blown cellulose attic insulation. Any recommendations on the roof venting?

    • Hi Rick,
      I have the same problem in my attic with blown insulation possibly covering the soffit vents. If you can find a way to see if the vents are clear, and you have enough soffit and ridge vents area combined (1 square foot of vent space for every 150 square feet of attic area), then you should be fine without a power vent. In fact, a power vent isn’t recommended for use with a ridge vent, since it reverses the natural air flow out the ridge vent, and might cause rain to be drawn in through the ridge vent. To find out more about the amount of attic venting needed, see our article on Adding Soffit Vents. For more on using an attic ridge vent with a power vent, see Combining an Attic Vent Fan with a Roof Ridge Vent.

  9. I just had attic foil barrier and more insulation installed. I had 2 differnt companies tell me that I needed to add a solar exhaust fan because my attic is still so hot and it’s making my upstairs warm. I thought the attic foil and more insulation would help, but it didn’t. I live in houston Tx. and my attic temp is 129. Any thoughts?? I also have ridge vents. Thank you.

  10. the way attic ventilation should work is cooler outside air enters the attic through the vented soffit naturaly forcing the hot air in the attic out through the ridge vent keeping your attic cooler and also keeping it dry. if there is insulation blocking the air infiltration through the soffit you need to place cardboard dams or a peace of rolled fiberglass insulation cut to fit and baffels wich extend out past the dam and at the other end past the level of insulation in the attic. the dams block the insulation from spilling into the soffit and the baffels allow the air to pass through the dam into the attic. more then one baffel may be required due to the pitch of the roof and the amount of insulation going into the attic. also make sure not to plug the baffels with insulation. for proper venting place baffels every other or every 2 rafters damming off every rafter leading into the soffit most diy stors cary a varity of differnt dams and baffels. i also recomend ridge vent over turtle vents because it lets the heat out of the attic faster and its at the peak of the roof, (heat rises). but the turtle vent still work just not as efficient. by following these simple procedures, you should not need a fan in the attic.

  12. I have 2 gables 22 in x 22 in. I want to put gable fans in both. What size fan do I need and where can I get them. What do you think of “solar fans? Thanks

  13. Similarly to Mirette, In my house we have two gables vents. What would be the best way to install the two fans in the attic? Should one pull air from outside into the house and the second one should push the air out?

    We do have central AC, so I’m wondering if the fan that’s is pulling the air out might take as well the air generated by the AC from inside the house.

    Please let me know your thoughts on the best approach two install two fans in the gable.

    thanks,
    Frank

  14. I am looking at having an attic exhaust fan installed, but I have a ridge vent on my house. The contractor said he typically closes 15 feet of the ridge vent around where he will place the attic fan. What are your thoughts on this? Also, should I have any concern over this fan being near the furnace in my attic? Finally, he wants to install a 1600cfm fan, but I may have a total of 1000 sq feet of attic space (there is a soffit running the full length). I do have a dark roof. Are there any negative effects of having a fan that is too strong?

  15. ..i would like your advice on installing 2 attic fans..my house is in 2 parts..regular pitched roof on 2 storys with a family room part that butts up against the regular house with its own roof at a lower level..one story..

    the upper roof is gable vented and is 700 cubic feet..
    the lower roof..regular pitch in the same direction as the upper has no vents and is 600 cubic feet..

    the lower roof is the challenge..i want to cut a gable vent
    in the open end and on the other end up against the house ..one window nas a small cutout in the roof where i may be able to install a small customized gable vent as a
    small triangle..if this fails..i need another idea for an intake source if i install a gable vent fan on the open end..
    appreciate your comments
    don

  16. PS
    i’m thinking of gable vent fans rather that cut holes in the roof unless they are much more efficient..

    don

  18. My one gable vent is divided by a 2×4. Each side has a measurement of 14 1/2″ x 24 1/2 “.
    Should the fan size be a 14″, 16″, 18″ or 24 ” fan blade size?
    Or should I get 2-14″ fans- one for each side of Gable opening?
    I understand all the rest about: amps, CFM, SF. Got that figured out.
    Just don’t know appropriate fan blade size.
    Thanks,
    Lydiah

  19. Hello,

    My house currently has several passive vents in the roof and I want to add a gable fan and have it draw air into the attic to help push the hot air out through the vents. Is this a good idea? Also would I calculate the size of the fan using the same formula above?

    Thanks,
    Jack

  20. Im sure I need some sort of vent in my attic. I have a townhome and when i look from the outside I see only gable vents. It is VERY hot upstairs and cold downstairs and my power bill is so high. Do I need a vent and is this something I could install myself?

  23. I CAN NOT FIND ANYTHING ON VENT FANS USED WITH TRIANGLE GABLE VENTS…MY HOME IS 100+ YEARS OLD AND THE VENTS ARE IN MINT CONDITION AND ARE COVERED IN WINDOW SCREENING. THIS IS THE ONLY VENTILATION ON MY 2ND FLOOR. IT IS CURRENTLY UN-USED, UNFINISHED WITH NO INSULATION. I NEED TO INSTALL A FAN…WHAT TO DO? OH AND THERE IS A SUPORTIVE 4×4 RUNNING DOWN THE CENTER OF THE “TRIANGLE” VENT!

    All I could come up with was installing (2) 10″inline fans, one on either side of the 4×4, with a thermostat.

    Please Help!!! Keith

  24. i have attic in garage with metal roof, 2 roof vents and 240 square feet of useable space to convert to wood shop. I want to install gable mounted exhaust fan. What size is needed for the space being used? North gabled wall has decorative window. The south wall will be the location of the fan. It will be located over open stairs. Will the effect the flow?

    Thanks Scott

  25. I understand the divide by 300 to get the intake venting required, but is this the same if I am only using a power exhaust fan ? It seems that less would be needed since there would be a lower pressure side the attic and air would have more speed through the intake vent.
    Thanks

  26. Thank you!!!! replacing original AV with solar. Used to sell tons of
    these but couldn’t remember formula for air draw space. We have
    same problem as Paul Z of 11/09/09…….1250 sq. ft. attic, 3/12 pitch
    very few 2″holes for outside air draw. Any suggestions? (65 yr. old
    house). Soffit space is very limited. No way have our vents met
    required air intake. (I figure approx. 42 2″ holes in existing soffits.)
    Will appreciate any advice you can offer. Thanks.

  27. Putting passive vents low on the roof to compensate for inadequate soffit venting is not a good idea. These vents are meant to be placed near the peak where the volume and speed of the water is not very great. if they are placed low on the roof, they have a high probability of leaking.

  28. Is there an issue when using a power roof vent in a garage attic if there is a ridge vent? Is there a minimum distance from the ridge the fan should be placed?

  29. I have a 32×40 ft wood arch rib quanset. Right now it’s all covered in asphalt shingles andit gets very hot in there. The inside is not insulated other than a small workshop.
    What would be the best way to get heat out or there?
    I have 2 large doors at one end of the building that really don’t close very well so no problem to get air in.

    Thank you

    • Hi, Rob,

      Danny says, “I would consider installing a 3-foot by 3-foot, wall-mounted exhaust fan that would pull air through the building. This can make a significant difference in cooling the building and ridding it of hot, moist air.”

      Good luck!

  30. I used to think a powered attic ventilator was unnecessary because I had R-30 insulation on top of the original insulation. I started noticing that most mornings, inside the house would heat up faster than outside, then last week a rare morning breeze picked up, blowing the hot air out of the attic and inside the house actually cooled below the outside temperature. I am now a believer in powered attic ventilation. However, I think the fan sizing above is excessive.
    A fan sized using the above formula would cause 10 to 30 air exchanges per hour depending on the height of the attic, more than the recommendation for a steam filled bathroom.

  31. If your inlet vents are screened with, say, 1/8 mesh screen, how much does that reduce the effective area of the vents?

    • Hi, Michael!
      Danny says, “It certainly depends on the type of mesh that is being used to cover the vents. But as a general rule, the screen will reduce the air intake by as much as 20 percent.
      The more soffit vents, the better.”

  32. I found it helpful when you said that you should multiply the width by the length of the attic floor in feet for you to get the size of your attic. My husband said that he’s interested in installing a triangle gable vent. He has been having a hard time in finding the right size based on our attic, so I’ll be sure to share your blog with him.

  33. I have a 1900 sf ranch home. The roof is 33 sq. I just re-roofed and now my power vent howls like a banshee. Roofers came back and quieted it down for about a day then it howled again. I didn’t have a ridge vent previously, they installed ridge vents with new roof. I’ve been told that a power vent system can be counter-productive with a ridge vent. If I should still have a power vent, what cfm should I purchase for best efficiency

    • Hi, Thomas,
      Glad to hear your home is getting the ventilation it needs!
      Regarding CFM, your best bet is to use our guide provided in this article.
      Good luck!

  34. It’s helpful to know that the size of attic fans we’ll need is based on the size of our attic. We’ve been noticing our air conditioning costs get pretty high during the summer and have been trying to find ways to keep it down. Maybe we’ll have to find someone to come take a look and see if it would be cost-effective for us.

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