Attic fan size calculation
Attic fan size (CFM) = 0.7 x Sq.ft area of the attic (Formula 1)
The size of an attic fan is calculated in terms of CFM. CFM (Cubic Feet per Minute) is a measure of the amount of air the fan moves in a minute.
The formula above gives an approximate value of the CFM needed for an attic fan. Some professionals multiply the square feet value with 0.6 instead of 0.7.
This is often used as a quick reference point. However, calculating the exact CFM for the attic fan takes a few more steps than this.
An attic fan is used to ventilate the attic. That means the air inside the attic needs to be exhausted to reduce attic temperature. The reduction in this temperature depends on how many times the air is changed inside the attic which is termed as air changes per hour (ach).
So to decide the CFM of the attic fan we need the following;
- The volume of air need to be vented (cubic volume of the attic area in cubic ft) (A)
- Number of air changes needed (N)
The volume of the attic space can be easily calculated using length x breadth x height. If it is a sloped roof the calculations differ (0.5 x l x b x h), but you get the idea.
The number of air changes depends on how hot your attic gets. But generally, it is 6 to 8 times per hour.
So, the total amount of air to be removed in an hour (V) = A x N
The total amount of air to be removed in a minute = V/60 = A x N/60= CFM (Formula 2)
eg: For a flat roof with size 40 x 40 x 10 ft the required CFM for a fan is;
40x40x10x6/60 = 1600
This is the exact CFM rating required for the attic fan.
Then why are people using the first formula for size calculation?
As mentioned before it gives an approximate size for the attic fan. Practically no attic is 10 foot high. If you replace the height in the above formula with 7 ft you get Formula 1 in effect. Depending on the number of air changes and the height of the attic the first formula makes sense.
For a steep roof
If your house has a steep roof then you will need a more powerful fan. Approximately it takes around 20% more power to vent a steep-roofed attic.
So you have to multiply the CFM value you got above with 1.2 (CFMx1.2)
For a dark roof
If it is a dark roof then you will need an additional 15% capacity. This is because attics with dark roofs get hot faster so more venting is needed.
So here the CFM of the fan = CFM x 1.15
Intake air vent size
For the exhaust to work properly the attic should have enough intake air vents.
Minimum intake air vent size = CFM/300 sq.ft
eg: if your attic requires a 1500 CFM fan then it should also have a 1500/300 = 5 sq.ft air vent.
Attic fan size chart for quick reference
How many attic fans do I need?
You can use one or more attic fans in your attic based on its CFM requirements. It is the total CFM of the fans that count. For example, if your attic needs a 2000 CFM fan as per the calculation above you can either use one fan of 2000 CFM or two fans with 1000 CFM each.