The size of an attic fan is measured in terms of CFM (Cubic Feet per Minute) which is a measure of the amount of air the fan moves in a minute. Attic fans are available in varying CFM ratings so how do you know what size fan you need?
The minimum size needed for an attic fan is 700 CFM for every 1000 square feet of attic space. For a steep roof, this size should be 840 CFM per 1000 sq.ft, and for a dark roof, it is 805 CFM per 1000 sq.ft.
So simply put, the size of an attic fan is calculated by multiplying the square feet area of the attic floor by 0.7. This number is further multiplied by 1.2 for a steep roof and 1.15 for a dark roof. This final number gives the CFM required for the attic fan for that particular attic.
Attic fan size calculation detailed
As mentioned before the size required for an attic fan depends on the area of the attic floor it needs to ventilate.
Attic fan size (CFM) = 0.7 x Sq.ft area of the attic (Formula 1)
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.
So for example, if you have an attic area of 2000 sq.ft you will require a 1400 CFM fan to ventilate it. For an area of 1000 sq.ft the required size of the attic fan will be 700 CFM.
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 feet 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.
Attic fan size calculation 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.
The minimum attic fan size for a steep roof is 840 CFM per 1000 sq.ft of the attic floor area. This is 20% more than that required for an attic with a normal roof. For example, if you have a 2000 sq.ft attic with a steep roof, it will require an attic fan of 1680 CFM.
So, the final calculation becomes;
Attic fan size for steep roof = Attic fan size for a normal roof x 1.2 = Attic floor area x 0.7 x 1.2 = Attic floor area x 0.85
Attic fan size calculation for a dark roof
For an attic with a dark roof, the minimum size required for the attic fan is 805 CFM for every 1000 sq.ft of attic floor area. This is because attics with dark roofs get hot quickly so the fan will need an additional 15% capacity to vent it faster.
So, CFM of the attic fan for an attic with a dark roof = Attic floor areax 0.7 x 1.15 = Attic floor area x 0.805
So if you have an attic of 2000 sq.ft with a dark roof the size of the attic fan should be no less than 1610 CFM.
Intake ventilation requirements
So far the calculations are for determining the size of the attic fan. As you know an attic fan is an exhaust fan ie, it removes hot air from the attic. This removed hot air needs to be replaced by cool air. otherwise, the attic will develop a negative pressure which can cause many problems including structural damage to the roof in a worst-case scenario.
So to achieve ideal ventilation there should be enough intake vents to match the exhaust.
According to the US Department of Housing & Urban Development, a ventilated attic should have a venting area of 1 sq.ft for every 150 sq.ft of the attic floor area. This can be reduced to 1 sq.ft for every 300 sq.ft of the roof has a vapor retarder and the vent areas are equally distributed between the top and bottom half of the attic.
But this is in the case of passive ventilation using static vents like soffit and ridge vents. When power attic ventilators are used, the rule of thumb is to divide the CFM of the attic fan by 300 to get the square feet area needed for the intake vents.
That means, 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
Given below is a chart showing the recommended attic fan size for a number of attic floor areas.
|Attic floor area||Recommended attic fan size|
|1000 sq.ft||700 CFM|
|1500 sq.ft||1050 CFM|
|2000 sq.ft||1400 CFM|
|2500 sq.ft||1750 CFM|
|3000 sq.ft||2100 CFM|
|3500 sq.ft||2450 CFM|
|4000 sq.ft||2800 CFM|
How many attic fans do you need?
You can use one or more attic fans based on the CFM requirement for the attic. 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.
Even though this is the case it is recommended to use just one attic fan if that is possible. This is because buying two fans of 1000 CFM instead of one 2000 CFM fan costs way more. Also considering the installation cost and maintenance I believe having a single attic fan is a better option.
Importance of finding the right size attic fan
It is important to have the right size fan as a fan more powerful or weak can cause ventilation problems.
Problem with using an attic fan that is too powerful
More power doesn’t necessarily mean more ventilation. Buying and installing an attic fan that is rated much above your CFM requirement can land you in trouble. A powerful fan will remove a lot more air than that is required to cool down the attic. Running these fans can create negative pressure inside the attic even if there are properly sized intake vents. This negative pressure can draw out the conditioned air in the house which will add an additional load on your air conditioner, which will reflect on your energy bills. This negative pressure can also cause back-drafting issues with chimneys and fireplaces even leading to carbon monoxide poisoning and fire hazards.
Another problem with this negative pressure development is that it can affect the roof’s structure. The constant presence of negative pressure in the attic can cause the roof to sink in causing structural damages.
Problem with using a weak attic fan
A weak attic fan means that it doesn’t have enough CFM rating to make the required number of air changed to cool down the attic. Having not enough air changes will cause the build-up of moisture in the attic which will lead to the growth of mold and fungus affecting the air quality inside it. In addition, the condensation of this moisture can drip back on the insulation of the attic floor making it less effective.
Another problem with using a weak attic fan is that it can affect the lifespan of the roof shingles. A hot attic will transfer some of its heat to the shingles which is why you can see the shingles on some houses greying even though they were installed only a few years ago. Even though a weaker attic fan can prolong the life of shingles when compared to having zero attic ventilation it is always recommended to use the right size attic fan.
Using a weak attic fan in winter can also cause ice dams. This is because since these fans don’t ventilate the attic space, it will remain hotter than the outside. This means that any snow on the roof will melt. Even though this may seem like a good thing this melted snow will refreeze at the eves building dangerous ice dams.
For further reading: Garage fan size guide