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 850 CFM per 1000 sq. ft, and for a dark roof, it is 805 CFM per 1000 sq. ft.**

This sizing calculation is the same for electric and solar attic fans. Both types of fans come with CFM ratings, so it doesn’t matter what powers them.

Furthermore, this calculation is the same irrespective of the placement of the attic fan. For hip and gable attic fans the size required is 0.7 times the attic floor area.

## Attic fan size chart for quick reference

Below is a chart showing the recommended attic fan size for a number of attic floor areas.

Attic floor area | Normal Roof | Steep Roof | Dark Roof |
---|---|---|---|

1000 sq. ft | 700 CFM | 850 CFM | 805 CFM |

1500 sq. ft | 1050 CFM | 892.5 CFM | 1207.5 CFM |

2000 sq. ft | 1400 CFM | 1700 CFM | 1610 CFM |

2500 sq. ft | 1750 CFM | 2125 CFM | 2012.5 CFM |

3000 sq. ft | 2100 CFM | 2550 CFM | 2415 CFM |

3500 sq. ft | 2450 CFM | 2975 CFM | 2817.5 CFM |

4000 sq. ft | 2800 CFM | 3400 CFM | 3220 CFM |

## How many attic fans do you need?

Most attics require only one fan. However, larger attic spaces may need two or more attic fans to match their CFM requirement.

For example, if your attic needs a 2000 CFM fan as per the calculation above, you can either use one fan rated 2000 CFM or two fans rated 1000 CFM each.

You can also use attic fans rated 1500 CFM and 500 CFM to match the 2000 CFM requirement.

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.

## Attic fan size calculation detailed

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.

Therefore,

**For a normal roof, 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 by 0.6 instead of 0.7.

So for example, if you have an attic area of 2000 square feet you will require a 1400 CFM fan to ventilate it. For an area of 1000 square feet, the required size of the attic fan will be 700 CFM.

### Let’s see how this formula is reached;

An attic fan is used to ventilate the attic. That means the air inside the attic needs to be exhausted to reduce the 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 on the CFM of the attic fan we need the following;**

**The volume of air needs 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 required 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.

Therefore, **the attic fan size for a steep roof should be 20% more than that for a normal roof**. So, the calculation becomes;

Attic fan size for steep roof = Attic fan size for a normal roof x 1.2

By replacing the attic fan size using Formula 1, the above equation becomes;

Attic fan size for steep roof = Attic floor area x 0.7 x 1.2

Thus the final calculation becomes;

**Attic fan size for steep roof = Attic floor area x 0.85**

Here is an example for your easy understanding;

For a steep attic with 1000 square feet of floor area, the minimum attic fan size is 1000 x 0.85 = 850 CFM.

### 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 area x 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.

## Importance of finding the right size attic fan

Choosing the correct size attic fan is crucial to avoid ventilation issues. Both overpowered and underpowered fans can lead to problems.

**Issues with Overpowerful Attic Fans**

Higher power doesn’t necessarily mean better ventilation. An attic fan exceeding your required CFM (Cubic Feet per Minute) can create trouble. Such fans remove excessive air, causing negative pressure even with proper intake vents. Negative pressure can lead to air conditioner strain, elevated energy bills, and risks of carbon monoxide poisoning and fires. Structural damage to the roof can also occur due to constant negative pressure.

**Issues with Underpowered Attic Fans**

Weak fans lack sufficient CFM to cool the attic effectively. Inadequate air changes result in moisture buildup, fostering mold and fungus growth and compromising air quality. Moisture condensation may damage attic floor insulation. Weak fans can also age roof shingles prematurely due to trapped heat. In winter, they contribute to ice dams by keeping the attic hotter than outside, causing snow to melt and refreeze dangerously.

In all cases, using the appropriate size attic fan is recommended to avoid these issues.

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