Good to know: Bathroom Exhaust Fans

Bathroom exhaust fans control humidity levels, preventing the moisture that causes problems. Fans draw moisture out of a room, pushing it into the sewer, window, or outside through a vent to help prevent damaging and unsanitary conditions. Fans mount low over the bathtub and shower enclosure with flexible ducting that allows you to route air where it’s needed most.

About bathroom exhaust fans
Bathroom exhaust fan

In short, a bathroom exhaust fan can address these problems by removing excess moisture before mold and mildew have the chance to take root, thereby preserving bathroom surfaces and improving indoor air quality.

Fans are essential components in any well-designed, energy-efficient bathroom ventilation system.

What to look for when selecting a bath fan?

There are many considerations involved when selecting a bathroom exhaust fan, the most important being proper air movement. Air movement is typically measured in cubic feet per minute (CFM). The higher the CFM rating, the greater the airflow. A bathroom that is 50 sq. ft. or smaller in size should have a fan rated at 50 CFM or higher. Bathrooms larger than 50 sq. ft. should have fans with CFM ratings of 100 or more.

Other ventilation considerations include:

  • An enclosed toilet should have its own exhaust fan
  • Fans approved for installation in wet areas should be located over (or very near) the shower or tub when possible
  • Bathroom doors should have at least ¾” clearance to the floor to allow for proper entry of makeup air
  • Bathrooms with greater than an 8′ ceiling may require additional ventilation

When selecting a bathroom fan, one important consideration is noise level. Bathroom fan ratings are measured in sones: 4.0 sones are the sound of standard television operation; 3.0 sones is typical office noise; 1.0 sones is the sound of a refrigerator; 0.5 sones is the sound of rustling leaves. An ideal bath fan would be rated at 1.0 sones or less for quiet bathroom ventilation to provide a restful atmosphere for bathing and showering.

Choose the right size for the bathroom fan exhaust

Choosing the right vent fan for your bathroom is important. A poorly ventilated bathroom is subject to mold and mildew problems, and a noisy vent can make even the most relaxing bath or shower a hassle. When sizing a vent fan, duct size and length are not negligible factors. Make sure the vent you choose allows a proper minimum CFM airflow based on the bathroom square footage.

Bathroom fan sizing

The bathroom requires constant ventilation for the health of occupants. The air changes in a bathroom every 10-30 minutes, which is why you hear ceiling fans and/or exhaust fans are either on or off; to prevent odor build-up. A bathroom has problems with humidity, moisture, mildew, and mold if venting is ignored. A bathroom fan without sufficient far for the square footage of the room might cause the fan to not move all the air out the duct and you can end up with soap scum, mildew, mold and germs.

The one thing you should not do is guess when sizing a bathroom ventilation fan. Thebest way for bathroom fan sizing is to use the Bathroom fan size calculator we’ve written under. Otherwise, ventilating your bathroom by guesswork could lead to mold and mildew problems, which can be expensive to correct.

Before using a bathroom fan size calculator, you will need to measure the square footage of the bathroom. Square footage refers to the area of your floor plan which is covered in current materials, like carpet or tile. For example, if your bathroom is 10 feet by 10 feet, it has 100 square feet of the floor plan area. The rest is easy, make sure to choose a bathroom fan that has declared Exhaust power.

Sometimes a second bathroom fan is needed

Toilet and shower areas that are completely enclosed by walls or doors require ventilation. If there’s a toilet area enclosed by walls, it must have a 50 CFM exhaust fan because of the long-term buildup of moisture. However, a bathtub or shower enclosure needs an introduction of makeup air for a short period of time during use. For example, if you run the shower at full blast until the room is humid, the fan should be enough to remove that humidity when you leave the room.

Bathroom fan size calculator

To properly size a bathroom fan, you must first measure your bathroom. Begin by measuring the length, width, and height of your bathroom. If you have an unusually shaped bathroom, it might be easier to draw out your floor plan and label each wall to keep track of your dimensions.

What does the 7.5 value stand for? The exhaust fan must be strong enough to restore the bathroom with fresh air every 7 1/2 minutes or 8 times an hour. This magical number (7.5) is the industry standard.

It is important to know that the amount of air moved by any bathroom extractor fan must be sufficient to replace the air in a bathroom with fresh air at least once every hour. In fact, this is the minimum requirement for your exhaust fan. If you have a square or rectangular bathroom, you can use the formula to calculate how much air your exhaust fan needs to move in order to replace all of the air in your bathroom: (Length X Width X Height) ÷ 7.5 = CFM needed.

An example calculation

For a standard 10ft by 10 ft bathroom with an 8 ft ceiling, you will need an exhaust fan capable of 106.67 CFM. Here’s how we get this number:

(10ft * 10ft* 8ft) = Volume = 800

Volume/7.5 = 106.67 CFM needed

The final Bathroom Fan Exhaust Size Formula:

Cubic Volume/7.5= CFM Exhaust Bathroom Fan Power Necessary

Questions & Answers

Can a bathroom fan have too much CFM?

Bathroom vent fans are rated for the amount of air they can move. Value is measured in cubic feet per minute, or CFM. Standard fan sizing applies to bathrooms that are 100 square feet or less. The rule of thumb is that you need at least 1 CFM per square foot of room area.