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How Bathroom Fans Reduce Humidity

When the water is running, the air in a bathroom with no circulation can feel heavy to breathe and can create a mess that needs to be cleaned later. There are many ways to reduce the humidity in a bathroom, but only one is the most effective.

Bathroom fans reduce humidity and condensation by extracting warm humid air via ventilation ducts to the outside of the house. The humid air is replaced by dry fresh air from outdoors through a fresh air intake vent.

A bathroom fan creates airflow while water is flowing, removing the moisture and heat resulting from the hot running water. That airflow is important to keep humidity at bay.  

Bathroom fans are the best option for reducing humidity in residential homes.

For best results place the bathroom fan as close to the water source as possible. In my article about placing a bathroom fan in a shower, I discuss this in more detail.

How Does the Bathroom Fan Work?

Bathroom fans effectively ventilate the bathroom, dispelling humidity caused by baths, hot water taps, and showers. The fans circulate the air inside the bathroom, bringing in air from the outside.


Bathrooms that do not have opening windows, benefit more specifically by the installation of a fan. In fact, if you do not have a window in the bathroom a bathroom fan is required by code.

Not only does the bathroom fan reduce the effect of high humidity, but it also reduces the potential for the growth of mold and mildew. In my previous article, I list the bathroom fan as the nr 1 prevention for mold in the bathroom.

Mold and mildew will eventually cause damage to the structure of the house if left unchecked.

The benefits of a bathroom fan include:

  • Reducing condensation
  • Eliminating odor by removing damp and stale air
  • Preventing mold from forming and thriving
  • Extending the life of the bathroom structure, fixtures, and fittings
  • Protects bathroom walls and paint from damage caused by moisture

It’s Not the Heat — It’s the Humidity!

Balancing the humidity inside the house is important. The indoor humidity level should ideally be between 30% and 50% (source).

The indoor humidity level should ideally be between 30% and 50%

Humidity sensor and a thermometer placed on a moist window

Humidity occupies the home as an unseen entity. Humidity in the bathroom is more prevalent than in any other room in the house. 

It is the one room where activities take place that introduces excess moisture into the air. Activities such as showering and the running of hot water probably occur more than once every day in the bathroom.

Related article: 6 Easy Steps To Choose The Right Bathroom Fan

Since the human body cools naturally through evaporating moisture from the skin, not even an air conditioner is guaranteed to cool the body sufficiently if the humidity is high. This alone is a good reason for installing an efficient bathroom fan.

Selecting the Best Bathroom Fan

It is imperative to install the optimal fan according to the unique specifications of the bathroom.

Bathroom fans are available in many different sizes, and precise measurements are a must to calculate which one is the perfect size fan for the specific bathroom.

So many types and models exist to choose from. Fans mounted on the ceiling are most efficient since they effectively remove the hot and damp air as it rises.


Measuring the Bathroom

The right size bath fan is important to adequately maximize efficiency and energy-saving and must fit the size requirements of the bathroom. An oversize bathroom fan will waste energy and is expensive to run.

Step 1: Measuring

Measure the length, width, and height of the bathroom using a tape measure. Multiply the length times the width times the height to calculate the volume. Include the shower space and bath space when calculating the volume.

Multiply the length times the width times the height to calculate the volume.

Alternatively, calculate the volume using the measurements provided on the architectural plans of the house.

Step 2: Calculating CFM

Cubic Feet Per Minute(CFM) is used to measure the amount of air a fan moves in one minute. The bigger the number the more airflow the fan will provide.

In my previous article, you can read more about how bathroom fans are rated.

The first and easiest option is to use the square footage of the bathroom as a guide. However keep in mind this will not be as accurate as it wont account for ceiling height.

Example: 10 ft.x 10 ft. bathroom = 100 sq. ft. And it would require approximately 100 CFM fan.

The second option would be to calculate the cubic feet per minute (CFM) by dividing the volume of your bathroom by 7.5.

For example, a bathroom measuring 10 feet long, 10 feet wide, and 10 feet tall has a volume of 1000 cubic feet (10 x 10 x 10). Dividing the volume by 7.5 calculates a CFM of 133.33.

A third option is to calculate the CFM by dividing the volume of the bathroom by the number of minutes in an hour and multiplying the answer by the number of mandatory circulations for the fan to work effectively, which is eight circulations: 

1000 ft3/ 60 = 16.67 
16.67 x 8 = 133.33 CFM

Step 3: Choosing the Correct Size

The CFM rating on the packaging of the bath fan must match or slightly exceed the CFM value calculated by the steps above.

A potentially easier way to determine the CFM will be to base requirements according to the number of fixtures in the bathroom. 

Rather than choosing a fan based on the CFM rating, installing a smaller fan right above the fixture will provide enough ventilation. In this way, a 50 CFM fan installed directly above the toilet will do the job (source).

Modern white bathroom with shower, toilet and a bathroom fan above it

The following are the requirements for each fixture:

  • Each shower, bathtub, and toilet needs 50 CFM
  • A jet tub needs 100 CFM

How Should I Use the Bathroom Fan to Reduce Humidity?

To check whether the fan is doing its job by reducing the humidity in the bathroom, simply carry out this simple test: turn the fan on just before turning on the shower and leave the fan on for the entire duration of the shower.

A fogged bathroom mirror indicates that the bathroom fan has too low airflow (CFM) for the size of the room.

A bathroom fan that is properly sized will reduce humidity effectively.

All you need to do to reduce humidity is remember to turn the fan on before you get in the shower and leave it on for at least 20 minutes after you get out. Although I recommend leaving it on a little longer since in most climates it will take longer to fully dry out the bathroom.

To ensure that the fan is suitably matched and adequate for your bathroom, follow the steps above to determine the CFM rating.

A simple way to promote air movement while reducing the risk of wear and tear to the fan motor is to leave the bathroom door or window slightly open while the fan is in operation.

How Long Should I Keep the Bathroom Fan On?

The general rule-of-thumb to clear humidity from the bathroom is for the fan to run for a minimum of 20 minutes after turning off the shower.


Regions that experience high outdoor humidity should make use of a timer to run the fan for the required time after vacating the bathroom. Running the fan for an extra hour should suffice to dehumidify the bathroom effectively under these conditions.

For those who cannot hang around while clearing the bathroom of humidity, the recommendation is also to install a timer to take the place of the standard fan switch. The timer will automatically switch the fan off after the set time.

If you are looking for a timer switch head out to my list of top 9 timer switches.

Should the Fan Stay on all the Time While Showering?

Running the fan while showering will mean less steam altogether because much of the steam will not build up in the first place. But running the fan while showering may not be preferential for those who like a particularly steamy shower.

For energy savers, running the fan while showering might seem wasteful during the colder months since it will expel hot air from the house. However, with fighting mold that will be the result of not using the fan will be much more expensive(source).

Should the Fan Be on for a Certain Time after Showering?

The bathroom fan should be on for at least 20 minutes after you get out of the shower. However, this will vary depending on your climate.

Should the Fan Be on only after Showering?

For individuals who prefer a really steamy experience in the shower, the suggestion is to leave the bath fan off while showering.

As mentioned previously, running the fan while the shower is on will mean less steam because the steam will not build at all. 

For those who prefer less steam in the shower, by all means, run the fan while in the shower. But keep in mind you will have to keep the bathroom fan running much longer after you get out of the shower.

How Effective is a Timer Installed on the Bath Fan?

It is easy to leave the bathroom without switching the fan off. Installing a good timer to the bathroom fan will eliminate human error as far as forgetting to turn the fan off before vacating the bathroom. 

A countdown timer switch of any sort programmed to a certain time will automatically shut the fan off. Many timer options offer a selection of preset choices as well as a reset button, allowing you to run it according to your preferences.

ENERLITES 4-Hour Countdown Timer Switch, 5-10-30-60 Min, 2-4 Hour, For Bathroom Fans, Heaters, Lights, LED Indicator, 120VAC 800W, No Neutral Wire Required, UL Listed, HET06-J-W, White

Many also feature a night-light to make it easier to locate the light switch at night.

Can I Safely Leave the Bathroom Fan on through the Night?

You should not leave the fan on for prolonged periods(source). Unless of course, you have a fan that is designed for continuous operation.

However, in most cases, those fans will have 2 speeds and the lower speed will be running constantly even when you turn the fan off from the switch. You can read my previous article if you would like to learn more about dual speed fans.

Final Thoughts

There really is no question that bathroom fans do reduce humidity. The very nature of this room in which humans conduct their all-important cleaning rituals means that it is, by default, a humid environment. 

Installing such a ventilation fan will not only keep the home environment healthy by preventing the formation of hazardous surface fungi, but it will also ensure a sound home structure and an optimal and fresh-smelling living environment for the whole family.

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