Bathroom exhaust fans are required by the NEC (National Electrical Code) and local building codes for any bathroom that doesn't have a window that can be opened to provide ventilation. In some jurisdictions, they are required even if windows are present. Bathroom exhaust fans must be vented to the outside through ductwork or sidewalls.
The Spruce / Margot Cavin
Installing a bathroom exhaust fan is worth the time and effort because it removes hot, moist air and odors. Moist air leads to mold growth and odors lead to general unpleasantness.
But installing a weak, undersized exhaust fan is almost not worth the effort, even if the fan meets the minimum requirements. It's important to understand how vent fans are sized and how to choose one with the proper capacity for your space.
Types of Bathroom Exhaust Fans
There's a difference between an exhaust fan and ventilation fan; an exhaust fan draws air out and a ventilation fan brings fresh air in. If you have to replace a bathroom exhaust fan, your best bet is to go with the type that's already in place. Four types of bathroom exhaust fans include:
Modern ceiling-mounted vent fans come in variations that may integrate ceiling lights, heat lamps, or sometimes even Bluetooth speakers. Accessories can generate extra heat in a bathroom, though not as much as older fans without all the bells and whistles. Check with the manufacturer to make sure you can put insulation over the box of a fan with accessories.
Standard Fan Sizing
Bathroom vent fans are rated for the amount of air they can move, 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.
To determine the square footage of your bathroom, multiply the length times the width. For example, if your bathroom is six feet wide and nine feet long, its square footage is 54. Therefore, it should have a fan rated for at least 54 CFM. But before you start shopping, there are a few things to consider.
When sizing a vent fan, a factor to consider is duct size and length. Most 50 CFM fans will run well with a 4-inch round duct. But as you get up into the higher CFM fans, duct size will have to be increased to a 5- or 6-inch round duct. The length of the run, as well as the number of fittings and elbows will also affect the amount of air your duct can effectively carry.
There are somewhat complicated formulas for calculating proper duct size for different length runs and configurations, but the instructions that come with the fan will outline the requirements. Just make sure there is sufficient space for the ductwork in the area where you're installing the fan. Trying to force too much air through an undersized duct will make the fan work too hard and will provide insufficient venting.
The Spruce / Margot Cavin
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Sizing for Large Bathrooms
For bathrooms over 100 square feet, exhaust fans can be sized according to the number of fixtures in the room. To calculate using this formula, add up the required CFM ratings for all of the fixtures:
For example, if your bathroom includes just a shower and a toilet, it needs a 100 CFM fan, while a bathroom with a jetted tub, toilet, and shower needs a 200 CFM fan.
Another calculation method with three steps is useful for large rooms with high ceilings (well over 8 feet). In this method, multiply the square footage by the ceiling height, divide by 60 (minutes in one hour), then multiply by eight (recommended number of air exchanges per hour). For example, for a room that's 120 square feet and has a 10-foot ceiling:
Fan Noise Ratings
Once you've chosen the right fan size for your bathroom, consider the sound rating, as demonstrated by the sone rating of each fan. (Sone rating is a measure of loudness.) A quiet fan might be a worthy investment, particularly in a bathroom you'd like to make peaceful and relaxing. The lower the sone number, the quieter the fan will sound when running. A sone rating of "1" is the quietest fan you can buy. Here are other sone ratings compared to other sounds:
A Second Fan May Be Needed
If your bathroom has a toilet or shower area enclosed by a door, it's a good idea to install a separate exhaust fan for that area. If the enclosed space is small, a 50 CFM fan should suffice. Otherwise, use the appropriate calculation based on the room size. Just remember that the fan needs airflow to do its job.
If the door to the enclosure is closed and there's no gap at the bottom of the door, the fan will be starved of "makeup" air and will perform poorly. If the gap under the door in an enclosure is less than 5/8 inch, you should leave the door slightly ajar when using the toilet or shower when the enclosure's vent fan is running. Another solution is to install a louvered door that allows airflow.
A bathroom exhaust fan with accessories—especially heat lamps—will likely require its own dedicated circuit. If the vent fan unit is just a vent fan with no add-ons, most local electrical codes will allow it to be powered by the same general lighting circuit that controls the room's lighting fixtures.
Your indoor air quality plays an important role in how we feel every day. So how do you improve it? A big part of improving your indoor air quality is controlling moisture in your home. Installing a bathroom exhaust fan is a big part of that. I’m going to explain how to choose a vent fan for your bathroom.
Bathroom fans aren’t just for removing odors. You can actually have serious issues from poor bathroom ventilation. This is especially important in a bathroom that does not have a window. Poor ventilation in a bathroom can lead to mold and bad indoor air quality.
The best bathroom exhaust fan will get rid of excess moisture properly. This means protecting your bathroom against water damage.
Some symptoms of poor indoor quality that you may notice are:
frequent headaches
Dryness and irritation in the eyes, nose, throat, and skin.
Fatigue
Shortness of breath.
Hypersensitivity and worsening allergies
Sinus congestion
Coughing and sneezing.
Dizziness
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There are a lot of options available with different installation styles, capacities, and noise levels when it comes to choosing your bathroom vent fan. You first need to decide on the type of bathroom fan you want to install.
You will probably come across one of three styles:
Ceiling-mounted,
Wall-mounted,
or Inline/remote.
Ceiling-mounted fans are installed on the ceiling. These are the ones we install on our projects and they make the most sense. Think about it. Warm, moist air rises so it makes sense to have a bathroom vent fan on the ceiling.
Ceiling insert fans pull the warm and humid air out of the bathroom along a duct and vent it through the roof. In some cases, it can be vented out the wall outside.
These are mounted on the external wall of the house and generally are used when there is no practical way to vent through the roof; for example, in a first-floor bathroom.
These have a fan unit located in a remote location such as the attic. Ductwork connects the fan to an opening in the bathroom ceiling covered by a grill. They are often installed when a homeowner wants to use one fan to vent multiple bathrooms, or to have multiple vents in one large bathroom.
For years I have been installing Panasonic ventilation fans on my projects. They are built right with high-performing motors. And you can expect a long life from your Panasonic ventilation fan.
If you have watched my shows, you know I HATE loud bathroom fans. The Panasonic fans are made to run quieter and are simple to install. I also like the speed-adjusting technology, you can change from 50, 80, and 110 CFM.
Here are the best options for bathroom fans from Panasonic:
By the way, the noise level for these Whisper Series fans is <0.3 sones —it’s super quiet you won’t even notice it’s there.
There are a lot of choices on the market when it comes to bathroom fans. Here are some things to consider:
Bathroom fan sound levels are measured in sones. To put this into context, 4.0 sones is the sound of a standard TV. 3.0 sones is your typical office noise and 1.0 sones is the sound of a refrigerator.
For quiet bathroom ventilation, the fan should be rated at 1.0 sones or less. The sone rating is an important factor.
A fan’s ability to move air is measured in cubic feet per minute (CFM). The CFM rating you need is based on the size of your bathroom. A fan should have a CFM rating high enough to replace the air in your bathroom at a rate of eight times per hour. Your fan should have 1 cfm for every square foot of floor space in your bathroom.
Some vent fan models let you choose the CFM. This means you have the ability to select your required airflow (50, 80, or 110 CFM) with the simple flip of a switch.
I like this because one fan can work for a range of ventilation needs.
How many fixtures do you have in the bathroom? Do you have a jetted tub? A shower? All these are a factor in choosing your vent fan. If it’s just a small bathroom with a toilet and a shower, you may only need a 100 CFM fan. If you have a jetted tub and shower, you will likely need a 200 CFM fan.
Most 50 CFM fans will run well with a 4” round duct. If you need a higher CFM, keep in mind you may need to increase the duct size to 6”. If you force too much air through a small (undersized) duct, your fan will be working too hard and not providing sufficient venting.
I recommend looking for a bathroom fan that is energy-star certified. Energy Star-rated bathroom fans use about 20% less energy than minimum federal guidelines.
It is also a good idea to look for a bathroom fan that has a timer and humidity sensor. That will cause your bathroom fan to only turn on when the room is humid. I like that because I always recommend running your fan for 30 minutes AFTER a shower.
I personally like bathroom ventilation fans with a night light. This WhisperFit DC Fan from Panasonic has a lighted model with one 10W dimmable LED chip panel. You can also find bathroom fans with Bluetooth speakers, motion sensors, heat lamps, and ceiling lights.
This is what I need you to keep in mind though. A vent fan with a bunch of accessories may actually need its own dedicated circuit. If it’s just a simple vent fan, most likely your local electrical code will allow it to be powered by the same general lighting circuit that powers your lighting fixtures.
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I don’t recommend installing a bathroom fan yourself. It is NOT a DIY project. This involves cutting a hole in your roof and electrical work. If this is a new installation, you may have to run electrical wiring and ductwork in your attic.
Cutting the hole in your drywall based on the size of the fan
Attaching the fan enclosure to the joist with screws
Cutting a hole in the roof (this also requires removing shingles)
Installing a roof vent
Connecting and wiring the fan
Wiring the switch
If you are replacing an existing fan, you need to make sure your existing wiring can handle the additional load. A licensed electrical contractor can guide you. Never vent to an attic or subfloor as moist air creates an environment for mold and mildew growth.
I always say this about home improvement projects, make sure to check on your local building codes before you start work. Your installation must be in full compliance with local regulations.
RESOURCE:
To find a licensed electrical contractor near you visit: myhome.eaton.com or if you are in Ontario, Canada visit findacontractor.esasafe.com
I always recommend installing a bathroom ventilation fan, even if you have a window.
In some areas, exhaust fans are required by building codes, especially if there’s no window.
Your bathroom exhaust fan should be connected to vent ducts that push the exhaust outside your home. This is required by most building codes.
Think about the damage moist air can cause in your home. If not vented properly, this humid air can get to the space between ceiling joists, and cause damage. Venting into an attic (which I have seen A LOT of) simply dumps excess moisture into a closed space. You are looking at mold growing and damaging your floors, walls, and building materials. And mold can cause serious harm to your health as well.
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