Bathroom Exhaust Fans: What You Need To Know Before You Buy

bathroom exhaust fan

Let’s face it: Modern homes need bathroom exhaust fans. The times of opening a window to ventilate the bathroom are over, and buyers expect exhaust fans.

Without a bathroom vent fan, moisture can buildup in the room, leading to mold growth, door hardware rust, and paint damage. Advanced features for new bathroom exhaust fans include bluetooth speakers, heaters, motion sensors, humidity sensors, LED lighting, and more.

And even the wall switch to control the exhaust fan can have unique features such as countdown timers, as well as built-in motion and humidity sensors.

Bathroom Exhaust Fan Installation: Is It Worth It?

Here are a few reasons you may want to invest in a new bathroom exhaust fan. Modern ventilation fans have DC or direct current motors that can be up to 30% more efficient than older AC exhaust fans — making them cheaper to run and noticeably quieter.

Modern bathroom fans also have multiple ways to turn on such as humidity and motion sensors. These sensors will automatically turn on and off the fan based on the humidity levels — or if it detects movement in the bathroom. And you can even get a vent fan that has a built-in bluetooth speaker to listen to your favorite podcasts or music while taking a shower.

Below is my guide on bathroom exhaust fans. In this guide, I will explain how bathroom fans work, tips on installing a new vent fan, the average costs, and how to choose the right one for your bathroom. I will try to help you avoid any surprises from buying a new fan.

Read Also: How To Vent A Bathroom Exhaust Fan Into An Attic?

How Do Bathroom Fans Work?

bathroom exhaust fan
  1. The bath fan gets turned on manually by a wall switch (1).
  2. It can also get turned on automatically through a humidity or motion sensor (2).
  3. An electric motor (3) in the range of 15 to 100 watts gets turned on.
  4. The motor spins fan blades or a squirrel cage (4) and it sucks in the moist bathroom air.
  5. The exhausted air is sent into a 4-6 inch diameter duct (5).
  6. Air travels through the duct (either smooth-walled or flex) and goes to the exterior vent hood (6) either on the wall or roof.
  7. The pressure of the air opens up the metal or plastic exterior hood flap (7), and it gets disbursed into the outdoor air.
  8. The bathroom gets turned off by using the wall switch or an automatic sensor detects lack of movement (motion sensor), a drop in humidity (humidity sensor), or a timer. The countdown timer may be on the wall switch or built into the ventilation fan itself.
  9. Once the bath fan turns off, the flapper closes on the exterior hood, preventing pests and outdoor air infiltration.

Read Also: What Are The Most Quiet Bathroom Vent Fans?

What to Know About Bathroom Exhaust Fans

bathroom exhaust fan

Bathroom Exhaust Fan Cost

Prices for bath fans range from $50 up to a few hundred. The average bathroom vent fan is probably around $100. The most expensive bathroom fans have heaters, lights, and bluetooth speakers.

The cost to install a bathroom exhaust fan by a contractor will be in the range of $100 on the low side, and up to several hundred dollars or more depending on the complexity such as installing new wiring or a new duct.

How To Install A Bathroom Exhaust Fan

Homeowners with some DIY skills won’t have too much trouble doing a bathroom fan replacement job, but advanced installations should be left to the pros. If the bathroom fan requires new wiring, wall switch, duct replacement, or drywall repair — a qualified contractor is recommended.

For exhaust fans above bathtubs or showers, they need to be connected to GFCI circuits, and high wattage bath fans may even need dedicated electrical circuits such as bath fans with built-in heaters.

For more details on installation, see the subsection below entitled “How To Install Bathroom Exhaust Fans”.

Bathroom Fan Maintenance

The most common maintenance task for modern bathroom fans is to clean the grill. The cover or grill can collect dust which will eventually impede air flow causing the bathroom exhaust fan to slow down. The dust can also make the motor work harder and make it louder. It also presents a potential fire hazard if it causes the motor to overheat.

It is pretty easy to remove the cover (uses spring clips) and to clean it under a sink with dish soap. Every few years you may also want to go a bit further and clean the fan blades as well as the motor which can also collect dust.

Oiling modern bathroom vent fan motors isn’t normally required because they are permanently sealed (no oil ports), but for old bathroom fans oiling is recommended.

Read Also: How To Remove Your Bathroom Fan?

What Size Bathroom Exhaust Fan Do I Need?

bathroom exhaust fan

Here’s how to correctly size your bath fan. Bathroom exhaust fan sizing (fan speed) is based on CFM or cubic feet per minute. This is a standard measurement of fan airflow — how it can exhaust every 60-seconds.

If you underpower your ventilation fan, it won’t properly exhaust the moist bath air, but if it is too powerful, you may be wasting money and contribute to negative air pressure problems such as backdrafting.

To find out whether the bathroom fan is right for your bathroom, you need to measure the square footage of your bathroom…

  1. The first step is to measure the floor space of your bathroom. Take a tape measure and multiply the width and the length to get your square footage.
  2. If the bathroom is 50 square feet or less, the CFM should be at minimum 50-cfm.
  3. If your bathroom is between 50-100 square feet, the CFM should be at minimum equal to or greater than the square footage.
  4. If the bathroom is greater than 100 square feet, you should add up all of the plumbing fixtures together giving 50-cfm to each one. Any jetted tubs you should add 100-cfm.

Read Also: Why Bathroom Vent Fans Leak When It Rains Outside?

New Bathroom Fan Technology

Bathroom exhaust fan technology is always improving, and below are some of the latest features…

Adjustable CFM

Some exhaust fans have adjustable CFM or fan speeds. There usually is a little switch on the bathroom fan housing that allows you to choose your CFM — usually 2 or 3 options.

If you think the bathroom fan is too weak, you can increase the CFM easily.

LED Lighting

You can buy bathroom fan with lights that have efficient LED lighting built into them. It may either buy an LED light bulb, or it may be a LED integrated chip panel.

If you already have a bathroom fan with a light, then you can use the same wiring to the wall switch. But if you are replacing a fan that doesn’t have a light, you will likely have an additional wire to the wall switch if you want to operate the fan and light independently.

Bluetooth Speakers

Bath fans that have built-in bluetooth speakers is a must if you are a music or podcast junkie. The speaker is usually on the perimeter of the bathroom fan cover, and it can be controlled using the phone app provided by the manufacturer.

Variable Speed Motors

A select few of modern bathroom vent fans have variable speed motors. This means that the fan motor will automatically increase or decrease to overcome resistance to the desired CFM.

For example, if you have an extra long duct with multiple bends, the motor will increase its speed to compensate. In addition, it may decrease the fan speed after an initial high speed to clear out the bathroom moisture — saving energy.

Flexible Installation Brackets

Buying an exhaust fan that includes an adjustable installation bracket is a great idea.

Installation brackets allow you to install bathroom fans that don’t span the width of your ceiling joists, so you don’t have to attach wood spacers to the joist. You simply adjust the bracket to fit in-between the joists, fastening it with screws.

The bathroom fan is secured on the bracket itself and not the joists.

Low Profile Bathroom Fans

If you have older ceiling joists without much ceiling cavity space, or if you have other obstructions in your ceiling — a low profile ceiling fan may be a good choice.

Low profile bath fans have much narrow housings that allow them to fit into tight ceilings. Some low profile bath fans are even designed to be able to fit in walls with a vertical install — it can fit in a wall cavity with 2×4-inch studs.

Ultra Quiet Motors

With modern DC or direct current motors, bathroom exhaust fans can be virtually silent. In fact, some of these ultra quiet ventilation fans have indicator lights that light up when the fan is turned on in case you can’t hear it.

These quiet bathroom exhaust fans usually have sone ratings from 1.0 and below. Sone is a measurement of loudness that is perceived by humans (different from decibels). Bathroom fans that are 3.0 sone or louder are considered pretty loud.

Read Also: What Are The Best Bathroom Fans With Lights?

Bathroom Exhaust Fan Control Options

Humidity Sensors

If you are tired of having to flip a switch, you can buy a bathroom fan with a built-in humidity sensor. This feature is also nice if you have kids or tenants who neglect to use the bathroom fan.

Humidity sensors have a set humidity level target, and when moisture rises above this level (such as 60% relative humidity) the sensors turns on the fan. And when the humidity drops below the target, the fan turns off.

One problem with humidity sensors is that if it isn’t calibrated properly, it may turn on and off your bathroom fan when it shouldn’t. The best option is to buy a fan that has an adjustable humidity sensor so you can tweak the sensitivity.

Motion Sensors

Motion sensors will turn on and off the bathroom fan based on movement detected. You may also be able to use a motion sensor in combination with a humidity sensor.

For example, you can program the exhaust fan to turn on when it detects movement, and then it will turn off when the moisture drops below a set target.

Similar to humidity sensors, the main problem for motion sensors is over-sensitivity and turning on when it really shouldn’t.

Wall Switch

The standard control for a ventilation fan is a wall switch. This can work fine if you have a simple bath fan, but if you have extra features such as a heater, humidity sensor, or light — you may want more than one switch.

Adding a switch will require adding another wire from the bathroom fan to the wall. For bathroom exhaust fans with humidity sensors, you can have a single switch and you will have to manually toggle the switch to manually turn on the fan (overriding auto mode).

Advanced Features

There are also advanced wall switches that include advanced features such as countdown timers, humidity sensors, and motion sensors.

Buying a switch with a countdown timer is great alternative to a humidity sensor to make sure that the bathroom fan is on for at least 20-min after a shower to remove all of the moisture.

Read Also: Why Did Your Bathroom Exhaust Fan Stop Working?

Bath Fan Exterior Vent Hoods

bathroom exhaust fan
bathroom exhaust fan

Exterior vent hoods or covers are required for properly installing a bathroom fan duct to the outside. The exterior vent cover has a flap or louvers that open and close based on the air flow from the ventilation fan. When the fan is turned off, the flap will closed which helps prevent outside air and pests infiltration.

There are three main types of exterior vent hoods…

1. Wall Hoods

These vent hoods are installed on the exterior wall, and are frequently made out of plastic.

2. Soffit Vent Covers

The soffit vent cover is a special type of vent cover that is installed on your soffit, and it has a spring-loaded damper instead of a flap.

3. Roof Hoods

Roof vent hoods are very similar to the standard wall vent hoods except that they are made out of metal. These roof vent covers are also usually galvanized and powder-coated in order to stand up to the harsh elements when exposed on the roof.

Read Also: How To Vent A Bathroom Fan Through The Wall?

Bathroom Fan Venting Options

bathroom exhaust fan

Even though there are only three types of vent hoods (soffit, wall, or roof) — there are multiple ways to actually vent the fan.

The first method is the most common and it is the standard ceiling bath fan to the exterior wall. The second vent configuration is to the roof, and the vent hose goes through an unconditioned attic space.

A third venting strategy is to install a special made ‘through-the-wall’ exhaust fan. These exhaust fans have very short ducts and it goes immediately to the outside — they are installed only on exterior walls of the bathroom. The duct may only be a few inches.

Similar to through-the-wall exhaust fans, you can also buy a low profile bathroom fan that can be installed vertically on the bathroom wall. They can even fit in-between 2×4 wall studs. You can also install low profile fans on the ceiling. These bath fans are vented to the exterior wall or roof. And lastly, the bathroom fan can be vented to the soffit with a special spring-loaded soffit vent cover.

Read More: What Are My Bathroom Fan Venting Options?

Bathroom Exhaust Fan Code Requirements

Even though state and city (or county) building code will always take precedence over national or international code — here are a few key codes from the International Residential Code or IRC…

1. Outdoor Venting

Probably the single most important code related to bath fans is that they need to vent to the outdoors.

Bath vent fans aren’t meant to vent into an attic, a soffit, crawlspace, or wall cavity. Bathroom vent fans exhaust a lot of moisture which can lead to mold growth or wood decay if it isn’t vented outside.

2. Ten Foot Rule

The 10 foot rule states that bathroom fans should exhaust at least 10-ft away from any intake vent openings such as for high efficiency furnaces.

There is an exception that if the bathroom fan is above the intake vent, then it should be at least 3-ft from the vent.

3. Windows

Bathroom exhaust fans need to vent at least 3-ft away from all windows and property lines. This code is similar to the 10-ft rule since a window is a type of intake as well.

4. Wet Areas

If you want to install an extractor fan directly above a shower or bathtub, it is supposed to be connected to a GFCI circuit.

GFCI is a special type of wall switch or electrical breaker that will instantly shut off power to the circuit if it detects any leaked voltage — preventing anyone from getting shocked. It is a common safety feature for bathroom, kitchen, and garage outlets.

5. Duct Length & Diameter

There are IRC code requirements for the length of the duct based on the CFM, duct diameter, and whether the duct is smooth-walled or flex.

As an example, if you have a 100-cfm bathroom fan, and you have a 5-inch diameter duct that is flex duct — the maximum duct length is 42-feet. You see the chart below for the guidelines…

Read More: Guide On Bath Fan Code Requirements

bathroom exhaust fan
Source: ICCSafe.org

Duct Condensation Problems

bathroom exhaust fan

One common problem with bathroom exhaust fan ducting is that they sometimes develop condensation problems. If the bathroom fan duct goes through an attic, garage, crawlspace, or other unconditioned area — moisture may form on the exterior or interior of the vent. The water inside the duct may even obstruct the air flow, and if it is dripping from the vent — may lead to mold growth or ceiling damage.

The easiest solution to prevent condensation is to install a bath fan duct that includes an insulated sleeve. These flex ducts are wrapped in fiberglass insulation and includes a vapor barrier.

When installing a bathroom fan duct, you also want to minimize sags in the vent which can accumulate water — and to also minimize turns.

Read Also: What Are My Top Picks For Bath Fans With Humidity Sensors?

How To Install Bathroom Exhaust Fans

Here are a few key tips for you and your contractor on the bathroom fan installation:

1. Turn Off Power

The first step is turn off power to the bathroom at the electrical panel. You can verify that power is off with a non-contact voltage detector.

2. Remove Old Bathroom Fan

You will need to pull down on the cover, and squeeze the metal clips holding it to the fan. Removing the fan assembly and housing is done be removing any screws and side clips holding it to the housing.

3. Disconnect Wiring And Duct

Before completely removing the fan housing, you will also need to disconnect the wiring and duct. To remove the duct, you may need to push the bathroom fan into the ceiling cavity or by using the attic.

4. Connect Duct & New wiring

Before fastening the new bathroom fan to the joists, you should first connect the wiring and the duct.

5. Secure New Bathroom Fan

Connect the new bath fan with the screws provided to the joists or to the installation bracket.

6. Turn On Power

Flip the breaker switch back on, and verify that the fan is working properly.

Read Also: How To Replace A Bathroom Fan Without Attic Access?

Installing A Larger Diameter Bathroom Duct

Many older bathroom fans are installed with 3-inch diameter flex ducting, but more powerful and modern bathroom fans usually require 4, 5, or 6-inch diameter ducting. You could just install an adapter that goes from the larger diameter collar to the existing 3-inch duct — but you won’t get the stated performance or Sone rating of the new fan.

If you have attic access, it should be pretty easy to replace it since you can do it in the attic. However, on a lower level of the home, you will first need to remove the existing bathroom fan, and the exterior vent hood.

After removal, you will need to pull out the existing vent, and tie it to one end of the new vent — snaking it through to the outside.


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Arie Van Tuijl

Arie Van Tuijl

I am a licensed home inspector in two U.S. states and the founder of Home Inspector Secrets. After performing hundreds of inspections, and seeing thousands of house defects, I realized people would love to have an online resource dedicated to home maintenance. I write detailed home guides and product reviews sprinkled with inspection tips. You can read my bio here.

About Home Inspector Secrets

Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.

Other Home Guides

Looking to learn more about home maintenance? Check out our other informative home product reviews and guides!