Bathroom Exhaust Fans

A bathroom exhaust fan is an electrically powered fan that moves air from the bathroom to the outside.

These fans are much more effective than just simply opening a bathroom window.

During my home inspections, I sometimes find that older homes still do not have bathroom exhaust fans — and I always call it out. Many times my clients thank me because they didn’t notice that the 1950’s era home doesn’t have exhaust fans (usually no bathroom outlets also).

Perhaps you have stayed in a home without an exhaust fan, and you got used to opening the bathroom window. The problem with this method is that during warm weather, you are allowing in hot and moist exterior air (vice versa during winter).

So it may remove some of the bathroom air, but it doesn’t really do anything about the moisture. Plus, you are screwing up your HVAC system by bringing in outdoor air when you have already paid money on conditioning the interior warm or cold air.

What's In This Guide?

Bathroom exhaust fans with humidity sensors are basically fans that will automatically turn on and off based on the amount of moisture in the bathroom.

These fans are great if you don’t want to bother with turning a fan on or off, if you have children who are neglectful of a bath fan, or if you have tenants who don’t use the fan.

In essence, if you want to force the use of a bathroom fan, these are the types of bath fans that you want.

The most common way to use bath fans with humidity sensors is by toggling the wall switch. What I mean by toggling is flipping the wall switch once to turn on the humidity sensor.

If you want to manually override the humidity sensor and turn on just the fan, then you flip the wall switch twice.

Besides using a single wall switch, you can also install a second switch so you have one switch for manual use of the fan, and the second switch is for the humidity sensing mode. In addition, you can even remove the choice of manually operating the fan, so it will only turn on automatically.

If you would like to read my review on the best bathroom exhaust fans with humidity sensors, check it out here!

Many times when I inspect homes for prospective buyers, I find that bathroom exhaust fans are improperly terminated inside of the attic.

Bathroom exhaust fans can be vented through an attic, but they should never actually exhaust moist air inside of the attic.

Venting a bathroom fan inside an attic can lead to wood damage and even mold growth.

I have only come across a few homes with mold growth in the attic, but trust me — it isn’t a pretty sight! It is a very costly process to remove mold that is growing on wood sheathing and rafters.

To clean mold from an attic, a dry ice blasting technique is used, and it costs a pretty penny.

However, there are ways to safely vent a bathroom fan into and through an attic.

Most often, when a bathroom fan is simply vented to the roof, where a special roof cover (with a flap) is installed. Likewise, the bathroom fan can be vented to a side gable wall.

Regardless of where it is vented on the outside, I always recommend insulating the bathroom vent so it doesn’t produce moisture in the attic.

If you would like to read my detailed guide on venting bathroom fans into the attic, check it out here!

Some bathroom fans are so loud, it sounds like some kind of otherworldly creature is lurking in the bathroom.

I can frequently hear these bath fans in other rooms or even down the hallway.

These loud bathroom fans usually are 4.0 sones or louder (sone is a loudness rating).

However, the quietest fans are whisper quiet.

Many times I find it difficult to even know that they are on. This is why the quietest bathroom exhaust fans have indicator lights to let you know that the fan is running! When that blue light is on, you know it is working.

These quiet and nearly silent bathroom exhaust fans are in the 1.0 sone range and less.

When buying and installing a very quiet bathroom fan, it’s important to realize that the vent size (the tube that goes to the outside) should be the same size recommended by the manufacturer.

Many new bathroom exhaust fans recommend 4″ diameter vent hoses. However, most homes with older bath fans have 3″ diameter vent hoses. If you install a newer bathroom fan with an older vent hose, the ratings specified by the manufacturer don’t apply!

Besides the vent hose size, having many twists and turns in the duct can also increase the bathroom fan noise.

If you would like to read my review on the best quiet bathroom fans, check it out here!

bathroom fans exhausting to outside

Installing a bathroom attic access seems like an impossible task, but it is actually straightforward if you know all of the steps.

Just by pushing the bathroom fan into the ceiling a few inches will allow you to disconnect the vent hose, and to pull the fan out of the ceiling cavity.

It isn’t always a good thing to have attic access.

And even if you have attic access, you probably shouldn’t.

I have seen many homes during my inspections where there wasn’t enough insulation (which means you can walk on the ceiling joists). And quite often the “storage platform” (plywood) is installed over the attic ceiling joists. 

Well, this isn’t the correct way to build a storage platform, it should be a raised platform above the joists.

Besides the danger of walking in the attic and poor insulation — you don’t need attic access to replace an existing bathroom fan!

The duct is already going through the attic, so you don’t need to install a new vent hose. You simply need to replace the bathroom fan in the ceiling cavity.

If you would like to read my detailed guide on replacing a bathroom fan without attic access, check it out here!

Sometimes there just isn’t enough lighting in the bathroom.

Perhaps the vanity light just isn’t bright enough or it is missing altogether.

Well, if you want to install a new bathroom fan, there really isn’t any point of installing a separate light fixture (if there isn’t one already).

There are many different bathroom exhaust fans that come with lights already installed. Some are even designed to look like a stylish light fixture instead of a boring fan. 

And if you want energy efficient lighting, there are bathroom fans designed to be installed with LED lights.

In regards to installation, you can install the light on a separate light switch or you can make the fan and light on one switch. Just remember that if you want an extra wall switch that isn’t already there, you will have to run additional wiring from the fan (through the ceiling/wall) to the wall outlet.

An electrician may come in handy for any extra electrical work.

If you would like to read my review on the best bathroom fans with lights, check it out here!  To read my review on the best bathroom fans that come with LED lights, click here!

sones rating for bathroom fan

If your bathroom fan is leaking water when it rains outside, then you know you have a problem.

As you might imagine, the issue has to do with the exterior vent hood. This is the little cover that has flap installed outside.

It could be on a vertical side wall, but it is most likely an issue with a roof vent cover.

If the roof cover isn’t installed properly, or perhaps the flashing and flap is broken, when it rains outside, water will leak into the vent hose, and run vertically (with gravity) towards the bathroom.

This is why I say that 99% of the time it has to do with a roof vent cover because water travels downward with gravity.

But, if there is some downward slope from a sidewall installation, and if there is enough rain or wind, then it could also be an exterior wall issue (not the roof).

When a homeowner has a problem of a leaking roof vent, it is important to carefully inspect the attic for any water damage or mold issues. It’s also a good idea to remove the bathroom fan and inspect the ceiling cavity for mold or water damage. 

Mold frequently grows on the backside of drywall because it likes to eat the drywall paper.

If you would like to read my detailed guide on leaking bathroom fans when raining, check it out here!

A cold bathroom in the early morning isn’t exactly the best way to start a day. Am I wrong?

Especially ceramic tile flooring tends to feel icy cold when half asleep. And then you sit down on a chilly toilet seat.

Well, there is a solution — bathroom fans with heaters.

These bathroom fans have heaters as part of the units — using either a coiled heating element or an infrared heat lamp. The coiled heating elements are generally in the 1000-1500 watt range, and the heat lamps are in the 200-300 watt range.

The higher the wattage heater you purchase, the faster and stronger it will heat your bathroom.

The main difference between the two is that the infrared heat lamp doesn’t heat the air, it heats the objects in the bathroom. However, the coiled heating elements usually produce more heat than the infrared lamps.

In addition, the coiled heating elements sometimes have separate fans to move heat throughout the bath even if the main bathroom fan isn’t on.

Just like with bath lights or humidity sensors, you can either wire the heater to a separate wall switch, or you can wire it to a single switch.

If you would like to read my review on the best bathroom fans with heaters, check it out here!

Choosing the correct size for your bathroom fan is a crucial part of the fan buying process.

An undersized fan won’t do an effective job of exhausting moisture from your bathroom.

However, an over-sized bathroom fan uses more energy (and is louder) than needed, and it may even cause negative pressure problems in the home.


Probably the most important rating is called CFM which stands for cubic feet per minute.

If a bathroom fan is rated at 70cfm, then it means that the fan can exhaust 70 cubic feet of air in one minute. As a rough calculation, you want a minimum of 1 CFM per square foot of bathroom floor space.

In essence, CFM is a rating for how powerful the bathroom fan is.

Bathroom exhaust fans generally range in the 50cfm and all the way up to 150cfm. It’s important to pick a reasonable CFM for your bathroom size. A 120cfm bathroom fan for a small 60 square foot bathroom is overkill.

If you would like to read my detailed guide on choosing the right CFM for your bathroom, check it out here.

Modern bathroom exhaust fans come with many different features, sizes, and levels of quality. It’s important to take an objective look at your bathroom, and the features that you want your fan to have.

Is the noise level most important to you? A low sone fan would be the way to go.

Or maybe it is your kids that don’t use the fan that bugs you? Humidity sensors may be a solution.

Or perhaps your bathroom needs more lighting? Then a bathroom fan that has a light is the right approach.

Bathroom exhaust fans make our lives much healthier and simpler. 

Opening a bathroom window just isn’t very practical or energy efficient. If it’s freezing cold or hot outside, frequently opening the window isn’t a good idea. Having a home in the 21st century just doesn’t make sense without a bathroom exhaust fan.

I hope you enjoyed this guide.