Last week we talked about lighting, lighting quality—how to buy the type of light bulb that will give you the color and brightness of light you’re looking for. When it comes to bathrooms, you’ve got a very important appliance that provides more than just light–the exhaust fan.

What’s it for?

A bathroom fan’s primary purpose is to remove the moisture-laden air from your bathroom. It also has the added benefit of removing unpleasant odors. A bathroom fan is not a luxury item.

Bathroom Exhaust Fan

Bathroom fans can have all kinds of bells and whistles, but the fundamental task of removing moisture is a building code requirement. The Residential Building Code requires all bathrooms to have either an openable window or an exhaust fan. And, the ventilation must exhaust directly outdoors – not just up into the attic, out of site.

Why is it required?

A bathroom brings together many destructive forces:
  • Heat and moisture in steam
  • Condensation created from the combination of steam and cold, tiled walls and floors
  • Warm air (because no one likes to shower/bathe in the cold)
  • Additional hot air from heat lamps, room heaters or radiant floor heating

Basically, you’ve created your own personal rain forest.

Moisture is the leading cause of many home and building problems. In the early stages, you may just notice annoying cosmetic issues like bumpy, flaky or chipping paint. As time goes by, you may notice problems with mold. But these are just surface issues. What you don’t see is the rotting and deterioration of ceiling, wall and floor surfaces behind the paint and tile surfaces and the studding.

In the case of fans that just exhaust into the attic and not outside, moisture filled insulation sits on your home’s ceilings making mold and rotting the gypsum board and ceiling joists. If you generate enough moisture, the trapped air can even rot away the structure of your roof. In addition to mold, the high heat/high humidity environment is a wonderful breeding ground for your own personal bacteria and microorganism zoo. So, not only is it destroying your home, it could be destroying your health as well.

Window vs. Fan

In a lot of older homes you may not find a bathroom fan – you may just find a window. Even in some newer homes the builder or homeowner may have opted for just a window. Depending on the location of the bathroom, installing an exhaust fan can be expensive and difficult. But seriously, a window? It’s hard enough to convince yourself to open the window when it’s 20 degrees outside, but how about those bathrooms where that openable window is actually in the shower? (As everyone knows, my two biggest pet peaves are being cold and wet. Combine them and it’s a double whammy. I wouldn’t shower until temps were above 70 degrees.) What happens is the homeowner doesn’t open the window because it’s uncomfortable. So, the builder met the code requirements, but bathroom doesn’t have any protection because the wet air is not going out the window.

Buying Guide

There are two factors to look at (actually three, but we’ll talk about lighting later): 1. Capacity and 2. Noise.


A fan’s ability to move air is measured in Cubic Feet per Minute. You’ll see this on the label displayed as CFMs.

For most bathrooms, the calculation is one CFM per square foot of bathroom area. So, if your bathroom is 7 by 10 feet, you have 70 square feet (7’ x 10’=70) of bathroom. That means you need a fan with a 70 CFM capacity.

The Residential Building Code requires a minimum of 50 CFMs. So, your standard hardware store, Home Depot, etc. is going to carry a range of fans usually from 50 CFMs to 110 CFMs. With fan capacity, the higher the CFM number is, the better. So, if your bathroom is 20 square feet and you get the minimum required 50 CFM fan, it just means the stale, moist air will be taken out of the room faster—and you want that out as fast as possible anyway.

Here’s an easy reference:

Less than 50 sq. feet — 50 CFM 50-100 sq. feet — 1 CFM per square foot of floor space

If you have a bathroom larger than 100 square feet, or multiple water fixtures, you’ll need to do some additional math. And, one fan is probably not going to do it for you. You’ll need multiple units in different locations. The Home Ventilating Institute (HVI) recommends that you use the following CFMs per fixture.

Toilet: 50 CFM Shower: 50 CFM Bathtub: 50 CFM Jetted Tub: 100 CFM

HVI also recommends the following:

  • If your toilet is in its own room within the bathroom, it should have its own fan.
  • Locate the fan as close as possible to the fixture it’s venting. For instance, if possible, put the exhaust fan in the shower rather than in the middle of the room.
  • Fans should also be located away from vents. In other words, don’t put your fan next to your furnace outlet. You want it to suck out the bad air, not to suck out the new air that’s coming into the room.
  • Your bathroom doors should have at least ¾” clearance to the floor to let fresh air in as the stale air is being pulled out.
  • Bathrooms with ceilings taller than 8 ceiling may require additional ventilation.
  • When you’re finished in the bathroom, don’t turn of the exhaust fan. Leave it on for 20 minutes after so that it will continue to pull out the moisture.

So, when it comes to the capacity, the higher the CFM number the better.


Noise is not damaging like capacity. If you pick the wrong capacity, you could have moisture damage in time. If you pick the wrong noise rating, you may just be annoyed and you may not enjoy showering or using your bathroom. Or, you could just pretend that you’re showering at the airport. Whether noise is a factor or not for you, rest assured that exhaust fan labels include this measurement. However, they wouldn’t want you to get by without being confused. So, it’s the noise value is the opposite of a CFM’s rating—meaning, the lower the number, the quieter the fan.

Fans are measured in “sones.” A sone is a measurement of sound in terms of comfortable hearing level for an average listener. The lower the sone measurement, the more comfortable the listening environment. Sones are not decibels or volume, but rather how sound is “sensed.” One sone is equal to the sound generated by a quiet refrigerator. I don’t know how they came up with this measurement, how they define an “average listener” or a “quiet refrigerator.” Does that “quiet refrigerator” include the noise that scares you to death when the icemaker drops the next load of ice when you weren’t expecting it? And, as many of us parents know, if they used a teenager as the “average listener,” they may as well have measured the wall’s reaction. Nevertheless, it is what is it. So… sones it is.

I’ve seen ratings as quiet as .5 sones. But quiet isn’t always the best thing. If it’s too quiet you may end up doing what I did with a hybrid rental car once—you just leave it running because you forgot that it’s on. I’ve also seen ratings as loud as 4.0 in which case I kept getting this feeling that I was missing my flight because I was in the bathroom.

Quiet bathroom fans will be rated as 1.5 or fewer sones. But ultimately you’ll want to consider the size and setup of your bathroom and not just look at the number. A 1.5 sone fan in an enclosed toilet room is going to sound louder than a 1.5 sone fan in a 7’ x 10’ bathroom. Likewise, if you have cathedral ceilings over the shower, you may not even hear the 4.0 sone fan.

Lighting and Other Functions

Fancy Exhaust Fans

As I mentioned before, the primary purpose of the exhaust fan is to protect your bathroom room from the destructive effects of heat and moisture. However, if you’re going to the effort and expense of installing one, you might as well see if there are some other functions for which you can use it.

Many fans feature lighting—heat lamps, ambient light, nightlight or some combination of the three. Unfortunately, you don’t get much say in the quality of the light (see last week’s post). Many fans today are incorporating LED lighting that can’t be replaced or swapped out. And the lighting temperature tends to be in the blue to bright white color range. So, if the quality of light you already have in the room is warm, you may not want to break that up with harsh, cold lighting. Some come with CFL bulbs with low lumens and a very yellow light quality. So, they don’t produce usable, attractive light. If either of these is a concern for you, I would recommend choosing a fan without a light, or, make sure that the lighting is a standard bulb size so that you can choose a bulb with the light quality you want.

Just as LED bulbs are incorporating other technology like Bluetooth and wifi, I wouldn’t be surprised if soon bathroom exhaust fans become wi-fi connections, Bluetooth speaker systems and a whole host of other things we haven’t even imagined yet.

I didn’t get into style choices and design considerations in this post. But just for fun, check out these awesome styles. You’d never guess some of them are exhaust fans.

There are also design considerations for placement and function of bathroom exhaust fans. So, if you need help, or you’d just rather sone out and have someone else take care of your bathroom function and design, email us or give us a call.