Bath Fan Guide

Your bathroom will naturally collect its fair share of moisture. With hot water coming from the shower, bathtub and bathroom sink, humidity and moisture can’t be avoided. But if that moisture isn’t vented out of the bathroom, it can quickly be a breeding ground for mold. A bathroom fan quickly and effectively vents the moisture out of your bathroom to leave you and your family with clean, breathable air.

Because every bathroom and its fixtures are different, there’s a wide range of bathroom fans available. For larger bathrooms or those that have multiple potential sources of moisture (like a Whirlpool tub or two sinks, for example), you’ll want a more powerful exhaust fan with higher CFM rating (exhaust fans are rated in terms of how much air can be exchanged throughout the bathroom in one minute, measured in Cubic Feet per Minute; hence, CFM). Another way to shop for a bath fan is to consider its number of sones, a measurement of sound.

Bathroom fans are available with a number of features. Some fans are equipped with a heater that kicks in soon after the fan is turned on. This gives the user another source of heat, a great feature for those colder mornings and evenings. Other fans come with a built-in light. This can be used as a primary or secondary light source for your bathroom while it’s getting vented. Still other bath fans with a light give your bathroom a design statement.

Bathroom exhaust fans are generally very simple to install and can make the difference between a bathroom that’s healthy in every way and one that’s ripe to allow mold to grow.

Bath fan FAQS

How do I calculate the size fan for my bathroom?

Here are two ways to calculate the minimum recommended CFM. For a standard 8-foot ceiling, take the bathroom’s square footage (multiplying its length by width), then multiply that by 1.1. For a bathroom with a ceiling height over 8 feet, determine the cubic feet of the room (length by width by height), and multiply that times 8. Take that number and divide it by 60.

Is it safe to install an exhaust fan directly over the bathtub or in a shower?

We can’t recommend this in any way. Bath fans are not UL-approved for this installation location.

If I’m installing a bathroom fan in the ceiling, where exactly should it go?

An exhaust fan should be installed right around the shower enclosure.

Can I install a bath fan in a wall?

Yes, but mount it as high as possible. But any fan without a light should be installed on the ceiling, not a wall.

Why would an exhaust fan not remove moist air from a bathroom?

This can be for a few reasons, including the size of the fan (it’s too small), wrong duct size, a duct obstruction, and an incorrect position in the bathroom.

What type of exhaust fan is recommended for a sauna or hot tub area?

A fan with a high CFM rating is recommended.

Do you recommend galvanized metal or flexible plastic duct?

When it’s possible, use rigid galvanized duct. It has less resistance to air flow and the fan will operate more efficiently. If flexible plastic is used, make sure the duct is as straight as possible.

Can I use a 3-inch duct pipe on a bath fan with a 4-inch duct connector? Can I use a 4-inch duct pipe on a bath fan with a 3-inch duct connector?

The first is NOT recommended. This will cause the fan to work harder, create excess noise, and reduce the fan’s performance. The second combination is OK—but only if you use flexible (or insulated) duct. Be sure to strap it tight with a long wire tie so it doesn't leak for less noise.

Can I use a multi-port bath fan in a single large bathroom?

This works great for larger bathroom, since you could install the ports over the spots where ventilation is most needed. The size of the room would determine the size of the multi-port bath fan.

What is “make-up air” and where does it come from?

Make-up air is the clean air needed to replace humid air being exhausted by the fan. This air is generally provided by a gap at the bottom of the bathroom door. A ½” gap between the bottom of the door and the floor or carpet should be enough sufficient for most installations.