Tips for Positioning a Bathroom Vent Fan

Choosing a location when installing a bathroom vent fan depends on where the fan will work best for removing moisture and how to run the ductwork efficiently. Read on to find out what a bathroom fan does, how best to run the ducts, and the steps you need to take.

Why Install a Bathroom Vent Fan?

A bathroom vent fan is necessary to remove excess moisture from the room during baths and showers. Without a bathroom vent fan, a stuffy room with no outlet for steam and condensation will develop mold, mildew, and water spots on the ceilings, walls, and flooring. A bathroom vent fan pulls the moist air out of the room to prevent muggy conditions that fog up windows and mirrors and create musty odors.

Building codes do not always require bathroom exhaust fans if the room also has a window. However, it's best to install a bathroom exhaust fan that can be utilized year-round because it can be uncomfortable or less energy efficient to open windows during especially cold or hot weather.

Follow the Moisture

First, the fan should be located in the area of the bathroom that is exposed to the most moisture. This is usually the area over the bathtub or shower. You can place the fan directly over the bathtub or shower base, but it's sufficient to position it anywhere near the bathing area.

The goal of the exhaust fan is primarily to remove wet, humid air that can cause mold or moisture damage, and your bathroom will exhaust most quickly if you position the fan over the area with the most moisture. If the bathroom has a tub and shower or a whirlpool tub, the fan should go somewhere between the two fixtures. 

If the bathing area is in its own room, that room needs its own fan, and you might want to add another fan for the main bathroom area or wherever the toilet is located. 

Plan Your Vent Ductwork Route

Bathroom exhaust fans must be vented through the roof or the side of your house. Plan the location for the bathroom exhaust fan so that the duct has the shortest and most direct route to the roof or wall vent. It's best if the fan unit is within 6 feet of the exterior vent cap, where all the moist air escapes to the outdoors using the straightest route possible. Longer duct runs make the fan less effective and increase the likelihood of condensation inside the duct. 

You can terminate the duct through the roof, a gable wall, or even a soffit (if you use a special vent cap). But avoid venting through a side wall below a soffit (the eave overhang of the roof), as this can trap rising, moist air under the eave overhang. 

Check the Attic

Head into the attic with a tape measure to scout your bathroom exhaust fan location from above the ceiling. If necessary, measure from an exterior wall, a plumbing vent pipe, or electrical boxes in the ceiling to find your bearings relative to the bathroom fixtures below. (It also helps to have a helper below measuring inside the bathroom and communicating measurements by cell phone.)

Ensure you can install the fan between two ceiling joists and that there is enough room to transition from the fan housing to the duct. Also, ensure the vent cap will be well clear of obstructions and trouble areas of the roof—things like roof valleys, skylights, chimneys, or furnace or water heater vents. 

Drill a Locator Hole

Before cutting the hole in the ceiling for the fan housing, use a drill and a small bit to drill a pilot hole through the ceiling material at the center of the fan location. Head back to the attic to confirm that the location will work and that no plumbing or wiring runs are in the way. Using a locator hole is much more accurate than measuring from neighboring features. Use the same technique when it's time to cut a hole in the roof or attic wall to install a vent cap to terminate the ductwork.