Bathroom Vent Fan Installation: A How-To Guide

A bathroom needs ventilation as much as it needs lights, so a bathroom vent fan installation will probably be part of your new bathroom or your bathroom remodel. In some places, a ventilation fan is only required by code if the bathroom doesn't have an openable window, but it's a good idea to install one anyway. It protects the walls, cabinets and framing from mildew and rot, and it improves the air quality.

You may want a bathroom exhaust fan with extra features, such as a light or a heater, in which case you'll probably need to install a dedicated electrical circuit for it. If it's just a fan, though, you can probably add it to the light circuit. Check with your local building department to be sure.

Extra features aside, when shopping for a ventilation fan, you'll want to make sure you choose one that has enough capacity for your bathroom and won't sound like a marching band when it's on.

Exhaust fan capacity is measured by the number of cubic feet per minute (cfm) it can exchange, and the minimum air flow required by code is 50 cfm. If your bathroom floor area is less than 100 square feet, the Home Ventilating Institute says the fan capacity in cfm should be the same as the area in square feet, but it's better to get a larger one. In a bathroom that exceeds 100 square feet in area, add the requirements for each bathroom fixture, which are 50 cfm each for the toilet, shower and bathtub and 100 cfm for a jetted tub.

The noise level of the fan is measured in sones, and it's displayed on the container next to the capacity. A quiet fan has a noise level of less than 1.0 sone, and a noisy one can be as much as 4.0 sones, the sound level in a busy office. The larger the fan, the noisier it is, so if you need a large fan, you'll probably have to settle for a noisy one.

The entire vent fan assembly consists of a housing to which the fan is attached and a duct system that directs air to the outside of the building. The housing has brackets that get fastened to a pair of ceiling joists. The positions of the joists determine the ceiling position of the fan, so you might not be able to put it exactly where you want it, although you'll probably want it as close to the center of the bathroom as possible.

The duct system runs from the fan and exits through a side wall, a soffit or through the roof. It can be challenging to install this if you don't have access to the attic, and you may have to remove some of the ceiling drywall. You want to plot the shortest course, and although code allows bends in the duct pipe, there should be as few as possible. Some local codes require rigid duct, but if yours doesn't, you can use flexible aluminum duct, which is easier to handle.

You'll need to have electrical wires in place for the ventilation fan. If you're replacing an old fan with a new fan, they'll already be there, but if you're upgrading the fan to one with extra features, you may have to run new circuit wires to a dedicated circuit breaker. If you aren't confident in your DIY electrical skills, you should have the electrical work done by an electrician before you begin your bathroom vent fan installation.

Planning the ductwork requires measurement of the distances involved so you can buy the pipes and fittings you need. You'll also need a vent cap for the outside of the building, and the style depends on whether you're exiting through a wall, a soffit or the roof. Vent fans tend to have a 3-inch outlet, so if you're using 4-inch duct, you'll need a reducing fitting.

The vent exit must be at least 3 feet from any opening, such as a window, that allows outside air into the building. When no attic access is available, it's best to run the duct along a joist and vent it through a wall or bend it downward so it can exit through a soffit.