Although older homes incorporated bathroom windows for ventilation, this passive form is inadequate for preventing moisture problems — you need an active ventilation system that will clear the air quickly. One of the most effective solutions is a vent fan that blows air through a duct leading outside. Installing a fan isn't difficult, but the project requires electrical, duct and roofing work. DIYers with intermediate to advanced skills should be able to tackle the job.
Select the right fan
A ceiling vent fan is the easiest option for most bathrooms, but it requires access to the ceiling joist cavity. I installed a ceiling vent fan in a single-story ranch-style house with a crawlspace attic. It's not fun to crawl through insulation and over ceiling joists, but an unfinished attic provides good access to the bathroom ceiling. If your bathroom is below a finished room, the installation will be more challenging: You must either remove the finished flooring and subfloor in the room above the bathroom or remove a portion of the bathroom ceiling. A better alternative in this circumstance is to install an in-wall vent fan (see "In-Wall Vent Fan Option," in PDF below).
A third option to consider, especially if you have multiple rooms that need ventilation, is an in-line vent fan. This type of fan is not installed in the room that is being vented. Instead, an inlet duct runs from the room to the vent fan housing, and an outlet duct continues from the housing to the outside. In-line vent fans often feature multiple inlets so that you can draw air from more than one location and vent the air through a single outlet. These fans typically require an unfinished attic or crawl space for installation.
Once you decide what type of fan to install, you'll need to determine the right size. Vent fans are rated by how many cubic feet of air they move in a minute (cfm). Adequate size is important to remove the humid air efficiently.
To determine the right size for a room with an 8-ft. ceiling, multiply the square footage of the room by 1.1. For example, a 100-sq.-ft. bathroom will be best served by a vent fan with a rating of at least a 110 cfm. If the bathroom has a vaulted ceiling, add 15 percent for every additional foot of ceiling height.
Another important consideration is the noise a vent fan produces, which is measured in sones. Not long ago bathroom vent fans sounded like an airplane passing overhead. Today, more powerful fans do not necessarily produce more noise. Improved motors, larger and more efficient blower wheels and larger exhaust outlets and ducts have resulted in fans that are both powerful and quiet. As a frame of reference, consider the noise level of a normal indoor conversation, which would be rated at about 4 sones, according to the Home Ventilating Institute (see SOURCES in PDF below). Modern vent fans that are considered quiet produce .3 to 1.5 sones, which is several times quieter than earlier models.
Vent fans of all power levels are available in a variety of styles, including models that look like traditional ceiling fixtures or even recessed lights (see "Fans in Disguise," in PDF below). Many also feature built-in lights and heaters.
One final piece of equipment not to be overlooked is the switch that controls the fan. You'll find almost as many switch options as vent fans. You can use a standard single-pole switch, but a timer switch is a better choice. This type of switch turns off after a set amount of time (typically 15 to 60 minutes), so you won't waste energy by leaving the fan on longer than is necessary. Timer switches are available with motion detectors that turn the fan on whenever a person enters the bathroom and then turn it off automatically. If the vent fan that you plan to install is equipped with a light or heater, consider adding a switch that lets you control each function individually.
Install the fan housing
Vent fans are most effective when they are located near the source of the humidity. You can install one directly above a tub or shower, but it must be UL-listed for over-the-tub installation and connected to a GFCI circuit. Most new vent fans are approved for such installations.
Once you have selected a location, drill a small hole through the ceiling and push a piece of wire or coat hanger up through the hole. (Tip: Tape a large garbage bag to the ceiling to catch any debris that falls when you're drilling and cutting.)
From the floor above, locate the wire and clear away any insulation surrounding it. Mark the cut lines for the vent fan opening between the joists, following the manufacturer's hole-size specifications. Cut the opening with a drywall saw, jigsaw or reciprocating saw.
Remove the motor and blower from the housing if possible. Next, apply a bead of caulk around the ceiling opening, and secure the fan housing in the opening (photo 1). The caulk helps create a tight seal, preventing air leaks.
Make the wiring connections in the fan housing and then fasten the housing to a ceiling joist with 1-1/4-in. screws.
Install the ductwork
Ceiling vent fans should be vented out through the roof or the wall. Venting through soffits is not recommended because the humid air that is expelled can be sucked back into the attic through nearby soffit vents.
The outlet that the duct connects to is called a wall cap or a roof cap. The advantage of installing a wall cap is that the duct can be pitched toward the outside, allowing condensation to drain away from the vent fan. A wall cap also will not be blocked by snow, a potential problem for roof caps. The advantage of installing a roof cap is that it is easier to achieve a properly flashed and watertight installation. Choose the outlet location that is easiest to work with in your installation.
The duct path from the vent fan to the wall or roof cap should be as smooth, short and straight as possible to provide the most efficient airflow and reduce noise. Obstructions such as ridges and turns in the ductwork create static pressure, amplifying noise. Rigid galvanized duct offers the best performance, but flexible insulated duct is also acceptable for runs that are less than 6 ft. long. When installing rigid duct, use 45-degree elbows to create gradual changes in direction. Also, whenever possible, install at least 2 ft. of straight run before the first elbow to allow the air to build momentum before changing direction.
Secure the duct to the fan damper with foil tape (photo 2). The silver fiber-back tape we know as duct tape is great for many applications, but sealing ducts is not one of them — the tape will break down over time.
Attach the duct to the fan housing with foil tape. Next, route the duct to the outlet location and drive a 3-in. screw through the roof or wall to mark the outlet outside the house.
Determine the exact outlet location, and drive a 3-in. deck screw through the roof or drill a small hole through the wall. Next, go outside and locate the protruding screw or hole on the outside of the house. (Note: Be sure to follow ladder- and roof-safety precautions.)
If you are installing a roof cap, carefully remove the shingles that directly surround the screw. Position the duct collar over the screw, and mark the center of the cap opening. Some models require you to install a duct starter collar separately. Mark the collar position and cut the opening with a jigsaw, reciprocating saw or 4-1/2-in. hole saw (photo 3).
Remove the shingles that will be directly under the roof cap installation location and then use a jigsaw or hole saw to cut a hole slightly larger than the roof cap starter collar.
Next, apply roof sealant to the underside of the roof cap flashing, and insert the roof cap collar into the newly cut opening, sliding the top edge of the vent flashing under the shingles above the roof cap. Secure the corners of the roof cap flashing with 1-in. galvanized roofing nails. Flash the top and sides of the roof vent with a piece of building paper (photo 4). Cut and replace the missing shingles to fit around the roof cap, working from the bottom up and being careful to properly overlap the higher shingles over the lower shingles (photo 5).
Make a cut in the existing building paper just above the top edge of the roof cap and slide a piece of building paper under the existing paper to flash the top and sides of the roof cap.
Replace the shingles around the roof cap, working up from the lower shingles. Secure the shingles with 1-1/4-in. roofing nails and then apply roof sealant over all nails and under the lower edge of each shingle.
A wall cap installation is the same as a dryer vent installation. The wall cap flashing must make as much direct contact with the siding as possible. The best way to install a wall cap is to fit the flashing under a piece of siding or trim that is directly above the wall cap, but this is not always possible. Wall caps must often rely on silicone sealant to fill in gaps and prevent water penetration.
When installing a wall cap, apply silicone adhesive around the hole, insert the collar through the newly cut opening and secure it to the siding with four 1-1/2-in. stainless steel screws. Then cover the screwheads and fill any gaps between the cap and siding with silicone adhesive.
Once the wall cap or roof cap is secure, go back inside the house and finish connecting the ductwork between the fan and the cap. Secure flexible duct connections with duct clamps, and wrap all connections with foil tape. To help prevent condensation in cold climates, cover ducts located in attics with loose insulation or apply duct-wrap insulation.
Make the wiring connections
If you are not familiar with current electrical code regulations, hire a licensed electrician to run 12-2 NM cable to the vent fan and switch locations. (Use 12-3 NM cable when installing a fan-light combination unit.) I replaced a single gang box that contained a GFCI outlet with a double gang box and tied into the GFCI outlet to supply power to the fan (photo 6 and "Vent Fan Wiring Options," in PDF below).
Connect the timer switch. I removed an existing single box containing a GFCI outlet, cut the opening larger and replaced it with a double box to hold both the GFCI and the vent fan timer switch.
If you removed the motor and blower before installing the housing, connect the power-supply cords and snap the motor and blower back into the housing (photo 7). Finally, install the grille to conceal the fan and then take the unit for a test run – after all that work, you probably need a hot shower.
Reinstall the motor and blower in the fan housing and plug the motor wire connector into the housing wire connector. Finish the installation by fitting the fan grille over the housing.