Installing a Bathroom Fan

Every bathroom needs an effective fan. Without one, the pleasure of a hot shower is usually followed by an aggravating slog through fog and heavy humidity. Besides, a bathroom fan helps you avoid all sorts of problems, from moldy grout and damp towels to mildewed walls, peeling paint, and even damaged insulation and framing members. To keep these disasters at bay, all you need is a fan and a plan.

Choosing Your Fan
The first issue to consider is how much air the fan will have to move. Air movement is measured in cubic feet per minute (cfm). To calculate how many cubic feet of air the fan will have to move, just round up the square footage to the nearest multiple of ten. For example, if your bathroom measures 6 feet by 8 feet, the square footage would be 48 square feet. For a bath that size, you'll want a fan that can move at least 50 cubic feet per minute, or 50 cfm.

The second fan characteristic to consider is noisiness. Fan noise is measured in sones. The lower the number, the quieter the fan. Fans rated at 2 sones and below will please all but the most sensitive ears. (For comparison, a fan running above 5 sones makes it impossible to have a conversation without shouting.) Quieter fans tend to be built better, too.

If you have a ceiling light in your bathroom, consider buying an all-in-one fan-light unit to replace it. You can easily tap power from it and use the same switch to turn on the fan and the light. However, if you want the fan to have its own switch or to operate on a timer (a good way to keep the fan working after you leave the bathroom), you'll need to fish an additional switch loop between the ceiling outlet box and the switch. If there's no power in the ceiling, you'll need to run cable from a nearby receptacle or bring in a new circuit from the service panel. (You may want to hire an electrician for this part of the job. But if you plan to do the work yourself, check out our articles on essential wiring skills and how to fish cable.)

As the drawing shows, there are three ways to vent your fan: through the roof, through a soffit and through an exterior wall. If there's an attic space above the bathroom, the shortest route is probably through the roof. If you have access to a soffit, though, the vent will require almost no weatherproofing.

If there's no attic above your bath, route the duct between ceiling joists and out through an exterior wall. Use a stud finder to determine the direction of the joists, and adapt your plans to what you find. Don't drill or notch the joists because it will weaken them.

For maximum efficiency, keep the exhaust duct as short and as straight as possible. You'll need to use ducting that's 3 or 4 inches in diameter and preferably sheet metal rather than flexible ducting. The smooth inside surface of rigid metal ducting promotes rapid air movement, quieter operation and less moisture buildup.

Place the fan housing next to the joist and trace its outline. Cut out around the outline with a reciprocating saw (if the ceiling is plaster) or a drywall saw (if it's wallboard). Clean up will be easier if you spread a drop cloth beneath the cutting area.

Tip from the pros: To reduce or eliminate damage to surrounding plaster and lath, securely screw a piece of scrap plywood (1/4-inch or 1/2-inch will work fine) to the ceiling, covering the area you need to cut. Saw through the ceiling and the plywood together, and you'll get a much cleaner cut.

If there's no attic space, use the stud finder to locate a joist in the area where you want to put the fan. Remove enough of the ceiling next to, but not under, the joist to let you see where the joist is. Place the housing or its template on the ceiling with one side against the joist, trace it and then cut out around it.

If you're replacing a ceiling light fixture with a vent fan, turn off the power and disconnect the light fixture from the ceiling outlet box. Disconnect the wires from the box, and locate the joists so you can trace the shape of the fan housing for the opening you'll need to cut. Before or after the opening is cut, you'll need to disconnect the wiring from the old box so it can be wired directly to the fan unit.

2. Route wires and install the fan housing. To keep repairs to a minimum, fish cable through the walls. Remove the fan from its housing and secure the housing with screws and brackets, following the manufacturer's instructions. Depending on your fan model and joist layout, you may have to install extra blocking around the housing.

Tip from the pros: Always use screws. Nails can work loose and contribute to rattling, a common problem with bathroom fans.

Connect the wires to the fan's terminals. Then mount the fan in its housing according to the directions, turn the power on and test its operation. Turn off the power when everything checks out OK.

3. Vent the fan. To get indoor air to the outside, you'll need to follow one of the three options explained below.

Venting through the roof. To cut through an asphalt composition roof, drive a nail through from the attic side to mark the vent location on the roof exterior. To avoid clogging your saw blade with tar, cut shingles away with a utility knife to clear an area about 4 1/2 inches in diameter. Then use a jigsaw or reciprocating saw to cut the sheathing away and make an opening into the attic space. (If your roof is something other than asphalt shingles, such as a cement tile or built-up roof, have a roofer install the ducting since these roofs are harder to cut and make waterproof.) Cut or carefully pry up enough shingles to allow you to slip a roof jack under them and position it over the hole in the roof. When the roof jack is in place, attach the piece of ducting that goes directly into the vent. Next, spread roofing mastic under the roof jack and around the junction between the jack and the piece of ducting. Nail the jack in place with roofing nails and seal the heads with mastic. Install a vent cap over the top of the ducting, securing it with sheet metal screws. Finally, reinstall shingles as necessary.

Venting through a wall. How you cut through an exterior wall depends on what it's made of. Wood, vinyl and aluminum siding can all be cut with a reciprocating saw or jigsaw after you drill a starter hole. If the wall is stucco, use a masonry bit to drill a series of holes in a circle where the opening for your ducting will go; space them about 1/2 inch apart. Then, using a masonry chisel, carefully chip away the stucco between the holes and remove it. This will leave the sheathing exposed; cut through it with a reciprocating saw to complete the hole for the ducting. Screw the duct cover in place, and caulk around its perimeter. If no grille or vent cap was supplied with your bathroom fan, a dryer vent cap will work well.

5. Finishing touches. To ensure proper venting, make sure that air can get into the bathroom to replace what the fan takes out. You can shorten the bathroom door so that there's a 1-inch gap at the bottom or even cut a hole in the door and cover it with a decorative grille. Don't ignore this important step.

Before you patch the drywall or plaster, remove the fan mechanism from the housing (or shield it well with plastic bags) so that dust and paint will not damage it. Also, before you put the grille over the fan, caulk between the fan housing and the ceiling to reduce air leaks when the fan isn't operating. Before you close up the walls and ceilings, take a moment to make sure that the fan and ducting work properly.