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Inch by inch, step by step, I vented my bathroom

House Doctor | Gary Dymski

April 28, 2005

Before I overhaul the bathroom in my daughter Melissa's bedroom, there's the pressing matter of an exhaust fan.

The bathroom needs one, so I'm going to spend less than $100 to install a fan-light combo. It's a relatively simple project, but you need knowledge of electrical wiring, the confidence to saw through an exterior wall and the ability to repair drywall. On my personal difficulty scale - with 10 hammers the sign to call the contractor - this rates about three hammers.

Why install a fan? The room does have a window to let in fresh air. But in cold weather, when the window is closed, air moisture can be trapped, causing mold and mildew to grow. If the bathroom needs overhauling, which means new tile shower walls, a glass shower enclosure, wallpapering and window treatments, a fan is necessary to control bathroom moisture. The Home Ventilating Institute recommends that a bathroom exhaust fan be capable of delivering eight air changes in an hour.

Because this is a ground-floor room, the fan will be installed over the tub, almost in the middle of the ceiling, and be vented through an exterior wall.

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Step 1: The bathroom has a switch for a light over the vanity mirror. We're going to replace the switch's existing single-gang electrical box with a double-gang box. The existing switch still will control the light over the mirror. The additional outlet will have two switches, one for the new fan and the other for the fan light.

Before working on any outlet, turn off power at the service panel. Remove the existing switch and wiring, making sure to label wires so you can properly rewire the switch. Once all wires have been loosened, remove the box. Use a reciprocating saw to cut away the nails that hold the box in place.

Step 2: Run the new wire. Make square holes in the wall above the box opening. One should be above the hole for the new outlet box, where the wall meets the ceiling. Another will be adjacent to this hole, on the ceiling, and will expose a joist. Another opening in the ceiling will be made at the fan location, and the wire will end up there.

But let me digress: Since extensive bathroom remodeling is planned, I'm not concerned about damaging drywall, including the ceiling. Most installation instructions will require tracing a template of the fan box, also called housing, on the ceiling. Instead of cutting out a small portion of ceiling, I plan to remove a larger section to make installing the fan and running duct work easier. Replacing and repairing damaged drywall is the simplest part of this project.

Be sure to use the correct gauge of electrical wire. My 15- amp switch means I will use 14-3 wire, which will allow me to have separate switches for the new fan and light.

Run wire through the existing switch hole to the ceiling. Use a spade bit to drill a hole through the exposed joist at the top of the wall. This will allow the wire to run behind the drywall and under the ceiling. Once the wire is through, pull it toward the hole in the ceiling that was created for the fan.

Step 3: Hang and wire the fan. Read installation instructions carefully. The manufacturer will provide several drawings of typical installations and wiring diagrams.

Step 4: Vent the fan. To do this, create another hole in the ceiling, near the wall where the vent will exit to the exterior. Determine the location of the hole on the outside wall by holding the vent pipe in place. While the vent pipe is being held in place, mark the wall from the inside with a pencil, and then drill at least four holes to mark the location of the vent. Before drilling, examine the exterior wall to make sure the vent can be mounted free of obstructions.

Step 5: Cut out hole for vent. Using the drill holes as a guide, draw a 3-inch circle on the exterior wall. Using a sabre saw, cut out the hole and insert the vent. Attach the vent, pipe and fan with ring clamps. Connections can be made first using duct tape, then tightening clamps over the taped areas.

Step 6: Wire the switch. Use a line tester to determine the power wire, and then wire the combination switch according to the installation instructions. You'll need to turn on the power at the panel to determine the hot wire. Remember to turn it off again before wiring the switch. One tip: The new box will be for two switches. Buy a box that secures to the drywall from the inside with two ear flaps. This outlet often is categorized as for "old work." Before installing the new outlet box, trace its outline on the drywall with a pencil and expand the opening with a drywall saw. As the flaps on the outlet box are tightened, they will flip out and secure the box to the drywall. Before installing the box, be sure the wire is long enough and that it is secured to the existing wire with cable ties.

Step 7: Test the fan. Inspect the wiring and compare your connections to the installation instructions before switching on power at the service panel.

Repair openings in the drywall and seal any openings on the exterior wall near the vent with caulk or expanding foam.