Bathroom air vents with fans move moist air from the bathroom to the outdoors. Installing a vent not only helps reduce foggy mirrors and that "always wet" feeling, but helps reduce mould and mildew that can make you and your family sick. The easiest and cheapest way to install a bathroom exhaust fan and vent is up through the ceiling where there is an existing light fixture, and then out through the wall.
Remove the bathroom light fixture. Take out the screws that hold the light fixture in place. Pull down on the light fixture until it is free from the ceiling. Unscrew the two electrical connectors that bind the wires from the light fixture to the electrical system. Set the light fixture aside, out of the way.
Enlarge the hole. Read the instructions that come with the vent/fan unit to determine the size of the hole you'll need in the bathroom ceiling. Use a framing square to draw a rectangle in the ceiling around the existing hole left by the light fixture. Cut out the rectangle with the jig or drywall saw
Attach the duct to the vent/fan unit according to the manufacturer's directions.
Attach the vent/fan unit. Push the vent/fan unit, with attached ducting, up into the hole. The ducting goes in first. Secure the unit against the ceiling joist, using screws provided with the unit. Affix the vent/fan cover, making sure it clicks into place.
Attach the wiring to vent/fan unit, using electrical connectors from the light fixture--black to black, white to white.
Cut a hole in the siding from the outside in, using a jigsaw. Make the hole just big enough for the duct and cap.
Install the ducting. Pull the ducting from within the ceiling space and attach it to the vent/fan unit, using the screws provided.
Install the duct cap. Apply sealant to the duct cap and siding wall. Screw the duct cap into the siding wall, using metal screws.
Turn on the shower and leave it running for several minutes with the fan running to test the system. Go outside and see if air is being expelled.
To determine the correct vent/fan size for your bathroom, multiply the room's square footage by 1.1 to give you the CFM rating. Your vent/fan unit should have a matching CFM rating. Install the unit on a sunny day when the weather is warm but not hot.
Turn off power to the bathroom before starting work Exhausting into an attic or crawl space will cause water damage and lead to mould problems. Use goggles to avoid eye injuries.