Show us a bathroom with peeling paint, rotting window trim and a mold problem and we’ll show you a bathroom without a vent fan. Bathrrom ventilation is the best way to prevent moisture-related problems, such as the growth of mold and mildew. Here, This Old House general contractor Tom Silva shows the proper way to install a fan, running the exhaust duct into the attic and through a sidewall to the outdoors.
Other venting options includes running the duct up through the roof or down through the soffit. Note that the fan must always exhaust to the outdoors; never allow the duct to simply blow into an attic, crawlspace or other enclosed area, or you’re certain to fail inspection.
Bath Vent Overview
A bathroom without a ventilation fan is like a fireplace without a chimney: If you fail to pull the moisture generated in the bathroom out of there, it will migrate into the walls and grow mold and mildew, or blister paint and peel wallpaper. One reason many households still don’t have bath fans is that they can be intimidating to install. That’s why we asked This Old House general contractor Tom Silva to show us how. The bathroom here is below an accessible attic, so Tom ran the exhaust duct across the attic and out a gable end.
Bathroom vent fans are rated by how many cubic feet of air they can move in one minute, known as the CFM rating. To determine which size fan to buy for your bath, multiply the room’s square footage by 1.1. For example, a 100-square-foot bath would require a 110 CFM-rated fan. Fans also have a sound rating, measured in sones. (A modern refrigerator operates at about one sone.) Vent fans range from as low as 0.5 sone up to about 6.0 sones. You’ll find both the CFM and sone ratings printed on the vent fan’s box.
Drill a Reference Hole
Draw a mark on the bathroom ceiling where you’d like to install the vent fan. For optimum performance, locate it between the shower and the toilet.
Use an extra-long, 3/8-inch-diameter spade bit to bore a reference hole through the ceiling and into the attic. Climb into the attic and clear away any insulation from around the hole. Now use the reference hole to determine the exact position of the fan.
Measure the vent fan housing. Try to position the vent fan directly between two joists near your reference hole. Take into account any nearby pipes or other obstructions. Note the final position of the vent fan in relation to the reference hole.
Mark the Ceiling
Back in the bathroom, measure the inside dimensions of the vent fan’s intake port to determine what size hole you need to cut in the ceiling.
Use the reference hole as a landmark to transfer your measurements from the attic to the ceiling.
Use a layout square or framing square to draw the rectangular outline of the intake port onto the ceiling.
Cut the Intake-Port Hole
Using a jigsaw (a reciprocating saw or drywall saw will also work), and wearing goggles and a respirator, cut through the ceiling on the layout lines.
When you’re almost finished making the rectangular cutout, support the waste piece with one hand to keep it from falling and possibly taking some of the surrounding drywall or plaster ceiling with it.
Position the Fan
Back in the attic, attach a 4-inch, 90-degree duct elbow to the outlet port on the side of the vent fan housing. Aim the elbow straight up and secure it to the port with foil duct tape.
Remove the knockout hole on the side of the fan’s housing and attach a cable connector.
Slide the four metal brackets into the tabs protruding from the sides of the vent fan.
Set the vent fan down into place between the joists, centered on the ceiling hole.
Tip: Use foil duct tape; unlike fabric duct tape, it won’t deteriorate over time.
Attach the Fan to the Joists
Fully extend the brackets until they come in contact with the sides of the joists.
Secure each bracket end with a single 1½-inch drywall screw.
Take the flexible duct that comes with the wall cap kit and slip one end over the elbow attached to the vent fan. Secure it with foil duct tape.
Feed the existing or new electrical cable through the connector, then tighten the connector screw to secure the cable.
Cut a Duct Hole Through the Sidewall
From inside the attic, select a spot on the sidewall for the flexible duct to exit the house. The location should be between two wall studs and within 6 feet of the vent fan.
Take a couple of reference measurements so that you can locate the spot from outside—from a nearby window, soffit, or other feature you can access from outdoors.
Mark the hole location on the siding, using the reference measurements.
Cut the duct hole through the house wall using a 4-inch hole saw.
Install the Wall Cap
Fasten the 12-inch-long connector duct to the wall cap with foil duct tape.
From outside, slide the wall cap into the duct hole and push it tight against the house siding. Cut a foam-rubber gasket to fit into any spaces where the wall cap doesn’t contact the siding.
Remove the wall cap and apply a bead of silicone adhesive to the siding and gasket around the hole.
Slide the wall cap into the hole and press it against the siding. Secure it to the siding with four 1½-inch stainless steel screws.
Attach the Exhaust Duct to the Wall Cap
After securing the wall cap to the outside wall, move into the attic.
Grab the free end of the flexible exhaust duct and carefully stretch it to the outside wall.
Attach the duct end to the wall cap’s connector duct with foil duct tape.
Make the Wire Connections
From the bathroom, unscrew and remove the blower motor from the vent fan housing. Save the screws.
Next, unscrew the built-in receptacle from inside the housing to expose its wiring. Again, save the screws.
Use wire nuts to join same-color wires with the ones from the electrical cable you fed into the housing in Step 5: white to white, black to black.
Wrap the bare copper wire under the green grounding screw inside the housing and tighten the screw. Place the electrical receptacle back into position and secure it with the two screws removed from it earlier.
Attach the Grille
Raise the blower motor up to the ceiling, then push its plug into the electrical receptacle inside the housing.
Secure the motor to the housing with the screws removed from it earlier.
Hold the fan’s plastic grille close to the ceiling. Slip the grille’s mounting wires into the slots inside the fan housing.
Push up on the grille until it’s tight against the ceiling.
Turn the power back on and test the fan.
Tip: If the grille doesn’t hold tight against the ceiling, spread apart its mounting wires to create more tension.