How to Install a Bathroom Vent Fan
It's the best way to expel moisture-laden air and odors, and we have step-by-step instructions for adding one
Installing a ventilating fan in your bath will help eliminate fogged-up windows, steamy mirrors and stale odors. But more importantly, it will help 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 bath vent fan. In this particular installation, Tom ran 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. Also, the installation of the bathroom exhaust fan will go much quicker if you ask a spouse or friend to help be in the attic while you work from below, or to hand you tools while you're on the ladder.
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.
Bath 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. Fan' 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.