Overview

Diagram: vent fan
Photo: Keller & Keller Photography
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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.
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    Tools List

    • caulk gun
      Caulking gun with silicone adhesive,
      for adhering wall cap to house siding
    • speed square
      Layout square
    • drill
      Drill-driver with 4½-inch hole saw,
      for cutting duct hole through sidewall
    • spade bit
      Extra-long, 3/8-inch-diameter spade bit,
      for drilling reference hole through ceiling
    • jigsaw
      Jigsaw,
      for cutting hole in bath ceiling
    • 16-foot tape measure
      Tape measure
    • eye goggles
      Goggles
    • extension ladder
      Extension ladder
    • ladder
      Stepladder

    Shopping List

    1. VENT FAN

    2. TARPS

    to protect floor and fixtures from dust



    3. 90-DEGREE SHEEET METAL DUCT ELBOW

    to connect flexible duct to vent fan



    4. 12-INCH CONNECTOR DUCT

    5. CABLE CONNECTOR

    6. WALL-CAP KIT

    7. FOIL DUCT TAPE

    8. DUST MASK