Green Building Library
Weatherization Tips

How to Install Exhaust Fan Ducts

  • Use smooth metal duct material.
  • Install short, straight runs.
  • Make a positive connection to the outside.

Exhaust Fans

Ducts and fittings must be smooth, 30 gauge metal or thicker. The diameter of the duct should match the outlet connection on the fan housing. However, always use at least 4 inch duct. If the fan has a 3 inch duct collar, add an adapter to accept a 4 inch duct.

Keep runs as short and straight as possible. When turns are needed, two 45º angle fittings create less resistance to air flow than one 90º. Avoid 180º turns and S-shaped runs.

Support the duct every 18 inches with galvanized wire, metal strapping or other approved duct support. Slope the duct so that any condensation will drain either to the outside or to the fan.  (A puddle in the duct could support mold or bacteria.)

Secure each joint with 2 sheet metal screws. Then seal it with duct mastic or other approved material.

Never dump exhaust air into an attic, garage, basement, or crawlspace. If the end of the duct simply hangs in the general vicinity of a vent, some moist air will stay inside.  Instead, install a fitting at the end of the duct to make a positive connection to the outside. These fittings can be purchased from sheet metal suppliers, or you can make one from metal flashing.

If converting an existing vent, be sure there is still enough net free area for adequate ventilation.

In cold climates, insulate the duct to reduce condensation.

(click below to see larger image)

exhaust fan ducts

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Kitchen Range Hood

Ducts for range hoods are similar to other exhaust fan ducts.  Remember these points:

  • Use smooth, 30 gauge or thicker galvanized steel duct.
  • The duct must be the same diameter as the connection on the fan.  A rectangular duct with the same area can be substituted.  (For example, 3 1/4 x 10 rectangular duct is equivalent to 6 inch round duct.)
  • If the duct is properly connected and supported, just seal it. If the duct needs to be extended to make the outside connection, install a metal roof vent.

Clothes Dryer Ducts

Existing flexible duct does not have to be replaced, unless it is damaged or incorrectly installed.  If repairs are needed, use 30 gauge smooth metal duct. Run the new metal duct from the repair to the outside end.

If replacing the entire duct, use 30 gauge smooth metal duct. This is important where the duct passes through a wall, floor, or ceiling. Flexible duct should be used only as a connector to attach the dryer to the duct.

Keep these ideas in mind:

  • Keep duct runs short with few bends.
  • Avoid sharp bends which can trap lint.  For example, use two 45º fittings instead of one 90º.
  • Don’t fasten the joints with screws, because the tips will catch lint that could block the duct.  Hold the joints together with foil duct tape.
  • Terminate the duct on the outside into a metal cap with a backdraft damper.