How should the flapper be installed in the bathroom fan?

Written by erin watson-price
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How should the flapper be installed in the bathroom fan?
After a hot shower, turn on the bathroom fan to remove the humidity. (girl covered in green towel in bathroom image by Nikolay Okhitin from

According to Tim Carter of Ask the Builder, the factory vent flaps on most bathrooms fans do not create a tight enough seal. This allows hot and cold air to leak through, raising your heating and cooling costs.One way to correct this problem is by adding a dryer vent flap to the bathroom fan's venting to provide a secondary seal. This vent flap will be installed in-line with the bathroom fan's existing four-inch flex duct vent system. This duct directs the humid air from the bathroom to the outside. All parts for this project can be purchased at a hardware store.

Skill level:
Moderately Easy

Things you need

  • Plastic draft blocker, 4 inch
  • 2 O-rings
  • Razor knife
  • Wire cutters
  • Ruler or measuring tape

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    Installing a Vent Flap on a Bathroom Fan

  1. 1

    Access the crawl space or attic above the bathroom.

  2. 2

    Follow the flex duct from where it connects from the outside vent to your bathroom fan. You may have to cut away the insulation around the flex duct to gain access near the bathroom fan box. Wear a dust mask if you must cut through insulation. Inhaling insulation fibres can be hazardous to your health.

  3. 3

    Measure approximately 6 inches above where the duct connects to the fan and cut the duct using a razor knife. You may need wire cutters for the coil inside the duct. Flex duct is springy, so measure the duct in a relaxed position. Do not compress or stretch the duct in order to measure it.

  4. 4

    Thread one of the O-rings over the duct connected to the outside and place the vent flap inside the duct. In the correct orientation, the vent should push open easily when facing you. Don't worry if your duct is larger than the vent flap. The O-ring will compress the duct to fit.

  5. 5

    Clamp the vent in place using an O-ring. O-rings slide through a buckle and clamp tight using a screw.

  6. 6

    Connect the other end of the vent flap to the duct connected to the bathroom fan using the other O-ring.

  7. 7

    Test the vent flap by turning on the bathroom fan. If the motor is running but there is no air circulation, the vent flap was installed backward. To fix it, just unscrew the O-rings and flip the vent flap.

Tips and warnings

  • If, for some reason, your bathroom fan does not vent to the outside but directly to the attic or crawl space, install a vent duct to prevent mould and mildew growth. See Resources for more information.
  • Don't be tempted to pull the bathroom fan from the ceiling. It is connected to both the vent duct and the electrical wiring, which, if accidentally disconnected, could cause twice the work and risk electrocution.

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