How to Insulate a Bathroom Fan Vent Pipe


Bathroom exhaust fans exhaust air out of your bathroom through a duct that is connected to the fan and runs out your roof. This duct is made up of single-walled sheet metal. Since the temperature of the air exhaust may vary greatly from the air temperature in the attic, an uninsulated bathroom duct can condensate. This condensation will drip off the duct and into your attic insulation or structural members. If these become moist, mold and mildew may grow. To prevent such issues, insulate the bathroom fan vent pipe with duct wrap.

Things You'll Need

  • Tape measure
  • Calculator
  • Duct insulation
  • Utility knife
  • Foil tape
  • Gloves
  • Dust mask
  • Safety glasses
  • Calculate the measurement around the outside of your exhaust fan duct. Typical exhaust fan ducts are 4 inches in diameter. To calculate the measurement around the outside, multiply the diameter (4 inches) by pi (3.14).

  • Mark out the above calculation on your insulation. Add a few inches to this measurement to allow for the insulation thickness and for seam overlap.

  • Cut through the insulation with your utility knife at the marked-out measurement.

  • Place the insulation on your exhaust duct. Tape one seam down to the sheet metal duct with foil tape. The insulation should face the duct while the foil backing faces outward.

  • Pull the other end of insulation around the duct until it meets up with the seam you taped down in the previous step. It should overlap just a bit. Tape this seam down with foil tape.

  • Attach each successive piece of insulation in the same manner. Tape the seams where they join together with foil tape.

Tips & Warnings

  • Fiberglass duct insulation is an eye and lung irritant. Wear proper protective clothing and equipment, including gloves, dust mask and safety glasses when working with duct insulation.

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