How to Insulate an Exhaust Fan Duct

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Insulating an exhaust fan duct is a particularly good idea for bathroom ventilation fans with ducts that are routed through an unheated space, such as an attic. When a vent fan does its job, it moves warm, moisture-laden air from the bathroom to the outdoors, but when that air runs through a cold section of duct, the moisture can condensate, leading to corrosion, ceiling leaks and possibly mold growth. You can insulate rigid and flexible metal duct with duct wrap insulation. If your existing vent duct is cheap, vinyl dryer hose, consider replacing it with metal duct, which is much more effective for moving air and water vapor.

Things You'll Need

  • Duct wrap insulation
  • UL 181-approved foil duct tape
  • Utility knife
  • Seal all of the joints along the duct run with UL 181-approved foil duct tape. Wrap around the duct a few times to create a strong air seal.

  • Secure one end of the duct wrap insulation roll around the duct where it meets the fan housing, using the duct tape (or other tape recommended by the manufacturer).

  • Wrap the duct in a continuous spiral fashion, overlapping the edges of the insulation by at least 1 inch (or as directed). Note: Some duct wrap products are installed by wrapping cut lengths of insulation perpendicularly to the duct, and taping each section together with duct tape. Other types of insulation require spacers to create an air gap between the duct and insulation. If you’re using fiberglass insulation, be careful not to pull and stretch the wrap; compressing the insulation compromises its effectiveness.

  • Complete the duct wrapping up to and around the collar of the vent cap at the exhaust end of the run. Seal the insulation to the vent cap with the duct tape.

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