How to Stop Condensation in a Bathroom Exhaust Vent

Written by philip schmidt
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Condensation in a bathroom ventilation fan system occurs most often when the exhaust duct is routed through an unheated space, typically an attic. When all that warm, moist air inside the duct enters the area where the duct sits chilling in the attic, the water vapour in the exhaust air condenses on the inside wall of the duct. In serious cases, the condensate can run back down the duct and into the fan housing and drop into the bathroom. The standard solutions to this problem are to insulate the duct and to run the fan longer after each shower, to completely expel the moist air.

Skill level:
Moderately Easy

Things you need

  • Duct wrap insulation
  • Tools and supplies for installing timer switch
  • Rigid metal duct and related supplies (as needed)

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  1. 1

    Insulate the vent duct with duct wrap insulation, following the manufacturer's directions. Whenever possible, insulate the duct along its entire length. Most duct insulation is designed for use with rigid metal and some rigid plastic duct materials. If you have flexible metal duct, check with the manufacturer about compatibility. Do not use insulation with flexible plastic ducts.

  2. 2

    Install a timer switch to control the fan, following the switch manufacturer's directions. If you're considering replacing the fan, look for a new model with a built-in humidistat that keeps the fan running until the room air is dry.

  3. 3

    Replace uninsulated plastic duct with rigid metal duct, whenever feasible, keeping the duct run as short as possible. If the duct terminates through a soffit or wall (as opposed to the roof), install the duct so it slopes downward from the fan location to the termination point (a slope of 1/4 inch per foot is plenty). This allows condensate to run out of the house instead of back down into the fan. If flexible duct is necessary, you can use insulated plastic duct designed for this type of application.

Tips and warnings

  • If the original ducting is flexible plastic or metal, you can improve the fan's performance and reduce condensation by replacing the old duct with smooth, rigid metal duct.

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