How to Install Bathroom Exhaust Fan Ducting

Updated February 21, 2017

Having a bathroom exhaust fan is important for a variety of reasons. Bathroom exhaust fans help prevent moisture damage to ceiling and walls, mould growth, and wood trim decay. Improperly vented bathrooms will produce wall stains, bathroom window condensation, wet insulation in attics, and damaged roof shingles over bathrooms. Bathroom exhaust fans need ducting to move the air from the bathroom to the exterior of the building. Once you install bathroom exhaust fan ducting for your exhaust fan, you will have a properly vented bathroom.

Locate a spot on the outside of the house where you'll want to locate the vent for your bathroom exhaust fan. Most bathroom exhaust fans transfer the bathroom air up into the attic and out through the side of the house using metal ducting.

Drill a 4 1/4 inch hole with your hole saw into the side of the house. Attach the exhaust fan hood by sliding the 4-inch duct stub through the hole and screw the face of the hood to the outside of the building. Caulk around the hood with a high quality exterior caulk.

Install 4-inch galvanised metal ducting from your bathroom exhaust fan outlet to the exhaust fan hood. Cut pipe to fit as needed and crimp the end of the cut pipe to fit into the next piece. Screw all joints together with sheet metal screws.

Support the ducting with galvanised pipe hanging straps. Pitch the ducting slightly down towards the exhaust hood. This will allow any excess moisture to run out of the pipe outside.

Insulate the ducting with duct wrap. This will keep your ducting from sweating when the warm, humid air moves through the pipe.


Do not use flexible "dryer vent" ducting. This ducting doesn't have a smooth interior and will significantly reduce your airflow. Avoid long runs and lots of bends. Keep the ducting as straight as possible. When you have lots of twists and bends, the air flow will be dramatically reduced.

Things You'll Need

  • 4 1/4 inch hole saw
  • Drill
  • 4 inch exhaust fan hood
  • Exterior caulk
  • Caulk gun
  • 4 inch galvanised metal ducting
  • 4 inch galvanised elbows
  • Tape measure
  • Aviation snips
  • Pipe crimper
  • Sheet metal screws
  • Level
  • Galvanised pipe hanging straps
  • Duct wrap
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