How to Install a Bathroom Vent Fan for a Mildew-Free Space

When building a new bathroom, you can't forget the vent! This important feature is responsible for keeping your bathroom free of moisture, mold, and mildew.

Vanity, sink
Project Overview
  • Total Time: 6 hours
  • Skill Level: Advanced

If you've ever been in a closed bathroom after a long shower, you know how important a bathroom vent can be. When left in the air, steamy moisture can ruin walls and wood features. Bathroom vents also do a good job at keeping people safe from the harmful fumes often found in bathroom cleaning agents.

When you install a new vent in a bathroom, it's a hefty task, but it can be done by homeowners with experience in siding, roofing, and electrical work. Below, we show you how to safely get the job done.

What You'll Need

Equipment / Tools

  • Voltage tester
  • Pry bar
  • Drill
  • Drywall saw
  • Jigsaw
  • Hammer
  • Ladder
  • Screwdriver
  • Strippers
  • Long-nose pliers
  • Lineman's pliers


  • Fish tape
  • Vent fan
  • Switch
  • Ductwork
  • Duct tape
  • Sheet metal screws
  • Cable
  • Clamps
  • Switch box
  • Wire nuts
  • Electrician's tape


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    Determine the Path

    Plan the installation, so the vent duct is as short as possible and makes as few turns as possible. In addition to a vent fan, a bathroom unit may have a light, nightlight, or heater. A heater uses more power than a light and fan, so it may need to be on its own circuit.

    If you can, run the duct straight out through a wall. Or go up into the attic, over, and out through the soffit. If there's no room above, the best solution may be a special vent fan that you can mount on the wall.

  2. Check Wiring

    A fan or a unit with a light and fan that come on simultaneously requires only two-cable wiring. However, wiring becomes more complex with separate function controls and additional functions. Check the wiring to replace an existing fan; you may need to replace two-wire cable with three-wire cable or even two cables.

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    Remove Existing Light

    To replace an existing ceiling light with a fan/light:

    1. Shut off power to the circuit.
    2. Remove the light and pry out the ceiling box.
    3. Cut carefully around the box before prying if you can't work from above. You may need to cut through mounting nails.
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      Mark Opening

      Disassemble the new fixture and use the housing as a template to mark the opening. The fan must be securely mounted; if there's no joist to attach it to, install blocking nailed to nearby joists.

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      Cut Opening

      If necessary, install blocking to keep the insulation away from the fan. Cut a hole with a drywall saw or reciprocating saw. If the ceiling is plaster, drill locator holes at the four corners and cut the opening from below.

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      Attach Fan

      Run new cable from the switch to the box if needed. (The fan shown has separate controls for fan and light and requires one three-wire cable.) Screw the fan to a framing member.

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      Cut Duct Hole

      For the wall vent, drill a locator hole from the inside through the outside wall. Outside, cut a hole for the duct.

      Editor's tip: To vent the fan out the roof, cut the hole from inside or drill a locator hole and cut the hole on the roof. Attach a short tailpiece onto the roof jack and install it. From inside, run flexible ducting from the fan to the jack, clamp each end, and cover the joints with duct tape. To prevent condensation from dripping near the fan or on the ceiling below, wrap the entire duct with pipe insulation.

      To install a roof jack, follow the manufacturer's instructions to prevent leaks. First, cut through the roof, then cut back shingles. Next, install the jack and cover its top half with shingles. Cover all nails with roofing cement.

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      Attach Duct

      Measure from the outside to the fan. Attach a piece of solid duct to the wall cap so it's long enough to reach the fan or as close as possible. Fit the duct to the cap, drill pilot holes, and drive sheet metal screws to hold it in place. Cover the joint with professional-quality duct tape.

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      Attach Cap

      Apply a bead of caulk around the exterior hole. Slide the duct through the hole and fasten the cap to the wall with screws.

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      Fill Gaps

      Fill any gap between duct and fan with another piece of solid duct or with flexible metal ducting. Clamp each joint and wrap with duct tape.

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      Run Cables

      First, turn the power off. Run the correct cable or cables to the switch box as needed. As shown above, power enters the switch box. If power enters the fan, see the manufacturer's instructions.

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      Wire the Switch

      To wire a fan/light switch:

      1. Connect the grounds and splice the white wires.
      2. Connect the red and black wires from the fan to the fan and light terminals.
      3. Connect the feed wire to the remaining terminal.
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        Wire the Fan

        Connect the grounds and splice the white wires. Splice the black wire to the black fan lead and the red wire to the colored lead. Attach the junction box cover. In the bathroom, install the light and the fixture canopy.

        Editor's tip: The more functions a switch controls, the more complicated the wiring. For instance, to wire a fan/light/night-light with separate controls for each, run two cables—one two-wire and one three-wire—from the switch to the fan.

        However, leaving the fan heater on wastes energy and creates a hazardous situation. To avoid this, install a timer switch and a two-function switch for the fan and light. To do so, install a double-gang box.

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