How to Install Bathroom Exhaust Fan Ductwork? | Done Right

Run bathroom exhaust duct straight to an exterior wall or roof cap, use the fan’s rated duct size, and seal every joint.

Bathroom fans only work as well as their ductwork. A fan that dumps humid air into the attic is worse than no fan at all, because it feeds mold on the framing and rains condensation onto the insulation below. Knowing how to install bathroom exhaust fan ductwork is more important than picking the fan itself, since the duct decides where every bit of moisture goes. The whole job rests on three rules: a short, straight run, the exact duct size the fan spec sheet lists, and airtight joints from housing to cap.

Route The Duct To The Outside — Nothing Else Counts

Bathroom exhaust duct must terminate at an exterior wall cap or roof cap. Discharging into an attic, vented attic, soffit, crawlspace, or just in front of an attic vent is not sufficient, and code requires exhaust air from bathrooms to go directly outdoors. Routing to an interior space doesn’t remove the moisture. It moves the problem into the structure.

Keep the run as direct as possible with the fewest bends you can manage. Locate the exterior termination at least 10 feet from a mechanical air inlet, or at least 3 feet from a gravity air inlet, so the moist air doesn’t get pulled straight back into the house. Install a backdraft damper between the fan and the cap so cold outside air can’t pour in when the fan is off, and pitch the duct slightly so condensation drains toward the outdoors. Wherever the run passes through unconditioned space, insulate it; for hard duct the U.S. Department of Energy recommends R-8, spelled out in its hard-ducted vent job aid.

The Installation Steps, In Order

Plan the run before you touch the fan: measure the straightest path to the nearest exterior wall or roof cap, and pick the termination point first. From there, installation follows a fixed order.

1. Snap the damper/duct connector onto the fan housing so the connector sits flush and the damper flap can close freely. A flap that can’t seat fully will rattle and leak.
2. Connect round ductwork to the connector using the diameter the fan manufacturer specifies. Smooth metal duct is preferred over flex; if flex is the only option, stretch it tight and keep it short.
3. Run the duct to the wall or roof cap with as few bends as possible. Keep at least 3 feet of straight duct from the fan outlet before the first elbow, and prefer gradual bends over sharp 90-degree turns, since an elbow placed immediately at the outlet can cut airflow significantly.
4. Secure every connection with mechanical fasteners or a clamp so joints can’t pull apart, then seal them with mastic or UL-approved metal tape. Ordinary duct tape breaks down here and isn’t accepted.
5. Seal the fan housing to the ceiling drywall so the fan can’t suck attic air through the gap around the housing.
6. Insulate duct that runs through unconditioned space to R-8 so condensation never forms on the duct skin.
7. Run 120 V AC house wiring to the fan location following the manufacturer’s instructions. A fan installed over a bathtub or shower must be UL listed for that use and connected to a GFCI-protected branch circuit.

When the job is done right, the damper flaps open fully the moment the fan starts, seat closed when it stops, and no gap shows between the housing and drywall. The room clears noticeably faster than it would with a short, sagging flex run, and the damper never rattles.

What Duct Size And How Many Bends Are Allowed?

Use the duct diameter the fan manufacturer specifies, usually 4 or 6 inches, and never step it down. Reducing the diameter below the fan outlet adds resistance and chokes airflow, which stretches dry time and raises fan noise. The same logic applies to bends: every elbow costs airflow, so fewer is better, and gradual turns beat sharp 90s.

Code sets the minimum ventilation for bathrooms and toilet rooms at 50 cfm intermittent or 20 cfm continuous. Size the fan for the room, then keep the duct at the fan’s rated diameter so that airflow actually reaches the cap. If you’re shopping for duct, flexible or rigid, our tested bathroom exhaust fan duct picks cover the options that perform well in real installs.

Smooth metal duct wins for this job; the DOE’s flex-duct job aid takes the same position and adds that flex must be stretched tight, kept short, and never left sagging, since sagging increases resistance and traps moisture. Flex also needs to stay at the fan’s specified diameter, never compressed through a tight space.

Work the job in this sequence: mount the fan with the exhaust port facing the termination point, connect the duct at full diameter, keep 3 feet of straight run before the first bend, seal every joint, insulate attic runs to R-8, then wire the fan. Follow that order, and the run stays short, dry, and quiet — and the bathroom actually gets vented.