How to Route Bathroom Exhaust Ductwork to the Exterior

Routing bathroom exhaust to the outside is the only way to actually remove moisture from your home—and it's the difference between a fan that works and one that just moves damp air into your attic where it causes rot. Most bathroom fans are installed with ductwork that either never reaches the exterior, dead-ends into an attic, or takes such a tortuous path that it barely moves air at all. A properly installed duct run gets moisture out cleanly, prevents mold in your attic, and keeps your bathroom dry without constantly running the exhaust for hours. The work itself is straightforward: you're essentially routing a tube from the fan housing through your home's structure to an exterior wall or roof vent. The key is planning the path first, using the right materials, and sealing everything so air doesn't escape along the way.

1. Plan the duct route and identify exit location. Locate the fan housing in the bathroom and trace a path that reaches an exterior wall or the roof with the fewest bends and shortest distance possible. Check the attic or cavity above for obstacles—electrical wiring, plumbing, joists, or rafters. Mark where the duct will exit the exterior wall or roof. Shorter runs are always better; aim for under 25 feet of total duct length, and reduce fan capacity if your run will exceed that.
2. Cut the exit hole and install the exterior vent cap. Drill a pilot hole through the exterior wall or roof at your marked exit point, then cut a hole sized for your duct diameter (typically 4 to 6 inches). Install the exterior vent cap—either a wall-mounted hood or roof vent—from the outside, sealing it with roofing cement or exterior caulk. Make sure the damper inside the cap moves freely; test it by hand before moving on.
3. Disconnect and prepare the fan outlet. Turn off power to the fan at the breaker. Disconnect any existing ductwork from the fan housing. Clean the outlet opening of any dust or debris. If the fan has a flexible connector, remove it; you'll replace it with rigid or semi-rigid duct for better airflow and easier installation.
4. Run the main duct from fan to exterior. Feed the duct from the fan outlet toward the exit hole, working through the attic or wall cavity. Use rigid duct (aluminum or galvanized steel) for the longest straightest stretch, then flexible duct for the final run through wall cavities if needed. Insert the duct through the exterior hole until the end reaches the vent cap. Do not compress flexible duct into accordion folds; keep it as straight as possible even if it means making gentle bends.
5. Seal and support the ductwork. Seal every joint between duct sections with UL-listed duct mastic or foil tape (not vinyl electrical tape, which deteriorates in attics). Support the duct with metal straps or hangers every 3 feet, angling it very slightly downward toward the exterior exit so condensation drains out rather than pooling. Do not use screws through duct walls; mastic and straps are sufficient and prevent air leaks.
6. Install an inline damper to prevent backflow. Place a damper in the duct run between the fan and the exterior vent cap—typically a flapper valve in a short duct section. Position it horizontally so gravity helps it close when the fan is off. This prevents outside air, insects, and weather from entering the bathroom when the fan isn't running. Test the damper by hand to confirm it swings freely without binding.
7. Test the fan and verify airflow. Restore power to the fan at the breaker. Run the fan for a few minutes and feel for air movement at the exterior vent cap. Hold a tissue near the bathroom fan inlet—it should pull toward the fan. If airflow is weak, check for kinks or compression in the flexible duct, loose joints, or debris in the cap. Listen for whistling, which indicates air leakage that needs sealing.
8. Insulate the duct if it runs through unconditioned space. If your duct passes through an attic or exterior wall, wrap it with pipe insulation or fiberglass wrap to prevent condensation from forming on the duct surface and dripping into insulation or framing. This is especially important in cold climates where temperature differences between the moist exhaust air and the cold attic space cause water to condense inside the duct.