Should a bathroom fan use a soffit vent, or vent through the roof or wall?

Roof cap

Pros: warm, moist air rises naturally, and the run is often short. Cons: another roof penetration that must be flashed correctly, and in winter condensation in the duct can run back toward the fan unless the duct is insulated.

Wall cap

Pros: no roof penetration, easy to service, good for bathrooms on exterior walls or near a gable. Cons: long horizontal runs through the attic reduce airflow, and the duct needs a slight slope toward the outside.

Soffit outlet

Pros: no hole in the roof or siding. Cons: exhaust can be drawn back in through nearby intake vents, and because the residential code (Section M1501.1) bars discharge into soffits, it only works as a dedicated outlet that truly terminates outdoors. Some inspectors will not accept it.

Duct details that matter more than the outlet

• Use the duct size the fan specifies, usually 4 inch, with smooth rigid or semi-rigid duct where possible.
• Insulate the duct in the attic to limit condensation.
• Seal joints with foil tape or clamps, not cloth duct tape.
• Keep the run short and avoid sags that collect water.

For roof caps added during a reroof, see roof replacement day and roof flashing explained.