The duct has to carry moisture outdoors
The Building America Solution Center’s bathroom exhaust fan guide says the fan should vent directly outdoors, not into an attic, crawlspace or the space between floors. It calls for the most direct duct route with as few bends as possible, not crushing or kinking the duct, insulating the duct to minimize condensation, and a wall cap with a damper that closes when the fan is off. Its separate checklist for exhaust fans venting outside explains that fans exhausting into an attic or crawlspace can cause mold and wood damage there. Ask an assessor to trace the full route and confirm the outdoor outlet; replacing only the fan leaves these problems in place.
How long the fan runs matters
The same guide recommends running the fan for one hour after a shower for effective humidity control, using a timer switch rather than a standard switch. Longer runtime still does not prove the airflow is adequate. The EPA’s guide to mold and moisture says that if mold keeps coming back in a bathroom, more ventilation and more frequent cleaning usually help. If humidity is a problem elsewhere in the house too, see what a dehumidifier does compared with a sump pump.
Exhausted air has to be replaced
The Solution Center’s introduction to air flow explains that air exhausted from a house must be replaced by air drawn in from outside, and that negative pressure can cause backdrafting from fireplaces, gas water heaters, furnaces and boilers. If the home has fuel-burning appliances, have a qualified professional consider combustion safety before installing a more powerful fan.
Cold surfaces can still be part of the problem
The same air-flow page notes that condensation forms when moist air meets surfaces colder than the dew point. Persistent condensation can point to cold surfaces as well as moisture levels, not just the fan.
A safe screening check, and when to call a professional
The Solution Center’s bath fan cleaning guide describes holding a tissue to the grille while the fan runs: if it pulls the tissue up and holds it, airflow is likely sufficient. Only do this if the grille is reachable from a dry floor, and treat it as a rough check, not a measurement, and not proof the air reaches outdoors. Leave covers, wiring, roof and attic work to qualified professionals. If condensation persists, ask an HVAC or ventilation professional to measure the fan’s airflow and report on the duct, the outdoor outlet and replacement air before proposing a replacement. Electrical work belongs with an electrician. More questions are on our electrical and breakers page.