· Michael Blair
A bathroom fan has one job: get moisture outside the building envelope. In Middle Tennessee, where outdoor humidity is already high for much of the year, an undersized or badly ducted fan is a reliable route to mould.
Sizing it properly
A common rule is one CFM per square foot of floor area for bathrooms up to about 100 square feet, with additions for a separate enclosed toilet, a jetted tub or a large shower.
Sound rating matters practically. A fan rated loud does not get used, and an unused fan achieves nothing regardless of its capacity.
Where it has to discharge
Outside. Through the roof or a gable wall with a proper hooded termination and a backdraft damper. Discharging into an attic or a soffit puts warm moist air into the roof structure, where it condenses on the sheathing.
Venting into a soffit is particularly counterproductive, because soffits are intake vents — the moisture is drawn straight back into the attic.
- One CFM per square foot as a starting point
- Insulated duct, sloped so condensate drains outward
- Rigid or smooth duct rather than long flexible runs
- Hooded exterior termination with a damper
- Never into an attic, a soffit or a crawl space
Run time is what actually clears it
A fan switched off with the light leaves most of the moisture in the room. Twenty minutes after the shower is the useful minimum, which in practice means a timer switch or a humidity-sensing control.
A humidity-sensing fan is the most reliable option in a Middle Tennessee bathroom, because it responds to the actual condition rather than to whether anyone remembered.
Signs yours is not coping
A mirror that stays fogged long after the shower, paint peeling from a ceiling, black spotting at the ceiling perimeter or in the grout, and a musty smell that returns after cleaning.
All of those point at ventilation before they point at cleaning products, and treating the mould without fixing the airflow means doing it again.