Bathroom Exhaust Fans, Ducts and Dampers

CFM, sones and duct size: sizing a bath fan

Airflow is rated in CFM, cubic feet per minute. A common guide is about 1 CFM per square foot of floor area, with 50 CFM as a minimum for small rooms, and bigger or higher-ceilinged bathrooms need more. Noise is rated in sones; lower is quieter, and a rating around 1 sone is very quiet while 2.0 is noticeable. Match the duct size to the fan outlet, commonly 3, 4 or 6 inches, because a short, straight, full-size duct moves far more air than a long run with bends.

Venting outside: caps, dampers and fan types

Always vent to the outdoors, never into an attic or ceiling space, where moist air causes mold and rot. A wall or roof cap with a damper stops cold air and pests coming back down the duct. Insulated duct reduces condensation in cold spaces. Radiation dampers for fire-rated ceilings are made for particular fan models, so match the model name. Shutter fans and portable blowers are different tools for garages, sheds or confined spaces, so check their voltage and mounting before assuming they replace a bath fan.

Questions about Bathroom Exhaust Fans, Ducts and Dampers

How do I calculate CFM for a large bathroom?

Take the room area in square feet for rooms up to 100 square feet. For larger rooms add up airflow by fixture: roughly 50 CFM for each toilet, tub or shower, and 100 CFM for a jetted tub. Very high ceilings call for extra airflow, so size up.

What are sones, and what is a good rating?

A sone is a unit of perceived loudness. One sone is about as loud as a quiet refrigerator, and lower numbers are quieter. For a bedroom-adjacent bathroom or a fan that runs for long periods, look for a low rating, and compare the decibel figure if the listing gives one.

Why does my bathroom fan seem weak?

Common causes are a duct that is too small, too long or crushed, a clogged grille or damper flap, and a fan rated too low for the room. Clean the grille, check the duct run for sags and bends, and make sure the vent cap opens fully.