Bathroom exhaust fan guide: CFM, noise and venting

A bathroom fan is the cheapest insurance against mold, peeling paint and swollen vanity doors. In New Hampshire, where houses are closed up half the year, it matters even more. The trick is sizing it right and venting it properly.

How much CFM

The basic rule: 1 CFM per square foot of floor, with a 50 CFM minimum. An 8 by 10 foot bath needs about 80 CFM. For ceilings over 8 feet, or bathrooms with a separate toilet room or big steam shower, size up. The bath fan calculator does the math.

How quiet

Fan noise is rated in sones. 1.0 sone or less is quiet; 0.3 to 0.5 is nearly silent. Quiet fans get used, loud ones get switched off early, which defeats the purpose.

Venting it right

A fan must vent outside, never into the attic. Use insulated rigid or semi-rigid duct, keep the run short with few bends, and slope it slightly toward the outside so condensation drains out in winter. Undersized or long duct runs cut real airflow dramatically.

Controls and features

• Humidity-sensing fans turn on and off by themselves.
• Timer switches keep it running 20 minutes after a shower.
• Fan and light combos and fans with night lights save a ceiling hole.
• ENERGY STAR fans use far less power running continuously.

Upgrading an old fan

Many old fans are loud, undersized and vented into the attic. Replacing one is often part of a bathroom remodel, and any electrical or duct work is quoted separately.

Common questions

What size exhaust fan for a bathroom?

About 1 CFM per square foot of floor area, 50 CFM minimum. An 8 by 10 foot bathroom needs about 80 CFM.

How long should a bathroom fan run after a shower?

About 20 minutes. A timer or humidity-sensing switch handles it automatically.

Can a bathroom fan vent into the attic?

No. It must vent outside through the roof, wall or soffit, or moisture will cause mold in the attic.