A bathroom fan removes the moisture of showers and baths before it condenses on walls and ceilings, where it causes mold, peeling paint and rot. It also clears odors. Fans are rated in cubic feet per minute (CFM), and the Home Ventilating Institute (HVI) gives two simple rules for choosing one.
The HVI rules
Bathrooms up to 100 sq ft: 1 CFM per square foot of floor, with a minimum of 50 CFM. This assumes an 8 ft ceiling.
Bathrooms over 100 sq ft: add up the fixtures:
| Fixture | CFM |
|---|---|
| Toilet | 50 |
| Shower | 50 |
| Bathtub | 50 |
| Jetted tub | 100 |
A separate toilet room or a closed shower enclosure should get its own fan of at least 50 CFM.
The air change method
Another approach is to replace the air 8 times an hour: CFM = room volume (cu ft) × 8 ÷ 60. It is useful for bathrooms with high ceilings, where the area rule underestimates the volume.
Example
An 8 × 10 ft bathroom (80 sq ft) needs an 80 CFM fan. With an 8 ft ceiling, the air change method gives 85 CFM, so a standard 80 CFM fan is the right choice.
Codes
The International Residential Code requires bathroom exhaust of at least 50 CFM for intermittent fans or 20 CFM for continuous ones, vented to the outside, never into the attic or a wall cavity.
Getting the rated airflow
A fan only delivers its rated CFM with a short, smooth duct. Long runs, flexible duct and many elbows can halve the airflow.
- Use the duct size the fan was designed for (often 4 in; 6 in for larger fans) and keep runs short.
- Prefer smooth rigid duct and insulate it in cold attics to prevent condensation.
- Terminate through the roof or a wall with a damper.
Noise and controls
Fans are rated in sones: 1 sone or less is quiet, 3 sones is like an office. A quiet fan gets used. A humidity-sensing switch or a timer keeps the fan running for 20 minutes after a shower, long enough to clear the moisture.