Exhaust fan for a 5 × 12 ft bathroom with a jetted tub and 2 elbows
| Rated size to buy | 300CFM |
|---|---|
| Airflow actually neededset by floor area | 200CFM |
| By floor area60 ft² at 1 CFM per ft², min 50 | 60CFM |
| By fixturestoilet, shower and jetted tub | 200CFM |
| Equivalent duct length10 ft straight + 2 × 15 ft per elbow + 30 ft for the wall cap | 70ft |
| Airflow lost to the ducta 300 CFM fan will move about 233 CFM here | 23% |
| Duct diameterrigid, not flexible — see below | 8″ |
| Room60 ft² | 5 × 12 ft |
- Width
- 5 ft
- Length
- 12 ft
- Fixtures
- a jetted tub
- Duct elbows
- 2
The number on the box is measured on a duct you do not have
Fans are rated at 0.1 inches of static pressure — short, straight, smooth duct. Real installations have elbows, a run to the roof or wall, and an exterior cap with a damper.
Each 90° elbow is worth about 15 feet of equivalent straight duct. The wall cap is worth about 30. With 2 elbow2 this bathroom's duct behaves like 70 feet of straight pipe, which costs 23% of the fan's rated airflow.
So: 200 CFM needed at the room, 300 CFM on the box, moving roughly 233 CFM in practice.
Rigid duct, not flexible
Corrugated flexible aluminium has around three times the resistance of smooth rigid pipe at the same diameter, and it sags between joists into low spots where condensate pools and eventually drips through the ceiling.
If there is one place to spend an extra thirty dollars in a bathroom, this is it.
Two ways to size, and the larger wins
By floor area: 1 CFM per square foot, with a 50 CFM floor from ASHRAE 62.2 — 60 CFM here. By fixtures: 50 each for a toilet, shower or tub, 100 for a jetted tub — 200 CFM here.
Below about 100 ft² the area rule governs. Above it, fixtures do, because floor area stops predicting how much moisture is actually produced.
Vent outside, never into the attic
A fan discharging into an attic puts the entire bathroom's moisture onto the underside of the roof deck, where it condenses. It is one of the most common causes of mould found during a roof replacement, and nothing indicates it until then.
Sone rating decides whether it gets used
Above about 1.5 sones people turn the fan off, and a fan that is off moves no air at any rating. Pair it with a timer switch and run it twenty minutes after the shower stops — the moisture is still in the air and the towels long after the water is off.
Notes
- **The number on the box is measured on a duct you do not have.** Fans are rated at 0.1 inches of static pressure — a short, straight, smooth duct. Yours has 2 elbows and an exterior cap, which together are worth about 70 feet of equivalent straight duct. That is why a 300 CFM fan moves roughly 233 CFM in this bathroom.
- **Rigid duct, not flexible.** Corrugated flexible aluminium has around three times the resistance of smooth rigid pipe of the same diameter, and it sags between joists into low spots where condensate collects. If there is one thing to spend the extra thirty dollars on, it is this.
- At 60 ft² the floor-area rule governs, which is the normal case for a bathroom this size. ASHRAE 62.2 puts a floor of 50 CFM under it regardless.
- **Vent it outside, not into the attic.** A fan discharging into an attic dumps the whole bathroom's moisture onto the underside of the roof deck, where it condenses. It is one of the most common causes of mould found during a roof replacement, and it is invisible until then.
- Run it 20 minutes after the shower stops. A timer switch costs very little and does more for the room than an extra 50 CFM — the moisture is still in the air and in the towels long after the water is off.
- Sone rating decides whether it actually gets used. Anything above about 1.5 sones is loud enough that people turn it off, and a fan that is off moves no air at any rating.
Nearby sizes
- Exhaust fan for a 4 × 12 ft bathroom with a jetted tub and 2 elbows
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- Exhaust fan for a 5 × 12 ft bathroom with a toilet, tub and shower and 2 elbows
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