An exhaust fan has one job: remove moist air faster than a shower produces it, and put that air outside the building envelope. Almost every failed bathroom ventilation system fails at one of those two clauses.
Sizing: how much CFM do you need?
CFM is cubic feet per minute of air moved. Two accepted methods, and you use the larger answer.
Method 1 — floor area (rooms up to 100 sq ft, 8 ft ceilings)
One CFM per square foot of floor area, with a minimum of 50 CFM. A 7 × 9 ft bathroom is 63 sq ft, so 63 CFM — round up to the next available size, typically 70 or 80 CFM.
Method 2 — fixture count (larger or unusual rooms)
| Fixture | CFM allowance |
|---|---|
| Toilet | 50 |
| Shower | 50 |
| Bathtub | 50 |
| Jetted or soaking tub | 100 |
A bathroom with a toilet, a shower and a separate tub needs 150 CFM by this method regardless of its floor area. For ceilings above 8 ft, multiply the floor-area figure by (ceiling height ÷ 8).
Bigger is not automatically better
An oversized fan in a tight, well-sealed house can depressurise the room enough to pull air down a water heater or fireplace flue. It also drags conditioned air out of the house faster than you need. Size it properly, then make it quiet.
Sones: the number that decides if you use it
Sones measure perceived loudness, and unlike decibels the scale is linear: 2 sones sounds twice as loud as 1 sone. The rating matters because a fan nobody switches on ventilates nothing.
| Sones | Comparable to | Verdict |
|---|---|---|
| 0.3–1.0 | A quiet refrigerator hum | Excellent. Worth the premium in a main bathroom. |
| 1.0–2.0 | Normal background conversation | The sensible target for most rooms. |
| 2.0–3.0 | Noticeable, slightly intrusive | Acceptable in a guest bath or laundry. |
| 3.5–4.0+ | The fan you turn off after two minutes | Avoid. |
Look for HVI-certified ratings; they are measured under a standardised test rather than estimated. Quiet fans achieve their rating with larger, slower impellers and a bigger housing, which is why they are physically deeper — check joist depth before buying.
Ducting is where installations fail
Manufacturers rate CFM at a specific static pressure. Real-world ducting adds resistance, and a fan rated 110 CFM can deliver under 60 through a poorly run duct.
- Terminate outside. Always. Venting into an attic, a soffit cavity or a crawl space moves the moisture problem somewhere you cannot see it until the sheathing rots. Roof cap, gable vent or wall cap only.
- Match the duct to the outlet. A 6 in. fan reduced to 4 in. duct loses a large fraction of its rated flow.
- Rigid or smooth-wall flexible duct. Corrugated flex has several times the friction loss of smooth pipe. If you must use flex, pull it taut — every sag is a resistance and a place for condensate to collect.
- Keep it short and straight. Every 90° elbow costs roughly the equivalent of 15 ft of straight duct. Two elbows and 20 ft of run is a realistic ceiling.
- Insulate any duct in unconditioned space and slope it slightly toward the outside so condensate drains away from the fan housing.
- Check the backdraft damper at the exterior cap. A stuck damper is the most common cause of "the fan runs but nothing happens".
Switches, timers and humidity sensors
Moisture keeps evaporating from surfaces long after the shower stops, so the fan needs to run for roughly twenty minutes afterwards. Three ways to make that happen:
- Countdown timer switch. Cheap, reliable, and the best value upgrade in the whole system. Set it for 20–30 minutes.
- Humidity-sensing switch or built-in sensor. Runs the fan when relative humidity rises sharply and stops when it falls. Excellent for guest bathrooms and households with teenagers.
- Continuous low-speed operation. Some fans run at 30–50 CFM constantly and boost on demand — the quiet, whole-house-ventilation approach used in tighter modern builds.
Replacing an existing fan
Kill the circuit
At the breaker, then confirm with a non-contact tester at the fan. Bathroom circuits are frequently shared with lighting and are often GFCI-protected.
Measure the existing housing and duct
Many replacement fans are designed to fit common housing footprints without cutting new drywall. Note the duct diameter and which side it exits.
Check joist spacing and depth
Quiet, high-CFM units are deep. A 2×6 ceiling joist bay will not take some of them.
Mount the housing to a joist
Never to drywall alone. Vibration transmitted through drywall is exactly what makes a fan sound cheap.
Seal the duct connection
Foil tape or mastic on the collar — not fabric duct tape, which dries out and lets moist air into the ceiling cavity.
Air-seal the housing to the ceiling
A bead of sealant around the housing flange stops conditioned air leaking into the attic and stops attic air condensing on the fan.
Electrical work
If the replacement requires new cable, a new circuit or a change to the switch box wiring, and you are not confident, hire a licensed electrician. Bathroom circuits carry specific GFCI and, in many jurisdictions, AFCI requirements, and permit rules vary by state and city.
Diagnosing a bathroom that still fogs
- Tissue test. Hold a square of toilet tissue against the running fan's grille. If it does not hold firmly, airflow is poor — suspect the duct, the damper or a clogged grille.
- Look at the exterior cap. Lint, a wasp nest or a seized damper flap block far more fans than motors fail.
- Vacuum the grille and impeller. Years of dust on the blades can halve the flow.
- Provide make-up air. A fan cannot exhaust air that cannot be replaced. Undercut the door by roughly ¾ in. or fit a transfer grille.
- Check the run time. If it is switched with the light and the light goes off with the person, the fan never runs long enough. Fit a timer.
If the room still fogs after all of that, the fan is undersized for the fixtures. Recalculate using the fixture method above, and while the ceiling is open it is worth reviewing the lighting layout at the same time — the two jobs share a ceiling and a circuit.
Frequently asked questions
What size exhaust fan do I need for a 5x8 bathroom?
Forty square feet gives 40 CFM by the area method, which is below the 50 CFM minimum — so fit a 50 CFM fan. If that bathroom contains a separate shower and tub, use the fixture method instead and size for 100 CFM.
Can a bathroom fan vent into the attic?
No. Venting moist air into an attic, soffit or crawl space causes condensation on sheathing and framing, and leads to mould and rot. It must terminate at a roof cap, gable vent or wall cap outside the building envelope.
How long should a bathroom fan run after a shower?
About twenty minutes. Surfaces keep releasing moisture long after the water stops, which is why a countdown timer switch or a humidity sensor outperforms switching the fan with the light.
Why is my bathroom fan loud?
Usually one of three things: a high sone rating, a housing fastened to drywall instead of a joist, or a restricted duct forcing the motor to work against pressure. Dust on the impeller adds to all three.
