Almost every underperforming fan fails on one of those three points, and the third — where the duct ends — is the one nobody checks.
Extraction is the cheapest moisture removal available, because it takes water out at the moment and place it is produced rather than after it has spread through the house. A shower puts a pint into the air in fifteen minutes; a fan sends most of it outside before it reaches a cold wall in another room.
It is also the most commonly misinstalled thing on this site. Fans get specified by price, wired to the light switch, and ducted to wherever was easiest.
- Sizing rule
- 1 CFM / sq ft
- Run-on after use
- 20 min
- Duct run
- As short as possible
- Running cost
- Pennies/day
50 CFM minimum; more for enclosed showers or high ceilings
When most of the moisture actually leaves the surfaces
Rigid or semi-rigid; every bend costs airflow
A fraction of what a dehumidifier costs for the same water
The three things that decide whether it works
Capacity. The rule of thumb is 1 CFM per square foot of floor area, with 50 CFM as a practical minimum. A bathroom with a separate enclosed shower, a jetted tub or a ceiling above 9 feet needs more — the fixture-based method adds 50 CFM for each of a toilet, shower and bath and 100 for a jetted tub.
Run time. Most of the moisture leaves wet surfaces after the shower stops, over the following fifteen to twenty minutes. A fan wired to the light switch turns off exactly when it is needed most, which is why a humidistat or a run-on timer is the single best upgrade to an adequate fan.
The duct. It has to terminate outside the building. A fan discharging into an attic, a soffit, a wall cavity or a garage moves the moisture from a room where you would notice it to a place where you will not — and attic sheathing is a far worse place for it. Short, straight, rigid or semi-rigid duct beats long flexible foil, which sags, collects water and strangles airflow.
| Failure | How to check | Fix |
|---|---|---|
| Duct ends in the attic | Follow it, or look for a stain on the sheathing | Re-route to an exterior or roof vent |
| Switched with the light | It stops when you leave the room | Humidistat or run-on timer switch |
| Undersized | Room still fogged 30 min later | Replace with the right CFM |
| Blocked or crushed duct | Tissue held to the grille does not stick | Clear it, replace flexible with rigid |
| Backdraft damper stuck | Cold draft when off, weak flow when on | Free or replace the damper |
| Grille clogged with dust | Visible lint on the vanes | Ten-minute clean |
When it does not work
The limits matter more than the benefits — this is where most money gets wasted.
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It cannot fix a cold wall
Extraction lowers room humidity. If mold keeps returning to one corner while the room reads normally, that surface is too cold and needs warming.
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A recirculating unit removes nothing
This applies to range hoods above all. If the ducting does not leave the building, it is a filter with a fan and every drop of steam returns to the room.
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It needs replacement air
A fan can only push air out if air can get in. In a very airtight room with a well-sealed door, an exhaust fan can stall against its own pressure. The undercut at the door is doing real work.
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It will not keep up with a whole-house problem
Extraction is targeted at source rooms. If the entire house sits above 60% in humid weather, that is a dehumidification or whole-house ventilation question.
Doing it properly
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Measure the room
Floor area in square feet gives you the CFM figure. Note the ceiling height and whether the shower is in a separate enclosure.
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Test what you already have
Hold a single sheet of tissue against the running grille. If it does not hold in place, the fan is not moving meaningful air and the problem is the duct or the unit, not the sizing.
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Follow the duct to its end
Into the attic, into a soffit, into a wall — all wrong. It has to reach outdoor air through a proper terminal with a damper.
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Fix the duct before the fan
Shorten it, straighten it, replace flexible foil with rigid or semi-rigid, and insulate any run through unconditioned space so it does not condense inside itself.
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Add a humidistat or run-on timer
This is the highest-value change to a fan that is otherwise adequate, because it removes the human from the decision.
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Use it properly
On before the shower, door closed, off twenty minutes after. The closed door makes the fan draw replacement air under it and out of the building.
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Re-check in the room, not the hallway
A hygrometer in the bathroom should be back under 60% within about twenty minutes of the shower ending.
What it costs
Prices are US retail ranges in 2026 and running costs assume roughly 16 cents per kWh. Treat them as the shape of the decision rather than a quote.
| Fan | Fitted cost | To run | Notes |
|---|---|---|---|
| Basic 80 CFM bathroom fan | $40–$90 plus fitting | ~$2/yr | Adequate for a small bathroom with a short duct run |
| 110 CFM with run-on timer | $80–$160 plus fitting | ~$3/yr | The configuration that fixes most bathroom condensation |
| Humidity-sensing fan | $120–$220 plus fitting | ~$4/yr | Starts itself; useful where nobody remembers the switch |
| Inline fan in the attic | $150–$300 plus fitting | ~$5/yr | Quietest option, and the one that survives a long duct run |
| Kitchen hood ducted outside | $150–$600 | Varies | Only counts if it vents outdoors — a recirculating hood removes no water |
How to tell it is working
The number to watch is recovery time, and a bathroom gives you an unusually clean experiment. Read the humidity immediately after a shower with the door shut, then read it again after twenty minutes with the fan running. A correctly sized fan on a short duct brings a bathroom from ninety-something back under 60% in that window. If it is still in the seventies, the fan is undersized, the duct is too long or too kinked, or there is no make-up air getting into the room.
That last one catches people out. A fan can only push out as much air as it can pull in. A tightly fitted door with no undercut starves the fan, and the measured extraction can fall by half. The test is to repeat the experiment with the door open an inch: if the recovery suddenly works, the room needs a gap under the door rather than a bigger fan.
Over a season, the signal that extraction has fixed a problem is that mold stops reappearing on the ceiling and around the window reveal after cleaning. Surface condensation on the mirror is not a fault — it is the coldest surface in the room and it will always fog. Water sitting on the walls twenty minutes after a shower is the fault.
What to check before you commit
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Size in CFM from the room, not the price
A common rule is one CFM per square foot of bathroom floor for an eight-foot ceiling, with a floor of about 50 CFM. A room with a separate enclosed shower or a large tub wants more. Undersizing by a third is the most common installation error.
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Insist on a run-on timer
Most of the water is still airborne when the light goes off. Fifteen minutes of run-on removes more moisture over a year than the difference between an 80 and a 110 CFM unit, and it costs almost nothing to add.
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Duct it outside, in a straight line, insulated
Every bend and every foot of flexible duct costs airflow. A run terminating in the attic rather than through the roof or wall simply moves the moisture into the roof space, where it condenses on the underside of the felt.
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Leave a path for make-up air
A gap under the door, a vent, or an inch of open window while it runs. Without one, the fan strains against a sealed room and moves a fraction of its rated volume.
Common mistakes
Buying on noise rating alone
A quiet fan that nobody minds running is genuinely valuable — but not if it was chosen at 50 CFM for a room that needed 90.
Leaving the bathroom door open
It spreads the steam into the hall rather than letting the fan pull it out. Door closed, fan running.
Venting into a soffit
Moist air discharged at the soffit is drawn straight back into the attic through the intake vents. It is one of the worst terminations available.
Long flexible duct with sags
Water collects in the low point, airflow drops, and eventually it drips back through the fan housing.
Common questions
What size exhaust fan do I need for a bathroom?
Roughly 1 CFM per square foot of floor area, with 50 CFM as a minimum. Add capacity for an enclosed shower, a jetted tub or a ceiling above 9 feet.
How long should a bathroom fan run after a shower?
About 20 minutes. Most of the moisture evaporates off wet surfaces after the water stops, which is exactly when a light-switched fan turns off.
Can a bathroom fan vent into the attic?
No. It has to terminate outdoors through a proper vent. Discharging into an attic or a soffit is a common cause of roof sheathing mold and rot.
How do I know if my exhaust fan is working?
Hold a sheet of tissue against the grille while it runs. If it does not hold in place, the fan is not moving useful air — usually a blocked, crushed or over-long duct.
Should the bathroom door be open or closed while the fan runs?
Closed, with an undercut or a vent for make-up air. A closed door keeps the moisture in the room the fan is serving instead of letting it spread through the house to condense on colder walls elsewhere. The gap under the door is what lets the fan work while the door stays shut — usually about three quarters of an inch is enough.
Why does my exhaust fan drip water?
Warm wet air is condensing inside a cold duct, usually one running through an unheated attic, and the water is running back down to the fan. The fix is insulating the duct along its whole length and giving it a slight fall toward the outside so any condensate drains away from the house rather than back into it.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions