Quick answer
Turn the fan on when you start the shower and leave it running for about one hour afterward. That is the recommendation of the Building America Solution Center, a U.S. Department of Energy resource, which also suggests a timer switch so the fan shuts off by itself. The run time only helps if the fan is strong enough (at least 50 cfm for a fan you switch on and off) and actually vents outdoors, not into the attic.
Why the fan needs to keep running after you leave
A shower fills the bathroom with warm, saturated air. When you step out, the steam doesn’t disappear: it’s in the air, on the mirror, on the walls and in the towels. If the fan stops as you leave the room, that moisture condenses on the coldest surfaces, usually the ceiling, the window and the corners of exterior walls, and stays there for hours.
Surfaces that stay damp are where mold grows. The EPA recommends running the bathroom fan or opening the window when showering, and keeping indoor humidity below 60%, ideally 30–50%. The extra hour gives the fan time to pull out the moisture that is still evaporating from wet surfaces after the water is off.
How long: the one-hour rule and when to adjust it
The Building America Solution Center recommends leaving the fan running for one hour after a shower for effective humidity control. Treat that as the default and adjust with what you see:
| Situation | What to do |
|---|---|
| Typical shower, fan vents outdoors | During the shower plus about 60 minutes |
| Mirror and walls still wet after an hour | Check fan airflow and duct; run longer until fixed |
| Several showers back to back | Keep it running and restart the hour after the last one |
| Mold already on ceiling or grout | Run the full hour every time, fix airflow, then clean |
| Fan is part of whole-house ventilation | It may run continuously at low speed; follow the system design |
A simple test: 30 and 60 minutes after the shower, touch the mirror and the ceiling paint near the shower. If they’re dry, your routine works. If they’re still beaded with water, the fan is weak, the duct is restricted, or the run time is too short.
Use a timer, not a regular switch
Almost nobody remembers to come back and turn the fan off an hour later, and many people switch it off when they leave to save noise. The Building America Solution Center recommends a timer switch rather than a standard wall switch for fans run intermittently: you set it once and the fan turns off automatically.
- Countdown timer switch: push a button for the run time you want.
- Humidity-sensing control: turns the fan on when humidity rises and off when it drops.
- Continuous low-speed fan: some fans run all the time at low speed and boost during showers; this is common when the bath fan is part of whole-house ventilation.
Watch out: replacing a switch involves household wiring. If you are not qualified to do electrical work, have an electrician install the timer or control.
Is your fan strong enough?
An hour of a weak fan does less than a few minutes of a good one. The Building America Solution Center summarizes the minimums from ASHRAE 62.2 and the International Residential Code, plus ENERGY STAR’s advice for choosing a fan:
| How the fan runs | Minimum airflow | ENERGY STAR sizing advice | Sound rating |
|---|---|---|---|
| Intermittent (you switch it on) | 50 cfm or more | Choose a fan rated at least 70 cfm | 3 sones or less |
| Continuous | 20 cfm or more | Choose a fan that provides more than 50 cfm | 1 sone or less |
Technical note: cfm is cubic feet of air per minute; a sone is a measure of perceived loudness, and lower is quieter. Real airflow is lower than the box rating because ducts, bends and grilles add resistance, which is why ENERGY STAR suggests buying a fan rated above the minimum you need. A quiet fan also matters for run time: people are more willing to leave a fan on for an hour if it’s barely audible.
Worked example: what an hour of fan costs
Assumed values: a fan drawing 20 watts, run 70 minutes per day (10-minute shower plus 60 minutes), electricity at $0.17 per kWh.
- Daily energy = 20 W × 70/60 h ≈ 23 Wh = 0.023 kWh.
- Yearly energy = 0.023 × 365 ≈ 8.5 kWh.
- Yearly cost = 8.5 × $0.17 ≈ $1.45.
kWh per year = watts × hours per day × 365 ÷ 1000
cost = kWh per year × price per kWhIllustrative example with assumed values. Check your fan’s label for its wattage and your utility bill for your price. In winter, the fan also exhausts some heated air, which adds a heating cost not included here.
Make sure it vents outdoors
Run time is wasted if the air goes to the wrong place. The Building America Solution Center says the fan should be ducted outside, not into the attic, crawlspace, garage, space between floors or wall cavities, and that moisture from exhaust fans venting into an attic or crawlspace can cause mold and wood damage there. It also advises against relying on an operable window for consistent bathroom ventilation.
- Check airflow: with the fan on, hold a single square of toilet paper to the grille. It should be pulled firmly against it.
- Check the outlet: find the vent cap on the roof or wall and see whether its flap opens when the fan runs.
- Check the attic: with a flashlight, follow the duct from the fan to the outside. The DOE guide suggests an N95 mask and coveralls for this.
- Clean the grille: dust on the grille and fan blades cuts airflow.
Frequently asked questions
Is 20 minutes enough?
Sometimes, with a strong fan in a small bathroom. The Building America Solution Center recommends about an hour for effective humidity control; if your mirror and ceiling are dry sooner, you can shorten it.
Can I leave the bathroom fan on all day?
Fans designed for continuous operation can run all the time; the continuous-rate guidance is at least 20 cfm and 1 sone or less. For an ordinary intermittent fan, check the manufacturer’s instructions.
Should I keep the bathroom door open or closed?
Keep it mostly closed during and after the shower so steam doesn’t spread through the house, but make sure air can get in (a gap under the door), or the fan can’t move much air.
Is opening the window enough?
The EPA lists opening the window as an option while showering, but the Building America Solution Center says windows shouldn’t be relied on for consistent bathroom ventilation. A fan works in any weather.
Sources
- Building America Solution Center (U.S. DOE / PNNL). Bathroom Exhaust Fans: one hour after showering, timer switch, airflow and sound ratings, venting outdoors.
- Building America Solution Center. Look at Exhaust Fans for Venting to Outside.
- U.S. EPA. A Brief Guide to Mold, Moisture and Your Home: bathroom fan while showering, humidity targets.
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