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A bathroom extractor fan does reduce humidity – but only under the right conditions. An undersized fan, one that vents into the ceiling cavity instead of outside, or one that’s switched off the moment the shower ends will make almost no difference to your bathroom’s moisture levels.
This article explains how extractor fans work, how to tell if yours is doing its job, and what to do if it isn’t.
How a Bathroom Extractor Fan Works
An extractor fan pulls humid air from the bathroom and pushes it through a duct to the outside. As humid air leaves, drier air from the rest of the house flows in to replace it – through the gap under the door or through a vent. This exchange reduces the total amount of moisture in the bathroom air.
The key word is outside. A fan that terminates in the ceiling cavity, loft, or wall space is moving humid air from one part of your home to another. It reduces bathroom humidity somewhat but deposits moisture where it can cause rot, mold, and insulation damage. Proper installation means the duct exits through an external wall or roof with a weatherproof vent cover.
How to Tell If Your Extractor Fan Is Actually Working
The toilet paper test
Hold a single sheet of toilet paper flat against the fan grille while the fan is running. If the paper is held in place by suction, the fan is pulling air. If it falls away, the fan is either clogged, the motor is failing, or the duct is blocked.
This is the fastest way to check airflow without any equipment. Do it with the bathroom door closed to simulate normal operating conditions.
The hygrometer test
The toilet paper test tells you the fan is moving air. The hygrometer test tells you whether it’s moving enough.
Check bathroom humidity with a hygrometer like the Unni Indoor Outdoor Hygrometer immediately after finishing a shower with the fan running. Check it again 20 minutes later. If humidity has dropped below 60% within that window, the fan is doing its job. If it’s still above 60–70% after 20 minutes, the fan is undersized or the duct has a problem.
This is the most useful test because it measures the outcome rather than just whether the fan is spinning.
Check where the duct terminates
If you can access the loft or ceiling space above the bathroom, follow the duct from the fan housing. It should terminate at an external vent on the roof or wall – not end loosely in the loft space or connect to a general roof void. A flexible duct that sags, kinks, or has become disconnected from the external vent will severely reduce airflow even if the fan motor is working fine.
Why Your Extractor Fan Might Not Be Reducing Humidity
It’s undersized for the room
Fan capacity is measured in CFM (cubic feet per minute) or m³/hour. Building regulations in most regions require a minimum of 15 litres per second (approximately 32 CFM) for a bathroom with a shower. In practice, most bathrooms need more – particularly if the shower is large, the room is small, or the ceiling is high.
A simple sizing guide:
| Bathroom size | Recommended fan capacity |
|---|---|
| Up to 4 m² (43 sq ft) | 60–80 CFM |
| 4–8 m² (43–86 sq ft) | 80–100 CFM |
| 8–12 m² (86–130 sq ft) | 100–130 CFM |
| Over 12 m² (130 sq ft) | 130+ CFM or multiple fans |
If your fan is rated below these figures for your bathroom size, upgrading is the most effective single change you can make.
It’s clogged with dust
Fan grilles and blades accumulate dust over time, reducing airflow significantly. A fan that hasn’t been cleaned in two or more years may be operating at 50–60% of its rated capacity. Remove the grille, vacuum the blades and motor housing, and retest with the toilet paper method. In many cases this alone restores adequate performance.
It only runs during the shower
This is the most common usage mistake. The fan running during a shower captures some steam but not most of it. After the shower ends, humidity continues to rise as remaining steam condenses off your body, off wet surfaces, and off the cooling walls. The fan needs to run for 15–20 minutes after the shower ends to clear what’s left.
A humidity-sensing fan or a timer switch eliminates the need to remember this. Humidity-controlled fans automatically switch on when humidity rises above a set threshold and off when it drops back down – they’re the most effective option because they respond to actual conditions rather than a fixed schedule.
The duct is too long or has too many bends
Every metre of duct and every bend reduces fan performance. A fan rated at 100 CFM in open air may deliver only 70–80 CFM through a 3-metre duct with two 90-degree bends. If your bathroom is far from an external wall and the duct runs a long route, the fan needs to be rated higher than the room size alone would suggest.
As a rule of thumb: add 20% to your target CFM for every 3 metres of duct length beyond the first metre, and for every bend in the duct.
The room is too airtight
Extractor fans work by pulling air out of the room. For that to work, replacement air needs to be able to get in. If the bathroom door has a tight seal at the bottom and there’s no air transfer grille in the door or wall, the fan creates negative pressure that limits its own airflow. Most bathroom doors should have a 10mm gap at the bottom or a transfer grille to allow makeup air to enter.
Humidity-Sensing Fans vs. Timer Fans vs. Standard Fans
Standard on/off fans – controlled by the light switch or a separate switch. Effective only if used correctly – run during the shower and for 20 minutes after. Most people don’t do this consistently.
Timer fans – run for a set period after the switch is turned off. A 20-minute timer set to run after the shower covers most of the critical post-shower window. Better than a standard fan because the timing is built in, not reliant on habit.
Humidity-sensing fans – the Panasonic WhisperSense turn on automatically when humidity rises above a set level (usually 70–80% relative humidity) and off when it drops back below. These are the most effective option for consistent humidity control because they respond to actual conditions. They also run only when needed, which extends motor life compared to a timer that runs on a fixed schedule regardless of conditions.
For a bathroom that’s had persistent humidity, mold, or condensation problems, a humidity-sensing fan is the upgrade most likely to solve the problem without any behaviour change required.
When a Fan Alone Isn’t Enough
In some bathrooms – particularly small rooms with poor natural ventilation, no window, or very high shower use – even a correctly sized and installed fan may not be sufficient on its own.
Supplementary measures that help:
- Leaving the bathroom door open after showering to allow humidity to disperse into the larger home volume
- Running a portable dehumidifier in the bathroom during and after showering – practical if the bathroom is large enough
- Improving heating so wall and ceiling surfaces are warmer and less prone to condensation even when humidity temporarily spikes
For the full picture on bathroom moisture sources and how ventilation fits into the overall fix, see Bathroom Humidity: Causes and Fixes.
What to Look for When Replacing a Bathroom Extractor Fan
When choosing a replacement fan, prioritise in this order:
- CFM/m³h rating – sized correctly for your bathroom (use the table above)
- Control type – humidity-sensing is best, timer is second, standard on/off is last resort
- Noise rating (sones) – 1.5 sones or below. Quieter fans get used more. A fan nobody turns on because it sounds like a hairdryer is doing nothing
- Duct size compatibility – most bathroom fans use 100mm or 125mm ducting. Check what your existing installation uses before buying
- IPX rating – fans installed within 600mm of a shower or bath need a minimum IP44 rating for safety
Related Articles
- Why Is Paint Peeling in My Bathroom? – what happens when ventilation fails over time
- Bathroom Humidity: Causes and Fixes – the full picture on moisture sources and ventilation
- How to Stop Condensation in the Bathroom – surface-specific fixes alongside better ventilation
- Mold on Bathroom Ceiling: Causes and What to Do – the end result of inadequate extraction
- Ideal Humidity Level for a Bathroom – how to measure whether your fan is actually working
Quick Summary
- A bathroom extractor fan reduces humidity effectively only if it’s correctly sized, venting outside, and running for 15–20 minutes after the shower – not just during it
- The toilet paper test checks whether the fan is pulling air; a hygrometer test 20 minutes after a shower tells you if it’s pulling enough
- Common failure reasons: undersized for the room, clogged with dust, duct venting into the ceiling cavity, or the room being too airtight for replacement air to enter
- Humidity-sensing fans are the most effective option – they respond to actual conditions and don’t rely on habit
- When replacing a fan, prioritise CFM rating, control type, and noise level (1.5 sones or below) in that order
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