I used to assume extractor fans were the blunt instrument of bathroom care. Switch on the fan. Wait ten minutes. Problem solved. Not quite. In hundreds of homes I have poked around the damp and the steamed mirrors tell a different story. Bathroom fans are supposed to evacuate moist air but in practice they often do less than you expect. The reason is simple and messy at the same time. Ventilation is a system not a widget and most of the time we treat it like a switch.
Fan power is only part of the story
Most people look at the fan and ask Is it spinning? and if it is they breathe a small sigh of relief. But airflow numbers on a product box do not translate cleanly into performance inside a strangely shaped, often cold, poorly insulated box called a bathroom. Duct length, bends in the pipe, where the air actually exits the building, and whether the fan is fighting against closed trickle vents or a sealed window all matter far more than the nominal extraction rate.
Installation choices that quietly sabotage extraction
Have you ever had a fan that vents into the loft rather than outside? That one error turns moisture into a slow leak into timber and insulation where it sits and causes trouble for months. Equally common is underpowered inline fans or ceiling units too far from the steam source. Location matters. A fan above the sink may look neat but it will miss the plume of steam rising from the shower. The industry talks about litres per second but no manual measures how often the installer chose the wrong place to put it.
Behaviour trumps hardware
The people using the bathroom are part of the ventilation system. Leave the door open, and moist air migrates into the rest of the house. Close the door, run the fan for a minute and switch it off immediately, and you have helped exactly nothing. Fans need time to clear the saturated layer of air; running them for a short interval simply moves damp air around without replacing it with dry outside air in any meaningful way.
And then there is the cost argument that haunts tenants and homeowners alike. In the current climate people rightly worry about running anything that adds to their bills. The result is a tendency to underuse the very systems that would prevent future damage and expense. There is an odd short termist logic at work here. Save a few pence today, pay for repairs and repainting later. Not a crime but hardly admirable planning.
Why humidity sensors are not a panacea
Humidity controlled fans sound brilliant on paper. They switch themselves on when humidity spikes and turn off later. Yet they are only as good as their location and calibration. Place a sensor in a draughty corner and the fan never sees the real steam. Set the threshold too high and it triggers only when condensation has already formed on surfaces. The idea is right but the practice often falls short because of small human choices and installation quirks.
Building fabric and heat levels change the game
Bathrooms in older stone or solid wall houses are cold beasts. Even if the fan works perfectly, moist air meeting cold plaster will turn into condensation. Insulation, heating strategy and surface temperatures are part of the moisture equation. Ventilation without attention to thermal behaviour is like trying to catch rain with a sieve. You can direct it but you cannot stop the basic physics unless you warm the surfaces or reduce the moisture source.
Our research is done in real homes so we understand how buildings perform in the real world not just in labs and models. Solid walled homes are among the least energy efficient and condensation damp and mould problems can be common.
Professor David Glew Director of the Leeds Sustainability Institute Leeds Beckett University
Real homes are stubbornly complicated
There is a temptation in shopping-centre wisdom to reduce solutions to single products. Buy a stronger fan and life will be bliss. That fails because houses are not test rigs. People have habits. Cupboards are pressed against external walls. Windows are double glazed with sealed trickle vents closed. Drying clothes indoors is common where outdoor drying is impractical. Those realities interact with ventilation to create stubborn pockets of moisture that a lone fan cannot chase away.
A failure of system thinking
When policy and guidance talk about extractor fans they usually place them in a list alongside opening windows and keeping doors shut. Those are sensible individual tips but they miss the emergent property of a whole-house ventilation strategy. Mechanical extraction must be thought of as part of a network. Where does the fan exhaust to. Is there a balanced intake of replacement air. Are walls too cold for the simple exchange to avoid condensation. Without a holistic view the fan becomes a token device that reassures without resolving.
Here is an uncomfortable opinion. The market sold us cheap fans for decades and everyone pretends they are a long term fix. I think that complacency allowed poor workmanship and sloppy design to persist. When the cheap fan stops being a cheap fix is when the conversation shifts to whole house ventilation strategies and insulation upgrades. Those are harder sells but they work.
Practical steps that alter outcomes
Start with how the fan vents. If it does not go outside that is the first problem to solve. Next place the fan where steam is produced rather than where a tile fitter leaves the most spare space. Think about overrun timers and humidity sensors but test them in the real spot they will operate in. If the bathroom is cold remember that the fan is only part of the strategy; warming surfaces or preventing cold spots is equally important. Finally, change the behavior. Run the fan long enough. Close the door while showering and leave it running afterwards. These are obvious but surprisingly rare instructions in real life.
Some possibilities I wish more people would consider
Don’t assume bigger is better. Excessive extraction can create negative pressure which draws moist air from elsewhere into the bathroom. Consider mechanical ventilation with heat recovery in homes where condensation is chronic. It costs more up front but it treats replacement airflow as part of the system rather than an afterthought.
I also think landlords should be more aggressive about commissioning post-installation checks. An extractor fan installed during a refurbishment should be proven to actually remove steam at the shower head not merely show up in an inventory list. Tenants often bear the consequences of system design flaws they never agreed to.
Conclusions and what I stand by
Bathroom fans do a job but they are not miracle workers. The moment you treat them as the whole solution is the moment damp quietly creeps back. Address routing and discharge first. Treat the bathroom as a thermal and ventilation system. And remember that the user is part of the system. You will get better outcomes by combining modest engineering sense with a little behavioural discipline. I am impatient with confident quick fixes. Moisture reduction rewards the patient and the methodical more than the flashy and the cheap.
Summary Table
| Issue | Why it matters | What to do |
| Exhaust routing | Venting into lofts or internal voids spreads moisture | Ensure fan exhausts directly outside |
| Fan location | Far from steam source reduces effectiveness | Position near shower or bath |
| Building fabric | Cold surfaces cause condensation despite extraction | Insulate or raise surface temperatures where possible |
| User behaviour | Short runtimes and open doors defeat extraction | Run fan long enough and keep doors shut when showering |
| System thinking | Fan alone cannot replace balanced ventilation | Consider whole house strategies for chronic problems |
FAQ
Why does my bathroom still steam up if the extractor fan is on?
Because extraction is only one component of moisture control. Steam rises from the shower and hits cold tiles and glass where it condenses. If the fan is not placed close enough to the steam plume or the air cannot escape the building efficiently the moisture will stay. Also short runtime or low power relative to duct losses reduces the effective extraction. The fan needs clear routing to outside and enough time to clear the saturated layer of air after bathing.
Can a more powerful fan fix persistent condensation?
Sometimes but not always. Bigger fans help if the existing unit is undersized and if the ducting and exit are correctly designed. However oversized or poorly balanced extraction can create negative pressure that drags damp air from elsewhere. A better strategy is to resolve routing issues and examine thermal behaviour of walls and windows as well as user habits before simply upgrading motor size.
Are humidity sensors worth having?
They can be helpful especially in buildings where usage patterns vary widely. The caveat is placement and calibration. If the sensor is in a draughty corner or placed away from the steam source it will not trigger as intended. Also thresholds set too high delay action until after condensation has happened. Use them but validate their operation in the real room.
Is venting into the loft acceptable?
No. Venting into lofts or internal voids traps moisture where it causes slow damage to insulation, timber and decking. Building codes and best practice require vents to discharge outside. If you suspect your fan vents into a loft have the routing inspected and corrected by a competent installer.
When should I consider whole house ventilation?
When condensation or mould repeats across rooms despite sensible extraction and behaviour changes. Whole house mechanical ventilation with heat recovery addresses replacement airflow and reduces energy penalties from opening windows. It is a larger investment but often the only durable answer in cold damp properties where isolated fans cannot keep up.
