Condensation & damp explained
Mould in the bathroom, even though there is an extractor fan
A fan running is not the same as a fan working. The rate printed on the box is measured on a bench with nothing attached to it — put that same fan on four metres of flexible duct with two bends in it and it can be moving less than half. Almost nobody ever checks, which is why so many bathrooms have a fan, a timer, and mould.
The number on the box is what the fan achieves with nothing connected to it. That is a legitimate way to test a fan and a useless way to describe an installation, because every metre of duct, every bend, every grille and every insect screen takes something off it.
A cheap fan on a short straight run through a wall may well deliver close to its rating. The same fan on four metres of concertinaed flexible duct, with a sag in the middle and two tight bends, plugged into a louvre with a mesh behind it, can be moving less than half — and it sounds exactly the same while it does it. Noise is not airflow. That is the single most common misunderstanding in this whole subject: people conclude the fan is fine because they can hear it working hard, when working hard against a restriction is precisely the problem.
The only figure that means anything is the one taken at the grille after installation, with a meter. It takes a few minutes, and in the great majority of bathrooms we are called to, nobody has ever done it.
Eight things that go wrong, in the order we find them
You can check most of these yourself. The first one is worth checking today, whatever else you do.
1. It discharges into the loft
Far more common than anybody expects, because from inside the bathroom it looks identical to a proper installation. The duct goes up through the ceiling and simply stops in the roof space, or is connected to a terminal that was never actually fitted. Every shower then puts warm wet air straight into a cold loft, where it condenses on the underside of the roof and soaks the insulation. Over a few winters that is genuine damage to timber. Put your head through the loft hatch and follow the duct with a torch. If it does not end at a terminal in the roof, in the wall or in the soffit, that is your answer.
2. Flexible duct, sagging
Concertina flexi has enormous resistance compared with smooth-walled duct, and worse, it sags between fixings. Every sag is a low point, every low point collects the water that condenses inside the duct, and a duct with water standing in it is partially blocked. Rigid or semi-rigid duct, run as short and as straight as the building allows and supported properly, is most of the difference between a fan that works and one that does not.
3. No backdraught shutter, or a stuck one
The shutter at the outside end stops wind blowing back down the duct. Left off, the fan spends windy days fighting the weather, and cold outside air runs back into the bathroom between uses — which chills the room and makes the condensation worse. Stuck shut, and the fan is moving nothing at all while sounding completely normal.
4. It runs for four minutes
Wired to the light switch with a short overrun, the fan stops long before the room has cleared. A hot shower puts something like 1.7 litres of water into a small room in ten minutes, and most of it is still there when the timer clicks off — it then condenses quietly on the ceiling and the walls overnight. A longer overrun, or a humidity-sensing fan that runs until the room is actually dry, changes the outcome more than upgrading the fan does.
5. Nowhere for replacement air to come in
A fan cannot pull air out of a sealed box. If the bathroom door has a good draught excluder and a thick carpet under it, the fan is pulling against a vacuum and moving a fraction of what it should. There needs to be a gap under the door — around 10mm is the usual figure — and the rest of the house needs its own way of letting air in, which is what trickle vents are for.
6. It is the wrong fan for the room
A standard 100mm axial fan is designed for a short duct run straight out through a wall. Ask it to push air along several metres with bends and it will not, whatever the box says. That run wants a centrifugal or an inline fan, which is a different product at a different price — and it is the substitution that quietly turns a two-hour job into most of a day.
7. There is no outside wall behind the room
Very common in conversion flats and in houses where a bathroom was added off a landing. It rules out the simple job entirely: what it needs is an inline fan in the ceiling void with a duct run out to the nearest external face. That is the biggest single price difference in this work, and almost nobody is shown it before the quote arrives.
8. Nothing is wrong with the fan at all
Sometimes the extract is fine and the moisture is coming from elsewhere in the house — washing dried on the landing, an unvented tumble dryer, a kitchen with a recirculating hood. Bathrooms are cold and tiled, so they show it first even when they are not causing it. This is why we measure the whole dwelling rather than just the room with the mould in it.
What the rate is supposed to be
Approved Document F sets minimum extract rates for wet rooms. For intermittent extract — a fan that runs when it is needed rather than continuously — a bathroom is 15 litres per second, a WC is 6, a utility room is 30, and a kitchen is 60 unless there is a cooker hood ducted to the outside, in which case it is 30.
Two things about that kitchen figure catch people out, and both are worth knowing. It is decided by whether the hood ducts outside, not by where the hood is: a recirculating hood sits directly over the hob, filters the grease and the smell, and puts every bit of the steam back into the room, so that kitchen needs 60. And an extract fan is not a cooker hood — a wall fan on its own in a kitchen is 60 as well.
These are minimums rather than targets, and they are the rate delivered at the terminal, not the rate on the packaging. A fan sold as 15 l/s is only a 15 l/s installation if somebody has put a meter on it and read 15.
A test you can do in ten seconds, and its honest limits. Hold a single sheet of toilet paper flat against the grille with the fan running. If it does not hold up on its own, something is badly wrong and it is worth a call. If it does hold, that tells you the fan moves some air — it cannot tell you whether it moves 15 litres a second or 6, and the difference between those two is the difference between a compliant bathroom and a mouldy one. That part needs a meter.
Start with what you can actually see
Most people arrive here with a symptom rather than a diagnosis, and the symptom is usually enough to narrow it down. Each of these explains what is happening and what genuinely fixes it — no form, no email address, just the answer.
My windows are wet every morning
Water running down the glass from October to March, and pooling on the sill. What it means, when it is ordinary and when it is not — and why the glass is the only surface in the room honest enough to show you.
Read the explanationThe mould keeps coming back
You cleaned it, you sprayed it, you painted over it, and six weeks later it returned in the same shape. Why that shape is the diagnosis, why anti-mould paint runs out, and how to tell which of the three damps you have.
Read the explanationThere is mould in the bathroom, and a fan
A fan running is not a fan working. The eight things that go wrong — starting with the one you can check yourself today with a torch and a loft hatch, which is the worst of them and the most common.
Read the explanationIt started after we had work done
New windows, insulation, a loft conversion, a new kitchen. Not a coincidence and usually not a botched job — every trade did their own work correctly, and nobody was responsible for the air.
Read the explanationQuestions people ask about bathroom fans
The rough test is to hold a single sheet of toilet paper flat against the grille with the fan running. If it will not hold up on its own, something is clearly wrong. Be aware of what that test cannot tell you though: it proves the fan moves some air, not that it moves the right amount. A bathroom needs 15 litres a second under Approved Document F, and a fan pulling 6 will hold a sheet of paper up perfectly happily while leaving your bathroom wet. The other useful check costs nothing and finds the worst fault there is — put your head through the loft hatch with a torch and follow the duct. If it does not end at a terminal in the roof, a wall or the soffit, it is discharging into your loft.
Considerably longer than most of them do. A hot shower puts roughly 1.7 litres of water into a small room in about ten minutes, and a fan wired to the light with a short overrun typically stops while most of that is still in the air — where it then condenses on the ceiling and the walls overnight. Fifteen to twenty minutes of overrun is a reasonable starting point, and a humidity-sensing fan that simply runs until the room is genuinely dry is better again, because it responds to a long bath or three people showering in a row rather than to a fixed timer. Extending the overrun is usually a cheaper improvement than replacing the fan, and it often makes more difference.
Yes, and a surprising number do not. Ducting a bathroom fan into a loft or a roof void rather than through a proper terminal takes every shower’s worth of warm wet air and delivers it into a cold space, where it condenses on the underside of the roof and soaks the insulation. From inside the bathroom it looks completely normal, which is why it goes unnoticed for years, and over a few winters it does real damage to roof timber and ruins the performance of the insulation. It is the first thing we look for and it is worth ten minutes of your own time with a torch.
Approved Document F sets 15 litres per second for a bathroom on intermittent extract, 6 for a WC, 30 for a utility room, and 60 for a kitchen unless there is a cooker hood ducted to the outside, in which case 30. The important part is that this is the rate delivered at the grille once it is installed, not the rate printed on the box — the box figure is measured with nothing attached. A fan sold as 15 l/s on four metres of sagging flexible duct with two bends may be moving half of that. So the honest answer to "what size" is that it depends on the duct run as much as on the room, and the only way to know you have it is to measure it afterwards.
Not for Part F purposes in a room being newly fitted out, and not in practice either. An openable window is not extract ventilation: it works only when somebody opens it, it is the last thing anyone does in a cold bathroom in February, and it does nothing at all for the twenty minutes after a shower when most of the water is still airborne. Where a window genuinely helps is as background ventilation on a trickle vent, which is a different job that runs continuously and costs almost nothing in heat.
Because the backdraught shutter is missing, stuck open, or was never fitted. The shutter sits at the outside end and closes when the fan is off, and without one the duct is simply an open hole in your wall — so on a windy day outside air runs back down it into the bathroom. That chills the room, which lowers the surface temperatures, which makes the condensation worse: you end up with a device installed to reduce damp actively contributing to it. It is a small component and usually a quick fix, and it is one of the most common things we put right.
Yes, and it matters more than people think. A fan can only push air out if the same volume of air can get in behind it, and a well-sealed door with a thick carpet under it leaves the fan pulling against a closed room and moving a fraction of what it should. Around 10mm of clear gap under the door is the usual figure. The same logic applies to the whole house: extract only works if the building has a controlled way of letting replacement air in, which is what trickle vents and other background ventilators are for. Sealing everything up and then fitting a bigger fan is the commonest way to spend money and change nothing.
The ducting is within reach of a competent DIYer. The electrical side is a different matter: a bathroom is a special location with defined zones that dictate what may be installed where and to what protection, and the work needs to be done and certified by somebody competent. The other reason to think twice is that the parts people get wrong are exactly the parts that are hard to inspect afterwards — the length and route of the duct, whether it sags, whether the shutter operates, and whether it genuinely terminates outside. Those decide whether the fan works, and none of them shows from inside the room. If you do fit one yourself, get the airflow measured at the grille when it is done.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, held by this business. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.