Black spots along the silicone. A window running with water. A ceiling that goes patchy no matter how often it is wiped. Every one of those is the same story told three ways: warm wet air was made in a small room, the room could not get rid of it fast enough, and it condensed on the coldest thing available.
It is worth being clear about this because a lot of money gets spent on the wrong end of the problem. Mould-resistant paint, anti-mould grout and a bottle of spray treat the symptom. The moisture does not care. If the air cannot leave, it will find another cold surface, and you will be back in six months. The fix is always the same shape: make less moisture, remove what you make, and stop the surfaces being cold.
Why a bathroom generates so much of it
A hot shower puts a significant amount of water into the air in a few minutes, in a room that is often the smallest in the flat. That air holds the moisture only while it stays warm. The moment it touches something colder — a window reveal, an external wall, an uninsulated ceiling under a cold roof void, a cold tile on a north-facing wall — it drops it as liquid. That is not a defect. That is physics working correctly in a room with no exit.
Central London stock makes this worse in specific, identifiable ways. Solid brick walls in Victorian and Georgian buildings are cold on the inside face in winter and cannot easily be insulated internally without consequences. Single-glazed or old windows in listed and conservation stock have reveals that run with water. Mansion block bathrooms are frequently internal, with no window at all, which means every cubic metre of moist air has to be moved mechanically or it stays. Period conversions often have bathrooms that were never designed as bathrooms — a box room or a return with an extract fan added later and ducted wherever was easy.
What the regulations require
Bathrooms engage Building Regulations Part F, which covers ventilation, alongside Part G for sanitation and hot water and Part P for electrics. Part F is why a bathroom needs mechanical extraction, and why an internal bathroom needs it regardless of anybody's opinion about how it looks. The fan is not a nicety. Our bathroom Building Regulations guide sets out the wider picture.
The important thing to understand is that meeting Part F is about what actually gets extracted, not about what is written on the fan. Which brings us to the single biggest failure in London bathrooms.
The duct, not the fan, is where it goes wrong
Almost every bathroom we look at that has a mould problem has a fan. Usually a perfectly decent fan. The problem is what happens after the fan.
- Flexible ducting, sagging. Concertina flexible duct has enormous internal resistance compared with rigid duct, and when it is left slack it sags into a series of dips. Condensate collects in the dips. The airflow drops. Eventually the water finds its way back to the fan, or drips through the ceiling, and someone concludes the fan is broken. It is not. It is drowning.
- The run is too long, with too many bends. Every metre and every bend costs airflow. A fan rated for a short straight run does almost nothing on a long duct with four elbows. In a mansion block, where the only route out may be a long horizontal run to an external wall, this is the norm rather than the exception, and it means the fan has to be selected for the actual route rather than off a shelf.
- It is not connected to anything. More common than you would like. The fan pushes air into the ceiling void, or the loft, where it condenses. Now you have a damp ceiling and a rotting joist rather than a wet mirror.
- The terminal is wrong or blocked. A grille painted shut, a cowl full of nesting material, a backdraught shutter that stopped moving years ago. Easy to check, rarely checked.
- Uninsulated duct through a cold void. The warm wet air condenses inside the duct itself, and the duct becomes a pipe carrying water back to somewhere it should not be.
The fix for most of this is rigid or semi-rigid duct, the shortest sensible route, supported so it cannot sag, insulated where it crosses cold space, with a proper terminal. That is unglamorous and it is the entire job. Getting the route right is a first-fix decision — it is far easier to do before the ceiling is closed up, which is why it belongs in the design of a bathroom refurbishment rather than being added afterwards.
The fan itself
Once the duct is right, the fan is a straightforward choice. Things worth knowing:
Continuous-running fans tick over at a low rate all the time and boost when triggered. They are quiet, they deal with moisture before it becomes visible, and in a small flat they generally outperform a fan that comes on for fifteen minutes twice a day. They are usually the better answer in an internal bathroom.
Humidity sensing beats a timer linked to the light, because the light gets turned off and the moisture does not know that. A humidistat runs the fan until the air is actually dry.
Inline fans sit in the ceiling void rather than on the wall, which puts the motor noise away from the room and lets you use a more powerful unit on a longer run. In a quiet bathroom in a quiet block, noise is the reason people switch fans off, and a fan that gets switched off is decoration.
Bathroom zones matter. Where a fan can go, and what it must be rated to, is governed by the zoning rules in BS 7671, and the electrical work falls under Part P. It needs to be on a properly protected circuit and certified. Our electrical work starts from £80 and is carried out to BS 7671.
And the detail everyone forgets: make-up air. A fan cannot extract from a sealed box. If the door has a tight seal and no undercut, the fan is fighting the room. A 10mm gap under the door does more for a bathroom than a bigger motor.
The other half: stop the surfaces being cold
Extraction removes moisture. It does not warm a wall. If a bathroom has a genuinely cold surface, condensation will form on it faster than a fan can clear the room, and the mould will track that cold surface exactly — which is a useful diagnostic. If the mould is in one corner, on one reveal, or along the top of one wall, you are looking at a cold spot, not a ventilation failure. Cold bridging at a window reveal, an external corner, or a ceiling under an unheated void will do this reliably.
Heating the room helps, and a heated floor helps more than people expect, because it warms the whole surface rather than one panel — see our underfloor heating guide. A heated towel rail dries towels, which is itself a moisture source removed. In listed and conservation stock, insulating an external wall internally is not a free decision: it affects fabric, it may need consent, and done badly it moves the dew point into the wall rather than removing it. That is a conversation to have properly rather than a job to do enthusiastically — see listed building consent.
Landlords, agents and the mould conversation
Mould in a let flat becomes a dispute quickly, and the dispute is usually about blame: the tenant is told to open a window, the tenant says the fan does not work, and nobody measures anything. The productive move is to establish whether the extraction functions at all. If the duct is sagging into the void, no amount of tenant behaviour is going to fix that room, and the conversation should never have been about drying laundry.
Proper extraction, sensible heating and a working fan with a humidistat remove the argument. It is also cheaper than repeatedly redecorating and far cheaper than a dispute. Our maintenance work covers exactly this sort of thing, and there is more for agents on our estate agents page and in our guide to refurbishing between tenants.
Diagnosing your own bathroom
- Hold a sheet of tissue to the fan grille. If it does not hold, the fan is moving almost nothing regardless of what it sounds like.
- Find where the duct terminates. Go outside and look. If you cannot find a terminal, that is your answer.
- Look at where the mould is. Even coverage suggests a ventilation problem. A stripe, a corner or one reveal suggests a cold surface.
- Check the door undercut.
- Check whether the fan runs on after the light, and whether anybody in the household turns it off.
- Check the terminal for paint, nests and a stuck shutter.
How we fix it
We look at the route before we look at the fan, because the route is usually the fault. If it is a cold surface rather than an airflow problem, we will say so rather than sell you a bigger unit. Fixed quote against a defined scope before any work starts — plumbing and electrical from £80, bathroom refurbishment from £2,800, and wider refurbishment per quotation. Send photographs of the mould, the fan and, if you can find it, where the duct comes out.