Grey sealant along the bath. A ceiling corner that speckles black no matter how often you scrub it. A mirror you can’t see yourself in for twenty minutes after a shower.
Nine times out of ten, when someone in Exeter calls us about mould in a bathroom, they’ve already decided what’s causing it. Rising damp. A leak somewhere in the wall. Something structural and expensive. And nine times out of ten they’re wrong.
What they’ve actually got is condensation. Water that was already floating around the room, landing on something cold enough to turn it back into a liquid. It’s a physics problem, not a plumbing one, and once you understand which two dials you’re allowed to turn, the fix becomes obvious. Skip that step, paint over the black, and you’ll be doing it again by the second winter. I’ve seen the same ceiling three times.
The bit nobody explains properly
Warm air carries more water than cold air. That’s the whole principle.
Run a hot shower in a small room and you’ve filled it with warm, saturated air in about ninety seconds. That air then goes looking for somewhere to dump its load, and it finds the coldest surface available. Below a certain temperature, called the dew point, the air simply can’t hold on any longer and the water comes out.
Which surface goes first is entirely predictable. Single glazing is usually the coldest thing in the room, so the window streams. Then an external wall, particularly a solid one with nothing in the middle. Then the corner where two external walls meet, which is the coldest spot in the whole house and, funnily enough, exactly where the mould always starts.
So you’ve got two levers. Get the moist air out before it can settle anywhere. Or keep the cold surfaces warm enough that the air never hits its dew point against them. That’s it. Every genuine fix is one of those two. Everything that fails – the wiping, the repainting, the fortnightly bleach ritual – does neither.
Devon makes this harder than it needs to be
There’s a version of this article that would work anywhere in the country. This isn’t quite that, because two local things stack up against us.
First, the weather. The South West is wet and, more importantly, mild. Cold air is dry air, which is why a proper frost sucks the moisture out of everything. What we get instead is ten degrees and drizzle for most of January, meaning the air coming in through your window already has a fair bit of water in it before you’ve even turned the shower on. Less headroom, faster saturation.
Second, the houses. A big chunk of the bathrooms we work in around Heavitree, St Leonard’s, Newtown and St Thomas sit in Victorian and Edwardian terraces with solid walls. No cavity, no insulation, and in winter those walls are properly cold to the touch. Head out toward Crediton and the Exe valley and you’ll find cob, which behaves differently again and needs handling with real care. Older houses also tend to have either no extractor or one that’s essentially decorative.
That isn’t an argument against old houses. I’d take a Victorian terrace over a new-build box most days. It just means ventilation and surface temperature have to be decisions you make on purpose, rather than things you get away with.
Your fan is almost certainly too weak
If you fix one thing, fix this. And be aware that most people who think they’ve already fixed it haven’t.
The numbers live in Approved Document F, the building regs guidance on ventilation, and they’re specific. A bathroom needs 15 litres per second of extract if the fan runs intermittently, or 8 litres per second if it runs continuously. It should sit as high as you can practically get it and no more than 400mm below the ceiling, because warm wet air rises and that’s where it pools. No openable window? Then it needs an overrun timer running at least 15 minutes after you’ve left the room. And a room with no external wall at all steps up to four air changes an hour.
Now think about the fan in your bathroom. Wired to the light switch, no overrun, mounted wherever the previous hole happened to be, roughly the diameter of a tin of beans. It ticks almost none of those boxes. But because something is technically whirring up there, nobody ever suspects it. Meanwhile it shifts a fraction of the air it should and dies the second you flick the light off, leaving a room full of steam to condense quietly overnight.
The other one we find a lot is a decent fan ducted straight into the loft. That doesn’t fix anything. It moves the problem into your roof timbers, where it does far more damage and costs considerably more to sort out. Ducting has to reach outside air, and it needs to get there without long sagging runs where condensate can gather and sit. Worth actually testing the flow rate once it’s in, too, rather than fitting it and hoping.
Warm walls don’t grow mould
Ventilation gets all the attention and the second lever gets forgotten.
Simple logic: if that cold corner never drops below the dew point of the air around it, nothing can condense there. Full stop. The most effective way to achieve that in a bathroom is underfloor heating, because it warms the room evenly and keeps it warm between uses. Surfaces don’t get the chance to go cold and clammy overnight. A heated towel rail is nice, and it dries your towels, but it warms a small area around itself and not much else. Underfloor lifts the baseline temperature of the whole room, which is what keeps the corners out of trouble.
Solid-walled houses sometimes justify internal insulation on the worst external wall. Sometimes. In a breathable construction like cob, the wrong insulation will trap moisture inside the wall itself, and you’ve swapped a visible problem for an invisible one that’s eating your building. That’s a genuine trap, and it’s why we’d never just batten and board a cob wall in a wet room without thinking hard about where the moisture is meant to go.
Replacing single glazing helps enormously, since that window is usually the first thing to stream. In a listed building or a conservation area you may not be allowed to, so check before you budget for it.
The rule that catches out even some luxury bathroom fitters in Exeter
This one is genuinely counterintuitive and it matters a lot in Exeter’s older housing.
Approved Document F says that if a room had no ventilation system to begin with, refurbishing it doesn’t oblige you to install one, though you may need to add ventilation if the work would otherwise leave things worse than before. It also confirms that swapping an existing fan for a new one on the existing cabling isn’t notifiable to building control.
Why should you care? Because it means a bathroom refit in a listed house in Topsham or Sidmouth doesn’t automatically force you to hack a new vent through a protected elevation to satisfy the regs. In a building where any external change needs consent, that’s a real relief.
To be clear, this isn’t permission to ignore ventilation. A period house is precisely where you want extraction working properly. It just means you’ve got room to solve it sympathetically. If an installer tells you the rules demand a new hole through a historic wall, they haven’t read them.
While we’re here, trickle vents deserve a mention. A small vent in the window frame keeps a trickle of air moving through the room all day, so it never sits stagnant between uses. Cheap, unglamorous, and it does more than you’d think.
Things people try that don’t work
The mould-resistant paint aisle exists because we keep buying from it. There’s a biocide in the tin that slows regrowth on that surface for a while, but it does nothing about the moisture or the cold. You get a year or two, and in the meantime you’ve painted over the early warning signs.
Wiping the window down each morning isn’t nothing, but you’ve signed up to a daily chore forever, and it does nothing for the corner hidden behind the towel rail.
Dehumidifiers do genuinely pull water out of the air, and in one stubborn room, running one can take the edge off. It’s still a machine you have to remember to switch on and empty, and it’s catching the water after it’s already in the room rather than removing it at source.
Then there’s the bleach cycle, which is the most common of the lot. Scrub, kill, admire, repeat in four months. It removes what you can see and leaves every single cause untouched. The NHS is clear that damp and mould can affect your health, especially for anyone with asthma or another respiratory condition, so it’s not purely about how the room looks. It’s worth fixing once properly instead of managing it forever.
Fix it while the walls are open
If you’re refitting a bathroom anyway, this is the cheapest moment you’ll ever get to solve condensation permanently. The fan, the ducting, the heating and every surface are all being disturbed regardless. Doing it then costs a fraction of coming back to each piece separately afterwards.
A fan sized to the room and ducted properly to outside. Underfloor heating holding the surfaces above dew point. Grout and sealant specified to cope with the moisture that does form. And, in the right property, a proper look at the coldest wall.
That’s the argument for treating ventilation as part of the design from day one, rather than a fan somebody picks in the final week.
Common questions
Why does the mould come back every time I clean it?
Because cleaning deals with the growth, not the two conditions that produce it. Until the air is being removed properly and the cold corner is warmer, everything the mould needs is still there. It’ll usually be back within a winter.
Is a good extractor enough on its own?
In a modern, well-insulated bathroom, often yes. In a solid-walled period house it frequently isn’t, because the walls are cold enough to condense water faster than the fan can clear it. There you want extraction plus some background warmth.
Can’t I just open the window?
It helps, but it depends on you remembering and on the weather cooperating. On a still, damp Devon morning an open window barely moves any air at all. A correctly sized fan with an overrun timer works every time, which is why the regulations are built around mechanical extract rather than a window.
My house is listed. Am I stuck?
No. Approved Document F doesn’t require you to add a new external vent when refurbishing a room that never had one, so you’re not obliged to cut through a protected wall. Reusing existing openings and internally sited systems are both usually options. Use someone who’s worked on listed buildings before.
Will underfloor heating stop it by itself?
It makes a big difference, because it keeps the room and its surfaces warmer between uses. It works best alongside good extraction rather than instead of it. Warm surfaces plus air that’s actually leaving the room is the combination that keeps a bathroom dry.