It happens every autumn in plenty of Barcelona flats. One November morning you raise the blind and the bedroom window is misted up from top to bottom, with a trickle of water running down to the frame. A few weeks later a grey shadow appears in the corner of the outside wall, behind the wardrobe, and the clothes inside smell musty. According to the climate normals from AEMET, Spain's state meteorological agency, outdoor air in Barcelona sits at around 70% humidity all year round, and October is the most humid month, at 73%. Indoors, we add the water from showers, cooking and our own breathing.

You don't fix mould by cleaning: it's water in the wrong place. It appears where humidity in the air meets a cold surface, and that can be measured long before you can see it. A sensor won't dry a single wall, but it knows before you do when a corner is about to get condensation, and your home can do something about it: switch on the extractor fan, start the dehumidifier or let you know it's time to air the room. This is the guide to what to measure, where to put it and what's worth automating.

Why mould appears: humid air meeting a cold surface

Warm air can hold more water vapour than cold air. When the humid air in your home touches something colder — a window pane, a corner of the outside wall — it cools suddenly, can no longer hold all that water and leaves it on the surface. The temperature at which it starts to drip is called the dew point, and it depends on how humid the air is: with the home at 20 °C and 60% humidity, any surface below about 12 °C gets wet.

But mould doesn't need droplets: it's enough for a surface to spend many hours very damp, even if you can't see any water. The Spanish building code's supporting document on condensation (in Spanish) uses a maximum average monthly relative humidity of 80% at the surface as its criterion, and warns that in a home in a coastal area indoor humidity can be much higher than the default value assumed, so condensation can form very easily. With the home at 60%, a wall goes over that 80% as soon as it drops below about 15 °C. That's why mould always appears in the same places — the coldest spots in the home:

  • Window panes and frames. Especially old aluminium frames without a thermal break: the frame carries the cold from the street right inside.
  • Corners and pillars on the outside wall. Where two exterior walls meet or a pillar runs through, the wall is colder than in the middle.
  • The roller-shutter box. A hollow box above the window that's hardly ever insulated.
  • Behind wardrobes and headboards. Pushed against an exterior wall, they shield it from the room's heat and stop the air from circulating.
  • The bathroom ceiling. It takes all the steam from the shower at once.

The water comes from everyday life: the shower, the cooking pot, laundry drying indoors, the breathing of whoever sleeps with the door closed. A butane heater adds more, because burning the gas releases water vapour; and in an unventilated room it's also a carbon monoxide risk that opening the window for a while or running the extractor fan won't solve — that's what a connected carbon monoxide detector is for. And there's a more recent factor: new windows. Spain's Ministry of Health puts it like this in its indoor air quality guide (in Spanish): new windows in existing buildings seal much better and encourage the moisture produced in the home to build up.

What humidity is right, and why airing the room dries it even when it's humid outside

The same guide considers 30–60% humidity acceptable in autumn and winter. In practice, a flat between 40 and 60% is fine. What's worrying isn't the ten-minute spike after a shower, but air that stays above 60% for days, because then the cold spots spend many hours above that 80%.

Airing the room in winter dries it, even when it's 70% humid outside. It sounds like a contradiction, but it isn't. January air in Barcelona averages around 9 °C and 70%; when it comes indoors and warms up to 20 °C, it carries the same water but could now hold much more, so its relative humidity drops to around 35%. That dry air is what carries away the moisture from the night.

Heating doesn't remove water. Turning up the heating lowers the percentage on the sensor, because warm air can hold more vapour, but the water is still in your home: it only leaves when you air the room or when a dehumidifier condenses it into its tank. And in summer it's the other way round: outdoor air comes in warm and humid, airing no longer dries, and then it's the air conditioning or the dehumidifier that does the job.

Measure before you change anything: where the sensors go

A temperature and humidity sensor is small, runs on batteries and sticks on with adhesive. What makes the difference is where you put it.

One in every room you live in. Bedrooms, living room and bathroom, at mid-height and away from anything that skews the reading: the shower, the hob and the radiator. That way you see the real humidity of each room, not of a hallway.

One at the cold spot, or a calculation that does the job. The sensor that really tells you whether mould is coming is the one in the corner of the outside wall, behind the wardrobe or next to the window. Where you can't stick anything, such as on a window pane, it's calculated. In Home Assistant, the system we work with, the Mold Indicator estimates the humidity at the coldest spot from the room's temperature and humidity and the outdoor temperature, using a factor measured once at that spot. Its documentation puts the risk of mould above about 70% and condensation at 100%; we send an alert when that 70% persists, not for a one-off spike.

One outside, on the balcony. It provides the outdoor temperature that calculation needs. We use a dedicated sensor rather than the weather forecast, because the forecast comes over the internet and the calculation has to keep working even if your fibre connection goes down.

The history stays at home. A week of curves explains everything: the seven o'clock shower, the night with the door closed, laundry day. That's precisely why it's personal data — it shows when you shower, when you sleep and how many people are in each room — and it stays on the device in your home, not on a brand's server.

What your home does on its own

Once your home is measured, the automations that work are few and very specific.

The bathroom extractor fan: triggered by a rise, not a threshold

The typical mistake is switching on the extractor fan when humidity goes above a fixed figure, say 70%. But the baseline humidity changes with the seasons: there are weeks when that figure isn't reached even with the shower, and others when the bathroom is already there without anyone having showered. What gives a shower away is the rise. If humidity climbs eight or ten points in a few minutes, the fan starts; and it stops when humidity is back where it was or, if it doesn't get there, after a maximum time, so it doesn't run all night.

It's done with a small module behind the switch you already have. If the fan shares a cable with the light, they need separating, and many modules need a neutral wire in the box: it's almost always a job with no building work, and we check it during the visit. The switch still works by hand as always. If the bathroom has no extractor fan, fitting one is a separate job — it needs an air outlet — and we quote for it separately during the visit.

The dehumidifier, only when it's needed

A dehumidifier left on twenty-four hours a day wastes electricity. Connected to a smart plug with energy metering, it starts when the room goes above 60–65% and stops at around 50%. That gap between the two figures, the hysteresis, stops it switching on and off every few minutes, and it's also given a minimum running time so the compressor isn't punished. If that room's window is open, it waits.

There are two conditions. The appliance must be one that starts again by itself when the power comes back, set to continuous mode — many electronic models stay off — so the model is checked first. And the plug also tells you when the tank is full: if it's on, humidity is still high and consumption stays close to zero for several minutes, the tank is full and you get an alert. A simple drop in consumption isn't enough, because defrosting causes one too. If the appliance has a drainage hose, you don't even need that alert.

Air the room for ten minutes, when it's needed

Airing the room is the cheapest tool there is; the problem is remembering. In the bedroom, a CO₂ sensor solves it. It has to be an infrared (NDIR) one, because sensors that estimate CO₂ from other gases are no good for this, and it usually plugs in, via USB or the mains. The Ministry of Health guide recommends exactly that — watching CO₂ to know when ventilation is needed: after a night with the door closed it goes up, and your home tells you in the morning that it's time to open the window, based on CO₂ or humidity.

There's no need to leave the window open half the morning. According to the practical energy guide (in Spanish) from the IDAE — Spain's national energy agency — about ten minutes is enough to air a room completely. Meanwhile, a sensor on the window means that within a few minutes only that room's radiator stops, and it starts again when you close it: airing the room doesn't have to mean heating the street.

No freezing rooms

The room nobody uses is the prime candidate for mould, because it's cold and nobody airs it. Keeping it at around 15 °C usually does the job, and the sensor in that room confirms it; we explain how to split the temperature room by room in our guide to what temperature to set in each room.

In the in-between seasons, on those mild, humid days when it isn't cold enough to put the heating on, the split air conditioner can help in dehumidifying (dry) mode, and your home can switch it on when the humidity calls for it. But that mode cools the room, so not in winter, and it follows the same window rule: if the window is open, it stops.

What a sensor can't fix

It's worth being clear: home automation controls the humidity produced inside your home. Other kinds of damp are a different matter.

  • A stain that grows when it rains, or next to a pipe, isn't condensation. It's water getting in or escaping, and neither a hygrometer nor a water leak sensor, which only detects water on the floor, will pick it up. That's a job for a plumber or whoever repairs the façade.
  • Rising damp in a ground-floor flat, coming up from the floor, is a construction problem.
  • A serious thermal bridge or a very old window can be monitored and compensated for, but the real fix is insulating or replacing the window frames, and that's another trade.
  • Mould that's already there needs cleaning off — with authorised biocides, says the Ministry of Health guide — and the cause needs fixing, or it comes back. The same guide links damp and mould with more respiratory symptoms, allergies and asthma: if anyone at home has symptoms, see a doctor.

What you need (and how we quote for it)

Not much, and almost all of it without building work: battery-powered temperature and humidity sensors in the rooms and at the cold spots, the plug-in CO₂ sensor in the bedroom, a module behind the extractor fan switch and a metering smart plug for the dehumidifier. It's all integrated into the same system as your smart climate control, so the window, the radiator, the extractor fan and the dehumidifier all work together. It runs locally: only the alerts on your phone and checking things from outside the house need the internet.

We don't give a fixed figure here because every flat is different: how many rooms need monitoring, whether the extractor fan has its own circuit, which dehumidifier you have. We quote for it during the visit, with no obligation, as a one-off payment with no monthly fees. It comes with the same warranty as everything else we install: we guarantee our installation and configuration for 5 years. The devices carry their manufacturer's warranty (2–3 years), and we handle it for you.

Common questions

What humidity is normal at home in winter?

Spain's Ministry of Health indoor air quality guide considers 30–60% acceptable in autumn and winter, and in practice a flat between 40 and 60% is fine. What's worth watching isn't a one-off spike, which happens with every shower, but humidity that stays high for days — and above all what's happening at the cold wall: the middle of the room can read a reasonable 55% while the corner of the outside wall is close to 80%.

Does a dehumidifier get rid of mould?

No: it takes water out of the air, which is half the problem. Mould that's already there has to be cleaned off — the Ministry of Health guide talks about authorised biocides — and if it comes back, the cause is still there: a cold wall, ventilation that isn't enough, or a leak. What a dehumidifier run by your home does well is stop the conditions that let the mould grow from coming back, and do it only when needed instead of twenty-four hours a day.

Is it bad to dry laundry indoors?

It isn't bad, but all the water that comes out of your clothes ends up in the air at home, and a full load of washing is a lot of water. If you can, hang it in the galería — the enclosed back balcony — or a laundry area; if not, in a room with the door closed and the dehumidifier running, and never in the bedroom. With a sensor in that room you see it straight away: humidity rises as soon as the washing goes up, and the dehumidifier can start by itself until the clothes are dry.

Does it keep working if the internet goes down?

Yes. The automations — the extractor fan switching on, the dehumidifier stopping, the radiator pausing when the window opens — live on a small device inside your home and don't depend on any outside server. You only need the internet for alerts on your phone and to check things from outside the house. That's why the outdoor temperature for the mould calculation comes from a sensor on the balcony rather than the weather forecast: that way it keeps working even if your fibre connection is down.

Can I have it if I'm renting?

Yes. The sensors run on batteries and stick on with adhesive, the CO₂ sensor and the dehumidifier plug in, and the little device that coordinates it all plugs into the router: none of it leaves a mark or needs the landlord's permission, and it all comes with you to your next flat. Only the extractor fan module, which goes behind the switch, is worth discussing with your landlord first, because it involves the electrical installation — although that, too, comes out without leaving a trace.

In short

Mould every winter isn't bad luck: it's humid air meeting a cold surface and staying there for too many hours. Measuring room by room, keeping an eye on the cold spot, letting the extractor fan and the dehumidifier work only when they're needed and airing the room for ten minutes when it's time solves most cases, with no building work and no new habits. And anything that isn't condensation — a leak, rising damp — is spotted sooner and goes to whoever needs to fix it. You don't need to battle mould every winter: you just need a home that sees it coming.