Condensation in a Spanish flat: where the water comes from

Two numbers decide whether a wall gets wet

Air always carries some water as invisible vapour, and how much it can carry depends on its temperature: warm air holds more, cold air holds less. Chill a parcel of air far enough and it reaches the temperature at which it can no longer hold what it was carrying. That temperature is its dew point, and the surplus has to go somewhere.

Indoors, the thing doing the chilling is a surface. The thin film of air touching a cold wall, a window reveal or the back of a wardrobe is cooled almost instantly to something close to that surface's own temperature. If the surface sits at or below the dew point of the room's air, the film gives up its surplus as liquid water, and the surface is where it lands.

That is the whole mechanism, and two consequences follow from it.

Nothing came in from outside. The vapour was already circulating in the flat. A wall that condenses is not letting water through; it is collecting water the household produced. Sealing or repainting its outer face changes nothing, and it will not dry out on its own while the same conditions keep repeating.

It is a threshold, not a slow decline. A fortnight can pass with nothing visible, and then three cold nights in a row put dark marks in half the flat. Mild weeks with no symptom do not mean the problem is solved; they mean the two numbers have not met recently.

Three levers, and why fixing one of them is often not enough

Every measure ever recommended against condensation moves one of three things, and only three. They carry equal weight, because water appears when the combination crosses the threshold — not when any single one of them is bad.

Moisture produced indoors sets how much water the air is carrying, and so pushes the dew point up. This is the lever of showers, cooking, laundry and how many people are in the flat.

Air renewal takes moist indoor air out and brings outdoor air in, pulling the dew point back down. In a mild, humid Mediterranean winter that outdoor air feels damp, and it is — but it is also colder, so each cubic metre of it carries less water than the air indoors. Bringing it in and letting it warm up is what dries a flat out. On a still, wet, mild day the same gesture achieves noticeably less, which is worth knowing before concluding that airing does nothing here.

Surface temperature sets the other number: how cold the coldest surface is allowed to get. That means heating — how evenly, more than how fiercely — insulation, and whether the heat in a room can physically reach the wall.

The reason careful households still get damp walls is that each lever has a ceiling:

• Laundry no longer dries indoors, but the spare bedroom stays shut and unheated all winter. Moisture down, surface temperature untouched, water in the same corner as before.
• The flat is aired conscientiously twice a day, while the bathroom door sits flush to the floor and the extract pulls nothing. Airing is a top-up; the vapour from each shower never actually left.
• The heating runs hard in the living room and the bedrooms are left at whatever temperature they reach. The warm room carries its vapour comfortably and passes it along; the cold rooms are where it gets set down.

So the useful question is not "what should I do" but "which of the three is weakest here". Where vapour has no way out of the flat, no amount of habit-tuning fully compensates. Where the air is renewed well but one corner is very cold, ventilating harder will not warm that corner.

Where the water comes from on an ordinary day

Nothing has to be faulty for a home to put a lot of moisture into its own air. What separates the sources is how much, how fast, and where.

• People. Breathing and perspiration release vapour continuously, including while asleep. It is the baseline, it rises with the number of occupants, and it cannot be switched off.
• Showers and baths. A large amount in a few minutes, released in the smallest room in the flat, usually behind a closed door.
• Cooking. Boiling and steaming release far more vapour than roasting in a closed oven. A lid on the pan is the cheapest intervention on this page.
• Drying laundry indoors. Every drop of water the machine left in the clothes ends up in the air of that room. All of it, slowly, with nothing visible while it happens. On a rack in a closed, unheated bedroom, all three levers work against you at once.
• Small, constant additions. Mopping floors, a tumble dryer that discharges into the room instead of outside, a group of houseplants, an aquarium.

None of this is a list of things to stop doing. Showering, cooking and breathing are not negotiable. What is negotiable is where the vapour is released and whether extraction is running at that moment. Taking vapour out at the point it is produced is always cheaper than taking it out of the whole flat afterwards, which is why DB-HS3 — the section of Spain's technical building code, the Código Técnico de la Edificación, that covers indoor air quality — asks for a specific extraction provision in the kitchen for cooking fumes, on top of the dwelling's general ventilation.

Why the patch appears behind the wardrobe

Vapour spreads into whatever air it can reach. A closed door slows it down; it does not hold it. The steam from a ten-minute shower is, a few hours later, distributed along the hallway and into the bedrooms. Where it finally settles is decided by temperature, not by where it started.

The code assumes a direction of travel: outdoor air is admitted in the dry rooms — bedrooms and living rooms — crosses the flat through the hallway, and is extracted from the wet rooms, bathroom and kitchen, to outside. The gaps and grilles that let air pass between rooms, the aberturas de paso on a Spanish plan, are part of that route rather than an afterthought. When it works, vapour leaves close to where it was made. When a link is broken, the vapour takes the alternative route, which is the rest of the flat.

Two things make that route harder to keep open in a Barcelona flat. Where a bathroom or kitchen gives onto an interior light well — the patio de luces, pati de llum in Catalan, a narrow internal shaft that several flats open onto — the air available is the shaft's own: stiller and often damper than street air, so the same opened window renews less. And where an extract discharges into an old shared vertical duct, a shunt serving stacked bathrooms up the building, the draw may not be what it was designed to be. That duct belongs to the building rather than the flat: if it is suspected of being blocked, that is a matter for the community of owners and a qualified professional, never something to open up from inside your own bathroom.

Once vapour is moving freely, the cold surfaces decide the rest. They are not evenly distributed, which is why the same marks come back in the same places:

• Corners where two external walls meet. They lose heat through two faces at once, and the room's warmed air circulates past them poorly. Geometrically they are the coldest point of the room, and they go first.
• The external wall behind a wardrobe or a sofa pushed up against it. The furniture insulates from the wrong side, keeping the room's heat off the wall and trapping a pocket of still, cold air behind itself. A musty smell inside the wardrobe often arrives before any mark on the plaster.
• Window reveals, jambs and the lintel. The wall thins around the opening, so the inner face there runs colder than the rest of the wall. The glass has tell-tale patterns of its own and is treated separately.
• Rooms kept closed and unheated. A spare bedroom, a box room, a study nobody uses in winter. This is the worst case, because the room becomes the collector for the whole dwelling: vapour made in the bathroom and kitchen arrives, finds the coldest surfaces in the flat and unloads there. It works like a dehumidifier nobody ever empties, which is why marks appear in a room where nothing wet ever happens.
• The inside of fitted wardrobes on external walls, and the wall behind a headboard. No air movement and a cold masonry face behind, so the first warning usually comes from the clothes or the bedding.

Read backwards, that list is a diagnosis. If the marks fall on the cold points, the mechanism is condensation. If one of them sits somewhere with no reason to be cold, that mark needs a different explanation.

Condensation, a leak, or rising damp?

Not all damp is condensation, and treating every kind the same way prolongs the damage. The other mechanisms behave differently.

Mechanism	How it behaves	What does not fit condensation
A leak: a pipe, a stack, the bathroom above	A stain with a defined origin, spreading outwards from one point	It grows regardless of cold weather and household habits, summer included
Rising damp	A continuous band low on the wall with a fairly level upper edge, often with salt deposits and lifting paint	It sits at floor level, not in high corners or behind furniture
Rain penetration	Appears or worsens after rain or a storm off the sea	Its calendar follows the weather outside, not the temperature or the use inside

The calendar is the cheapest clue available at home. Condensation follows cold and habits: worse with the flat shut up, better when it is aired and heated, and largely absent through a hot Mediterranean summer when no indoor surface is cold any more — unless air conditioning is busy making one. A leak keeps its own rhythm. Rain penetration follows the sky.

The limit has to be stated honestly, though, because these mechanisms overlap. A small leak keeps masonry wet; wet masonry conducts heat better and therefore runs colder; and a colder surface then condenses on top of the original fault. The shape of a stain and its calendar orient you, but they do not reliably separate one real case from another. That takes an on-site inspection, and persisting with ventilation while water is genuinely getting in only lets the damage advance.

Where damp has gone on long enough to leave visible mould, it is not worth living alongside. The World Health Organization's guidelines on dampness and mould support a cautious link between persistent indoor dampness and mould and respiratory symptoms; that is as far as the evidence is taken here, and nothing on a screen can diagnose a person. Removing mould that is already there and correcting the air that produced it are two separate jobs, and doing the second does not guarantee the first will not return.

What heating, fans, dehumidifiers and air conditioning each do

Each of these does something real. The trouble starts when one of them is asked to do the work of all three levers.

What it is	Which number it moves	What it cannot do
Heating harder	Raises surface temperature in the rooms that are heated	Nothing for moisture or renewal; heating one room while others stay cold concentrates the water in the cold ones
A bathroom extractor fan	Air renewal, if replacement air can reach the room	Nothing at all behind a door fitted flush to the floor, with no undercut and no transfer grille
A dehumidifier	Lowers the moisture in the air of the room it stands in, while it runs	It renews no air, and does nothing for the rest of the flat
Air conditioning	Cools, and wrings some water out of the air as a side effect	It brings in no outdoor air at all

The last row is the assumption that costs international residents the most. An ordinary domestic split draws in the air already in the room, treats it and returns it to the same room; what travels to the outdoor unit is refrigerant, not your air. A flat can be perfectly cool and completely unrenewed at the same time — and because cooling means keeping the windows shut, comfort and renewal end up pulling in opposite directions.

There is a fifth response, and it goes backwards: sealing things up. Taping over a wall grille, blocking a trickle vent in a window frame or fitting draught excluders under internal doors reduces the feeling of cold and makes condensation worse, because it removes renewal without touching the moisture being produced.

And there is the case blamed most confidently of all: condensation appearing the winter after new windows go in. Airtight units create no moisture. What they remove is the uncontrolled leakage the flat had silently been relying on for its air, exposing a planned route that may never have been adequate. That case has its own page, because what fixes it is different.

The words you will need, and where your flat stops

Describing the problem precisely in Spanish or Catalan changes the answer you get back. Humedad, in Catalan humitat, covers everything and separates nothing: saying "tengo humedades" tells a professional only that something is wet somewhere. The distinctions that matter have their own names, and they are the ones on this page:

• Humedad por condensación / humitat per condensació — the mechanism described here.
• Filtración / filtració — water arriving from a leak or from outside.
• Humedad por capilaridad / humitat per capil·laritat — rising damp.
• Punto de rocío / punt de rosada — dew point, the term a technical report will use for the first of the two numbers.

The second thing worth establishing early is where your flat stops. What happens inside the dwelling — habits, internal doors, the extractor in your own bathroom, how the place is heated — is yours to change. Shared vertical ducts, façade openings, roof terminals and the fabric of the building are communal elements, and what may be done to them is decided through the comunidad de propietarios, the comunitat de propietaris, usually via the administrador de fincas who manages the block.

Locate that line before commissioning anything. A damp problem whose real cause sits in a communal duct or in the façade cannot be solved from inside your own front door, however much is spent in there — and knowing which side of it you are on turns a vague complaint into a specific request to the right people.