Ventilation in a coastal home that stands empty for months

An empty house is not a dry house

Shutting up removes the main source of water vapour at a stroke. Nobody showers, cooks or dries laundry in there. Which is exactly why finding the place worse at Easter than you left it in October feels like a contradiction.

Three people leave with you, and only one of them was ever a problem.

• The one who opens a window. Spain's building code, the Código Técnico de la Edificación, describes a dwelling as a route — air admitted in the dry rooms, bedrooms and living room, crossing the house through transfer openings, extracted from the wet rooms, bathroom and kitchen. That section is DB-HS3, and it is the reference you will see on paperwork. The route is still there in an empty house. The daily gesture that makes it do anything is not performed for months.
• The one who heats. With no heating, interior surfaces drift towards outdoor temperature. Persianas down — the external roller shutters — take away the occasional hour of sun that might otherwise warm a wall back up.
• The one who notices. In an occupied house a stain is seen the week it appears. In a closed one, the first witness is the smell on the day you walk back in, by which point it has had a whole winter to happen.

The building itself does not pause. It carries on exchanging air and moisture with the outside, slowly, and on this coast the outside air is humid for a good part of the year: mild, humid winters and hot, humid summers. That exchange is steady enough to keep the house loaded and far too weak to flush out any corner of it.

What decides where mould appears is never the average condition of the house. It is the pockets. Inside a fitted wardrobe on the seaward wall. The gap behind a headboard. The spare bedroom whose door was closed because nobody was using it. Air does not move there and the surface is cold. And everything you left inside — mattresses, cushions, bed linen, books, the chipboard back of a wardrobe — takes up moisture when there is plenty and releases it when there is less, without a gram of it ever leaving the building.

Why a mild February day is worse for the house than a cold one

An August weekend with the house shut smells stale and does little else. The house is warm and the walls sit well above the temperature at which water settles on them. The summer exception is the ground floor: the slab is in contact with the ground and stays cool while warm, humid air comes in, which is why a tiled floor can be wet in August with no leak anywhere in the building.

The long winter closure is a different animal. Unheated, the whole house ends up working at outdoor temperature, and inside that average there are surfaces colder still — the exposed façade, the corners, a partition against an unheated store or stairwell, the ground-floor slab. In a Mediterranean climate that does not mean frost. It means weeks of mild, wet weather with a cold house sitting inside it.

This is where an instinct brought from further north misfires. In a continental or a British winter, cold outdoor air is dry, so throwing the windows open on a raw day genuinely dries the place out. Here the damaging day is the opposite one: heavy, mild, with wind off the sea, arriving at a house still holding the temperature of the previous three weeks. That is when the most water is deposited on the walls, and it is precisely the day that tempts you to open everything wide. Ventilation timing written for other climates does not transfer to this coast unchanged.

So the harmful combination belongs to closure itself: cold, still, dark, humid, unwatched. Remove any one of the five — a little background warmth, or somebody walking through — and the rest lose much of their effect.

Locking up: the half hour that decides the next six months

None of this costs money and none of it is technical.

What you touch	On the day you close	Why it matters
Interior doors, wardrobes, drawers	Leave open or ajar	The house should be one volume of air, not a set of sealed pockets
Tall furniture, headboards, mattresses	Pull off the outside walls; stand mattresses on edge if you can	A cold wall behind a solid object stays cold and unventilated
Wall grilles, trickle vents, bathroom extractor	Leave them clear, tape nothing	The small amount of air movement that remains is exactly this
Stopcock (llave de paso, clau de pas)	Close it	A flexible hose that gives way in an empty house leaves weeks of water, not a damp patch
Bathroom and kitchen traps	Fill them with water; for a long absence, ask a plumbing professional about compatible protection	Unused traps can dry out and the house can greet you with a drain smell
Fridge	Empty, defrost, prop the door open	A closed empty fridge is a box of damp for the whole winter
Towels, mop, bucket, bin, plants	Take them out	Anything wet left behind evaporates for months into a house nobody opens

One item on that list is a judgement rather than a task. Shutters down protect against sun, weather and casual break-ins, and they also cost the house its only free source of surface warmth. That is a trade worth making room by room and deliberately, rather than by reflex on the way out.

What can safely be left running, and what only looks useful

If the house already has mechanical ventilation designed to run continuously — a VMC, ventilación mecánica controlada, the term you will hear from a Spanish installer — then a long closure is exactly when leaving it on earns its keep. Switching it off to save money while the house is alone deactivates it during the months it is doing the most work.

A dehumidifier is a partial tool and worth seeing for what it is. It dries the air in the room it stands in, while it is running, and renews nothing. On a tank, it stops on the day the tank fills and spends the rest of the winter as furniture. It only makes sense plumbed to a permanent drain — and leaving any appliance powered for months in an empty house is a decision with risks of its own. If the property is insured, the policy, not an assumption, is the place to check what it says about unoccupied periods.

Background heating at a low setting, where the house has it and it can be left safely, dries nothing at all. What it does is keep surfaces above the worst of the cold, which is the other half of the problem.

A simple hygrometer with a maximum-reading memory, left in the living room, fixes nothing and tells you a great deal in March: how high the humidity climbed while you were away. It replaces "it felt damp" with something you can compare against next year.

And the reflex to resist is sealing. Taping over grilles, blocking the extractor duct or stuffing the trickle vents before you leave feels like protecting the house and does the reverse. It removes the renewal without removing a gram of the moisture already held in the air, the furniture and the walls.

The first evening back is where most of the damage is manufactured

The mistake has a very recognisable shape. You arrive after dark, it is cold, you close the door behind you, put the heating on full, take a shower and cook dinner. In the morning there is water on every pane and the walls are wet. You have not discovered the problem; you have just made it. The walls are still at the temperature of the whole winter, and every bit of vapour from that evening went straight onto them.

The order that works runs the other way round:

1. Open up before anything else, on two opposite sides, interior doors included, so air crosses the house. Judge the closed-up smell after that, not on the threshold.
2. Start heating early and keep it steady rather than in bursts. For the first hours you are not trying to warm the air, you are trying to stop the surfaces being the coldest thing in the house.
3. Run the extraction for the first showers and the first meals, and leave a gap under the bathroom door so air can get in to replace what goes out. The house is not yet in a state to absorb vapour.
4. If the day you arrive is mild and heavy and the house is cold, do not leave everything open for hours. Air in shorter bursts and put the effort into heating instead. Opening wide does not always dry a house; on some days it is how the water gets in.

Then look, before you wipe anything

• Inside the wardrobes on the outside walls. Cold surface, completely still air, fabric touching both. This is a pocket, not a verdict on the whole house.
• Behind the headboard, the sofa, anything pushed up against a wall. The furniture keeps that patch colder than the rest of the room. Pull it out and look again in a week.
• Mattresses, cushions and stored bedding. They absorb, and they sit against a wall or the floor. Air them before anyone sleeps on them.
• The bathroom ceiling corners and the window reveals. The coldest edges of the coldest rooms. If they are marked, the air route will need attention once the house is lived in again.
• The ground-floor slab and the skirtings. The ground keeps them cool, so this may not be condensation at all. Look at the same spots again in a dry week.

Photograph whatever you find before you wipe it, and long before anyone paints over it. A stain dried on the first morning can no longer be compared with anything.

If there is visible mould, taking it off and correcting the air are separate jobs. The World Health Organization's guidance on dampness and mould supports linking persistent indoor damp and mould with respiratory symptoms, so it is not something to live alongside all summer in the hope that it dries out on its own.

Wind-driven rain, leaks, and the water meter

A house on the coast takes something a sheltered inland flat does not take with the same force: storms driving rain horizontally at one particular façade — a llevant, the easterly, is the one that does it here. That water is not falling on the building, it is being thrown at it, and it finds carpentry joints, sills, terrace thresholds and outlets that cannot keep up. With the house shut, the same episode can repeat all winter without anybody seeing it once.

Signs that point to water getting in rather than condensation:

• the stain appears or grows in the days after a storm, then stops;
• it is there in August too, in dry weeks with the house warm;
• it runs vertically down from one point, or rises from the floor in a continuous band;
• it sits under a terrace, a joint or a window, rather than on the cold zones of the house;
• with every tap closed and nothing running, the water meter still moves.

That last one is the cheapest check in existence and worth repeating every time you open the house after a long closure. If the dial turns with everything shut off, what you have is not a question of air.

Ventilating touches none of this. A leak, damp rising from the ground and condensation are corrected in different ways, and they cannot be told apart reliably without an on-site inspection. Airing a house while water comes in with every storm only extends the time the fabric stays wet.

There is also one visit that does not belong on a calendar: the one after a hard easterly blow. Between finding water three days later and finding it three months later, the difference is not really the size of the stain. It is whether anything underneath ever had a chance to dry.

What to ask of whoever holds the keys

If somebody is going in while you are away — a neighbour, family, a keyholder you pay — it is worth agreeing what the visit actually consists of instead of leaving it to their judgement. A useful version is short:

• open two windows on opposite sides on arrival, interior doors open, and leave them like that for the whole visit;
• walk through every room, including the ones nobody uses, and open the wardrobes;
• check three fixed points: the bathroom ceiling and corners, the exposed façade wall, and the ground floor at the skirting;
• read the water meter with everything closed;
• take the same photographs from the same positions each time, and only get in touch if one of them has changed.

What should not be on that list: going up onto a roof or a terrace, clearing a high gutter or outlet, opening a communal duct or shunt — the shared vertical flue serving stacked bathrooms and kitchens in a block — or touching anything electrical. In a block of flats, roofs, façades and shared ducts are generally not an individual owner's to work on in the first place; they are handled through the comunidad de propietarios, the owners' association, and its administrador de fincas, the managing agent. That is not a formality. It is also the difference between asking a favour and causing an accident.

Everything left on the list is a slow walk around an open house — which is, unglamorously, the most useful thing that happens to the building between one visit and the next.