Damp and mould in a second home on the coast south of Barcelona

What a closed-up winter actually does to a wall

An empty, unheated home does not sit still. Its air drifts towards the air outside — a modelling study comparing an occupied and an unoccupied dwelling found indoor humidity in the empty one tracking outdoor humidity with roughly a half-day lag. That study is East Asian and measures no Catalan house, so take the mechanism and none of the numbers: removing the occupants removes a moisture source, not the moisture.

And the threshold that matters is surface humidity, not room humidity. BS 5250, the British code of practice on condensation, treats growth as likely where surface relative humidity holds above about 80 % for more than two or three days, and germination near that limit is slow rather than instant. So the exposure behind the mark you find in April ran quietly from November. By the time anyone opens the door, it is a record of something that already finished happening.

That is the difference from the Barcelona-flat problem. In an occupied flat moisture is produced daily and dealt with daily, or not. In a closed house nothing is produced and nothing is removed, and the building simply equilibrates for months.

Why this coast, and why Sitges catches owners out

Sitges has the mildest winter and driest year on this stretch of coast — a mean January minimum of 8.7 °C and 502.9 mm of rain, on the Atles Climàtic de Catalunya’s 1991–2020 municipal normals. Milder in January than Barcelona itself.

It also had 6,368 of its 19,230 dwellings recorded as non-principal in the 2021 census — 33.1 %, against 13.6 % in Castelldefels. That count mixes second homes with empty stock, since Idescat publishes no split. Sitges’s annual full-time-equivalent population runs at 115.6 % of registered residents; Castelldefels runs at 95.2 %, which is to say Castelldefels empties by day, into Barcelona, not by season.

So the problem here is the occupancy pattern, not the climate. “It is a dry, mild town, it cannot be damp” is exactly the reasoning that produces a surprise in spring.

Nobody can honestly quote a humidity figure for these towns: Meteocat has no station in Sitges, Castelldefels or Gavà. The nearest to Sitges sits at Sant Pere de Ribes inside the Garraf park, 164 m up and well off the seafront, where daily mean relative humidity was 80 % or above on 21.3 % of days over 2015–2024, against 6.4 % at Barcelona el Raval. Hillside outdoor readings are not the humidity in your hallway — but that gap is real.

The first hour after you open the door

Photograph before anyone cleans. A dated shot taken before the wipe is worth far more to a manager, an insurer or a comunidad than a description of it afterwards.

Where	Why it fails there	What you are looking for
North-facing bedroom walls	Coldest surfaces in the house, all winter	Fine speckling in the top corners; a wall cold to the palm
Inside and behind fitted wardrobes	A still, unventilated pocket against an external wall	Marks on the back panel, on stored fabric, on the wall
Behind a bed head or a sofa	The same pocket, made by furniture nobody moved in October	A rectangle of speckling matching the furniture
Bathroom with no window	Nothing dried out since the last shower in August	Dark grout and silicone, blistered ceiling paint
Ceiling under a terrat or a terrace	Roof and terrace are common elements; failure shows below first	Brown-edged rings, re-skimmed patches, a repainted band
Trastero, garage, semi-buried rooms	Cool, unventilated, below the level of everything else	Musty smell, bloom on stored boxes, salts low on a wall
Under the sink, behind the washing machine	A slow drip nobody was there to hear	Swollen chipboard, lifting skirting

Then check the smell before opening every window. Shut the door on the smallest room, do everything else, and go back to it last. Odour alone does not prove hidden mould: stale closed-up air, drains and damp materials can smell similar. A localised or recurring musty smell still deserves investigation; the musty-smell guide explains how to read that clue without treating it as a diagnosis.

Why the mark is behind the wardrobe and on the north wall

Both are questions about one patch of surface, not about the room. A north wall, an external corner losing heat in two directions at once, a slab edge, a window reveal: each sits colder than the air beside it, so the air touching it condenses sooner.

A wardrobe pushed flat against that wall adds a second effect: the pocket behind it neither warms convectively nor dries. Room-average humidity can look entirely reasonable while the surface a centimetre behind the wardrobe back sits above the threshold all winter. That is why mould behind furniture is so reproducible — same corner, same wardrobe, year after year — and that repeatability is itself a diagnostic signal.

Airing it out on arrival can make things worse

The instinct on arrival is to open everything. In a cool, heavy-walled house opened on a warm humid morning, that admits air whose dew point sits above the internal surfaces, and it condenses on them. The mechanism is documented — CSIRO’s condensation guidance covers it — and it is the opposite of what the person at the window believes is happening.

The rule that holds up is to ventilate when the outside air is drier in absolute terms, not merely cooler or fresher: mid-afternoon on a dry day rather than at dawn after a humid night, once surfaces have warmed a little. Heating the house before you air it does more than airing it harder. And either way, a day of open windows cannot undo an exposure that ran for months. Why a cleaned mark returns covers the occupied-flat version.

Before you lock up again

This list is worth more than the arrival list, because it governs the months nobody is watching.

• Leave interior doors open, and wardrobe doors ajar.
• Pull furniture a few centimetres clear of every external wall.
• Take out damp textiles: towels, bed linen, the laundry basket, a wet mop head, anything in a sealed box.
• Wipe down the shower and the sink cupboard, and prop the fridge open if you empty it.
• Arrange continuous low-level control rather than a burst of activity on your next visit — background ventilation if the property can be left secure that way, a deshumidificador on a humidistat if it cannot.
• Give whoever checks the property the arrival table above, and ask for photographs of the same places each time. Two dated shots a season apart tell you more than one alarming one.

We do not supply, fit or maintain ventilation equipment, so read that fifth point as what the physics supports, not as something we are selling.

When a cleaning visit will not be enough

A cleaning company can take a mark off tile, grout and silicone, and after a short closure that is often the right call. It stops being enough when:

• The mark comes back in the same place every season.
• The paint lifts, bubbles or feels soft when you wipe it.
• There are several marks in one room, or the mark is on plaster and paint rather than on tile.
• A smell persists in a room where you cannot see anything.
• The plaster sounds hollow or crumbles — yeso muerto.

At that point the work is saneado: taking the surface back to something sound, treating it, and using pintura antimoho as a finish on a prepared wall rather than as the fix. When to call a specialist sets out the line; mould removal and anti-mould painting describe the work.

Two honest limits. NoMoho treats visible mould and the condensation side of damp — not obra, waterproofing, injection against capilaridad, leak repair, ventilation installation or reports for disputes. If the ceiling below a failed terrace is marked, the ceiling is ours and the terrace is not. And on the delta side of this corridor the documented shallow-groundwater record concerns basements, car parks and lift pits; it does not document rising damp in ordinary flats, and no water-table depth is published for Castelldefels at all. A mark in a semi-buried trastero is a different conversation from one in a first-floor bedroom.

We do not promise mould will not return. How a property is closed up, and how often it is opened, keep mattering after we leave.

Running it from another country

Send a wide shot of the room, a close-up of the mark, what is on the other side of that wall, whether this is the first year, and how long the place stands empty. That is usually enough for a straight answer on whether a visit is worth arranging. The first photo review is free — write to us in English.

The rest is other people’s language. The Spanish–English damp glossary gives you the words a builder or an administrador de fincas will use; damp, your comunidad and your insurance covers whether the cause is yours, the building’s or a neighbour’s; and if you are buying rather than closing up, what to check before you sign.

Where we actually cover

NoMoho is based in Barcelona and has no office in Sitges, Castelldefels or anywhere else on this coast. We travel out from the city — one reason we would rather see photographs before agreeing a visit than after driving to one.

South, that means Castelldefels and Gavà Mar, Sitges and its outlying urbanitzacions, Vilanova i la Geltrú and Sant Pere de Ribes. North, up the Maresme as far as Lloret de Mar.

The town-by-town pages carry local detail this one deliberately does not repeat, and they are in Spanish only: Sitges, Castelldefels, Vilanova i la Geltrú, and northward Lloret de Mar and the Alt Maresme, with the full list at zonas. The parallel English journey for a home through Maresme towards Blanes and Lloret is the north-coast second-home guide; it is a corridor guide, not a translation of a city page. Write to us in English regardless; we reply in English.