Bathroom mould usually has one of three causes: condensation from showering without enough ventilation, water escaping from a leak behind the finishes, or damp coming from the structure or the roof. Condensation is solved with extraction and airflow; leaks with repairs to pipes or waterproofing; structural damp needs a diagnosis by a building professional. A renovation can deal with the first two but should not be used to cover up the third.
Why are bathrooms in coastal homes prone to mould?
A shower fills a small room with warm, humid air. When that air touches a cooler surface it condenses into water. Many apartments on the Costa del Sol have inside bathrooms with no window, only a grille to a shared shaft; others are closed up for long periods as second homes. Both situations leave moisture with nowhere to go. Mould needs moisture, a surface to grow on and time, and a poorly ventilated bathroom supplies all three.
How do you tell condensation from a leak?
Condensation appears on cold surfaces after showering: the ceiling above the shower, corners of the outside wall, the window frame, grout lines. It tends to be spread out and worse in cooler months, and it improves when ventilation improves.
A leak usually shows as a localised patch that grows over time, often near the shower, the WC or a pipe run, and it persists regardless of ventilation. In apartments, it often appears first on the ceiling of the flat below.
Structural damp, such as water entering through the façade, the roof or rising from the ground in older houses, shows as patches at the base of walls, near windows or on top-floor ceilings, and may come with salt deposits or flaking plaster.
What does Spanish building regulation say about bathroom ventilation?
Spain’s Código Técnico de la Edificación includes a basic document on health and hygiene, DB HS, whose section HS 3 covers indoor air quality. For new dwellings, it expects bathrooms, toilets and kitchens, the so-called wet rooms, to have extraction openings, and air to circulate from the dry rooms towards the wet ones, where it is extracted.
How far HS 3 applies when an existing bathroom is renovated depends on the scope of the intervention (to be verified case by case). Whether or not it is a legal requirement in your case, the logic is sound: moist air should be extracted where it is produced.
What can improve ventilation in an existing bathroom?
- An extractor fan, ducted to the shaft or outside, on its own switch or linked to the light with a run-on timer.
- A gap under the door, so air can be drawn in from the rest of the home when the fan runs.
- Using it: running the fan during and after showering, and leaving the door ajar afterwards.
- Clearing the existing grille into the shaft, which is often blocked by paint or dust.
The electrical point for an extractor must respect the zones around the shower defined in ITC-BT-27 of Spain’s low-voltage regulation; our authorised partner installers position it accordingly.
What does a renovation fix, and what not?
A bathroom renovation can renew leaking pipes and waste, add a proper waterproof membrane in the shower, improve extraction, replace mould-damaged plaster and finishes, and use materials that clean easily. That deals with condensation and with leaks originating inside the bathroom.
It cannot fix water entering through the façade, the roof or the ground. Covering structural damp with new tiles, plasterboard or paint hides it until it reappears, often worse. If we see signs of structural damp during the visit, we tell you and recommend an assessment by an architect or building surveyor before any renovation. For villa façades and roofs, see our sister site reformavillascostadelsol.eu.
How should existing mould be dealt with?
Surface mould on tiles and silicone can be cleaned, but mould that has penetrated silicone generally returns; the silicone needs replacing. Mould in plaster or paint on the ceiling often means the surface layer should be removed and redone with a suitable finish after the cause has been addressed. Treating mould without fixing ventilation or the leak only buys time.
Our maintenance guide covers the routine care that keeps mould from returning once the cause has been solved.
What about homes that stand empty?
Second homes closed for long periods are particularly vulnerable. Without anyone opening windows or running a fan, humidity builds up in warm months. Leaving internal doors open, keeping a window on a secure vent where possible and asking a key-holder to air the property during visits all help. Turning off the water when leaving reduces the risk of an unnoticed leak.
Which bathroom designs help prevent mould?
Design choices during a renovation can make mould less likely. A walk-in shower with a single screen and fewer corners dries faster than a tray with a curtain. Large tiles or a seamless finish reduce the length of grout lines where moisture lingers. A niche with a slight outward fall at its base does not hold water. Ceilings finished with a moisture-resistant paint or board cope better with occasional condensation. And an extractor placed high, away from the door, draws air across the room rather than straight out again.
None of these replaces ventilation, but together they make a bathroom more forgiving of real life.
When should you ask for a renovation quote?
When the cause is understood and it is something a renovation addresses: an old shower without waterproofing, pipes behind the tiles that have started to weep, or a windowless bathroom that has never had extraction. Send photos of the affected areas, the ceiling, the shower and any grille or fan, plus the approximate floor area. We reply with a first assessment. See bathroom renovation and small bathrooms for the scope of the work.
Official sources
- Código Técnico de la Edificación, DB HS Salubridad (HS 3 indoor air quality)
- REBT, Real Decreto 842/2002 (ITC-BT-27, rooms with a bath or shower) – BOE consolidated text
Bauman Raymond Attila · Reforma Baños Málaga · Avenida Antonio Belón 3, Marbella. Last reviewed: 05/11/2026.
