Removing mould (moho) and stopping it coming back

Terms used

• Lejía — Lejía is household bleach.
• Fungicida — A fungicida is an anti-mould cleaning product sold in Spanish shops.
• Yeso — Yeso is plaster, the porous coating on most interior walls.
• Puentes térmicos — Puentes térmicos are cold spots in a wall where heat escapes faster and moisture condenses, causing repeat mould.
• Cajones de persiana — Cajones de persiana are the boxes above windows that house roller shutters, often poorly insulated.

Key rules

• DIY limit: Clean only patches under 1 square metre yourself; anything larger, returning or inside walls needs a professional.
• Humidity: Keep indoor humidity at 40–60%.
• Daily airing: Air every room for 10 minutes a day, even in winter.
• Cleaning products: Never mix lejía with ammonia or other cleaners.
• Damp source: Fix the cause of the damp first, or the mould will return.

Can you clean it yourself?

Clean mould yourself only on patches under 1 square metre. Larger areas, mould that keeps returning or mould inside walls need a professional. Call one too if the person cleaning has asthma, allergies or a weakened immune system, or if the mould followed flooding or sewage. Spain has no national size limit for homes, so this is common international practice.

How to clean a small patch

1. Find the damp source first. A leak, condensation or a cold wall will bring the mould back if you only clean the surface.
2. Protect yourself. Wear gloves, eye protection and an FFP2 mask. Open the windows.
3. Wash it. Use detergent and water, diluted white vinegar or a commercial anti-mould product (fungicida).
4. Use lejía only on hard surfaces. Diluted lejía suits non-porous surfaces such as tiles. Never mix it with ammonia or any other cleaner.
5. Do not dry-brush. It spreads spores through the room.
6. Dry it fully. Wipe with a cloth and keep the room ventilated. Repaint with anti-mould paint if needed.

On porous materials such as plaster (yeso) or wallpaper, mould often goes deep. You may need to scrape it, replace the material or call a professional.

Health: what to know

Living with damp and mould is linked to cough, wheeze, rhinitis, allergic reactions, and asthma starting or getting worse. Children, older people, and people with asthma, allergies, chronic lung disease or weak immunity are most affected. Serious infections mainly affect people with weak immunity. Claims of "toxic black mould" poisoning in ordinary homes are not well supported, but any mould is worth removing. If someone in the home has symptoms, see a doctor.

Stop it coming back

• Air the home. Open windows on opposite sides for 10 minutes a day, even in winter, ideally at a cooler time of day with the heating off. Several short airings beat leaving a window ajar.
• Run extractor fans while cooking and showering. Keep kitchen and bathroom doors shut meanwhile.
• Keep ventilation openings (aberturas de ventilación) clear. Do not block them.
• Watch humidity. A cheap hygrometer shows it. Aim for 40–60%.
• Keep it warm enough. Typical winter advice is about 20–21°C in living rooms and 16–18°C in bedrooms. Do not let rooms stay cold.

Cold spots (puentes térmicos)

If mould returns in the same place, look for a puente térmico: a spot where the wall is colder, so indoor moisture condenses there. Typical places are corners, pillars, window and shutter boxes (cajones de persiana), balcony junctions and top-floor ceilings. It is common in older buildings without insulation. Lasting fixes are external insulation, insulated shutter boxes or better windows, and these need your landlord or building community. Ventilation and heating control still help meanwhile.

The "what if" and FAQ below cover repeat mould and common questions.

What to watch out for

• Mixing cleaning products: Mixing bleach with ammonia or other cleaners can release harmful fumes. Dry brushing mould also spreads spores into the air. Use one product at a time, wet-wipe instead of brushing, and wear gloves, eye protection and an FFP2 mask with the room ventilated.

• Cleaning without fixing the damp: If the moisture source stays, the mould returns, and living with damp is linked to coughs, asthma and allergies, especially for children, older people and people with lung problems. Fix the leak or condensation first, then clean, dry fully and repaint with anti-mould paint if needed. People with asthma, allergies or weak immunity should leave the cleaning to a professional.

• Letting humidity stay high: Mould risk rises when indoor humidity stays above about 60–70%. Aim for 40–60% humidity, measure it with a cheap hygrometer, and ventilate and heat to keep it there.

• Treating porous surfaces like tiles: On plaster (yeso) or wallpaper, mould often penetrates, so surface cleaning alone will not remove it. Scrape back or replace the affected material, or ask a professional to treat it.

What if mould keeps coming back after cleaning?

Cleaning only removes the surface growth. If mould returns, the moisture source is still there. Work through the causes below in this order, and call a professional if the first three fixes don't work.

1. Condensation from indoor humidity

This is the most common cause. Cooking, showers and drying laundry indoors put moisture in the air, and it settles on cold surfaces.

• Buy a cheap hygrometer and keep humidity within 40–60%.
• Mould mostly in cold corners and on windows, and worse in winter regardless of rain, points to condensation.
• Try the foil test. Tape a square of aluminium foil to the wall for 24 to 48 hours. Moisture on the room side of the foil means condensation. Moisture under the foil means the damp comes from inside the wall.

2. Poor ventilation

Check whether vents are blocked, whether air can cross the home, and whether the bathroom and kitchen extractors actually work. Open windows on opposite sides for a few minutes every day, even in winter. Don't dry laundry in closed rooms.

3. Thermal bridges

A thermal bridge is a spot where the wall is colder than the rest because of missing insulation or concrete or steel in the structure. Moisture condenses there, so the mould returns in the same place every time.

• Typical spots are corners, edges of floor slabs, pillars, lintels, shutter boxes, balcony junctions and top-floor ceilings. Uninsulated north-facing walls are also common.
• Older, uninsulated buildings are the most affected.
• Real fixes are external insulation, insulated shutter boxes and better windows. Insulating on the inside is a weaker option. Good ventilation and steady heating help in every case.
• Tenants should ask the landlord about any work on the building itself.

4. Hidden leaks

A leak keeps feeding the mould whatever the weather or the season.

• Look for a stain that spreads on its own and appears whatever the weather.
• Check the water meter with every tap closed. If it keeps moving, a pipe is leaking.
• Look at gutters, window and terrace sealant, the roof, and the flat above. A neighbour's leak is a common source.
• Rising water bills are another warning sign.

5. Building defects

If condensation, ventilation and thermal bridges are ruled out, suspect the building. A low band of damp along the base of a wall, often with white salt deposits, suggests rising damp. Cracks, failed waterproofing and a poor facade can also let water in. Mould that is worse after rain points to an outside leak.

When to get a diagnosis

Call a professional if you have fixed the first three causes and the damp persists. Call sooner if you suspect a leak or a building defect.

• Building engineer or architect (arquitecto técnico, aparejador or arquitecto): gives an independent written report. This helps in a dispute with a landlord or community, before a purchase, or for faults affecting the whole building.
• Damp specialist company: diagnoses and treats the problem, but may sell its own remedy. Get more than one opinion before paying for a big job.
• Plumber or leak-detection firm: best for a suspected pipe leak. They use thermal cameras or gas tests.
• Waterproofing contractor: suits roofs and terraces.
• Home insurance: sudden water damage may be covered, and the insurer may send an assessor. Report it quickly.

Take photos, a log of humidity readings and the results of the tests above. They make any diagnosis faster and strengthen your case if you need the landlord or community to act.

Frequently asked questions

Why does mould keep coming back in the same corner?

Mould returns in the same corner usually because of a thermal bridge (puente térmico), a spot where heat escapes faster than through the rest of the wall. That surface stays colder, so indoor moisture condenses on it.

Is black mould toxic for people at home?

No, ordinary black mould in a home is not well shown to cause poisoning, but damp and mould are linked to coughs, wheezing, asthma and allergies. Children, older people and people with asthma, allergies or lung disease are most affected. Serious infections mainly hit people with weakened immune systems.

Can I use bleach to clean mould off my walls?

Yes, diluted bleach (lejía) works on hard non-porous surfaces such as tiles, but never mix it with ammonia or other cleaners. On plaster (yeso) or wallpaper, mould often goes deeper, so you may need to scrape, replace the surface or call a professional. Avoid dry brushing, which spreads spores.

What indoor temperature helps keep mould away in winter?

Spanish consumer and health bodies suggest about 20–21°C in living rooms and 16–18°C in bedrooms in winter. The WHO has recommended at least 18°C. These are guidelines, not legal limits for private homes.