How does mold form on walls? 3 causes and a permanent solution

Mold needs three things: moisture, moderate temperature and a breeding ground. These are dust, plaster, wallpaper and ordinary paint. You can always give it warmth and food at home. So the humidity is decisive, and not the humidity of the air in the room, but the humidity directly on the surface of the wall.

Dew point: when does air turn into water?

The dew point is the temperature at which water vapor in the air turns into droplets. At a room temperature of 20°C and a relative humidity of 60%, it is approximately 12°C. If any part of the wall is below 12°C, water will start to condense on it, even if you can't see it with the naked eye. When the wall surface remains damp for several days (relative humidity at the surface around 80% or more), mold spores will start to grow.

Thermal bridges: cold spots in the wall

A thermal bridge is a place where heat escapes from a house faster than through the surrounding structure. This can be above a window at the lintel, at the corners of an exterior wall, near a ceiling cornice, or behind a cabinet that is close to an exterior wall. These places are several degrees cooler than the rest of the wall in the winter. That's why mold almost always appears there first.

Insufficient ventilation after insulation and window replacement

The old windows were not airtight and the house was “ventilating” itself. New plastic windows and an insulated facade stop this leakage. Moisture from cooking, showering, drying laundry and breathing (a household of four produces approximately 8 to 12 liters of water per day) then remains inside. What causes mold on the walls in newly insulated houses? Most often, this imbalance: the tight envelope of the house and the original ventilation habits.

Before you start removing it, find out where the water is coming from. Each of the three main causes is helped by something different, and a bad diagnosis means the mold will return in a few weeks.

Condensation appears as spotted mold in corners, behind furniture, and near windows, most often in winter. The spots do not have sharp edges, and you often see fogged windows in the morning. The cause is the air in the apartment, not the structure.

Leaks form limited areas with yellow-brown edges, usually under the roof, near the balcony or under the window. The stain grows after rain, regardless of the season. Neither ventilation nor a dehumidifier will help here until the roof, flashing or window sill are repaired.

Rising damp rises from the ground through masonry that does not have functional waterproofing. You will see it as a damp strip at the bottom of the walls, about 30 to 100 cm high, often with white salt blooms and falling plaster. It is typical of older houses and basements.

The most common mistake is this: spray the product, wait for the stain to fade, and then paint the wall. After two months, the mold is back, sometimes even under new paint. The spray destroys the surface layer, but the mold fibers go deeper into the plaster and the cause remains.

Painting a damp wall makes the problem worse. The paint will trap moisture in the plaster and create ideal conditions for further growth. When asked what really works on mold on walls, the answer is always the same: remove it and remove the source of the moisture.

You can handle smaller areas up to approximately 1 m² yourself. If the mold is more extensive, returns despite the measures, or if there is an allergy or asthma sufferer living in the household, call in a professional.

Safety first

Mold spores irritate the respiratory tract, so wear an FFP2 respirator, rubber gloves and safety glasses when working. Ventilate the room while working and close the doors to other rooms. Never mix chlorine products with vinegar, acids or ammonia, as this produces toxic gases.

Removal procedure

• Moisten the affected area first to prevent the spores from spreading while you work.
• Apply an antifungal (antifungal) product and let it work as directed, usually 15 to 30 minutes.
• Remove the softened coating with a spatula or brush. If the mold has penetrated deeply, scrape off the plaster as well.
• Let the wall dry completely. This is a step that most people skip.
• Treat with the product again and apply antifungal, vapor-permeable paint to the dry substrate.

However, the key to permanently removing mold from a wall is not the four steps with chemicals, but the fourth: thorough drying.

Plaster after leaks or long-term condensation contains a lot of water. Ventilation alone in damp weather in autumn or winter dries it out for weeks. Technology can do this faster and more reliably.

A condensation dehumidifier sucks in air, cools it, and collects the water in a container or drains it through a hose into the drain. Professional machines with a capacity of 40 to 59 liters per day are sufficient for an apartment or a smaller house after a leak. Machines with 60 or more liters per day are designed for new buildings, fresh plaster and screed, or flooded basements. If you don't know what capacity to choose, the article How to choose a dehumidifier for construction or the home will help you, which compares machines according to the type of space and the desired drying speed.

Hot air drying

Warm air holds more moisture and the wall evaporates it more quickly. Therefore, a combination of a heater and a dehumidifier is more effective than either device alone. First, warm up the space slightly and then let the dehumidifier run in a closed room so that it does not dry out the air in the entire house.

Air circulation

The air near the wall, especially in corners and behind furniture, is stagnant. A fan directed at the damp surface constantly exchanges it and accelerates evaporation. Move furniture at least 5 to 10 cm away from the wall while drying.

Practical example: a family from Trnava insulated their family home in the summer and replaced the windows. In January, black spots appeared in the corner of the children's room and behind the wardrobe in the bedroom. The hygrometer showed 72% in the rooms. After removing the mold, they left a rented dehumidifier with a capacity of around 50 liters per day running for five days, which extracted several liters of water from the air every day. When they started to ventilate regularly, the humidity stabilized at around 50% and the mold no longer appeared.

Mold in the bathroom

The bathroom produces the most steam in the entire apartment in a short time. If you want to know how to remove mold in the bathroom from the joints, reach for a product designed for silicones and joints. It is better to cut out and replace heavily attacked silicone, because the mold grows through the entire mass. After each shower, wipe the walls of the shower enclosure, let the fan run for at least 15 minutes and open the door a little.

Humidity in the basement

When asking how to remove moisture in a basement, first check whether it is rising damp or condensation. In summer, warm outside air cools below the dew point in a cool basement, so ventilation on hot days paradoxically increases humidity. Ventilate in the morning or at night when it is cooler outside. With rising damp, additional waterproofing will help, and a dehumidifier is only a temporary solution in this case.

High humidity in the house

What should the humidity be in the apartment? In living rooms, 40 to 60% at a temperature of 20 to 22 °C is recommended, in the bedroom, the lower limit. A value above 60% for a longer period is a warning sign. How to reduce humidity in a house without technology? Do not dry laundry in rooms, use a hood with exhaust to the outside when cooking, and do not turn off the heating completely in unused rooms.

Prevention during winter

Winter is the time when mold grows the fastest because the temperature difference between the inside and the wall is the greatest. Instead of opening the windows all day long, ventilate them intermittently: 3 to 4 times a day for 5 to 10 minutes with the windows wide open, ideally with a draft. The air will be exchanged, but the walls will not cool down.

Maintain a consistent temperature throughout the house, at least 18°C even in unused rooms. Do not place cabinets and sofas close to the perimeter walls. For 10 to 20 euros, buy a hygrometer. In a few days, it will show which room is the problem, and you can react before the first stain appears on the wall.

If you need to quickly dry out a space after a leak, renovation or mold remediation, buying your own professional machine is not worth it for a few days. In the Dehumidifiers category at Ramirenta , you can rent a dehumidifier 40–59 l/day for apartments and smaller houses or a dehumidifier 60 l/day and more for new buildings, basements and larger areas. Rental starts at approximately €18 excluding VAT per day.

For hot air drying, you will find electric heaters and for air circulation, fans . You can contact your nearest branch to help you choose the right power.

Once you understand how mold forms on walls, it becomes clear that chemicals only address the consequences. You will achieve lasting results if you identify the source of moisture, dry the wall thoroughly, and ventilate and heat properly in winter.