Mould on walls: how to remove it safely and why it returns

You can remove mould from a wall in two steps: first treat the affected spot with a product that has a fungicidal effect and remove the residue mechanically, then find and eliminate the cause of the damp. Without the second step, the mould will return. You can deal with a smaller area yourself using gloves, a respirator and goggles, but extensive or recurring mould is a job for a specialist company and a builder.

Why mould grows on walls

Moulds need moisture to grow. In its position statement on the mould problem, the National Institute of Public Health (SZÚ) names a certain water content in the material they grow on as the basic condition, and distinguishes two groups of causes:

• Building defects: most often water leaking into the building, damp rising from the ground and condensation on cold spots, known as thermal bridges. A one-off incident, such as a burst pipe or flooding from neighbours, can also soak a wall.
• How the flat is used: cooking, washing and drying laundry without sufficient ventilation, inadequate or irregular heating, and too little airing. According to SZÚ, stagnant air also encourages mould growth, typically in corners and behind furniture.

The place where the mould appears can offer a clue. Condensation and stagnant air show up in cold spots, in corners and behind furniture, while leaks and rising damp are linked to the building structure. Only a building specialist can determine with certainty which cause applies in your home.

Health risks, briefly and without alarmism

In the same position statement, SZÚ states that mould spores are serious allergens in indoor air and can trigger allergies, including asthma. The US Centers for Disease Control and Prevention (CDC), in its information on mould, adds that people with weakened immunity or chronic lung disease are at greater risk. This article is not a substitute for medical advice; if you have health problems that you associate with mould, consult a doctor.

Small or large area

In its guide to mould and moisture in your home, the US Environmental Protection Agency (EPA) states that most people can clean an area smaller than about 10 square feet, or roughly one square metre, themselves. For larger areas, or after flooding with contaminated water, it recommends professional help. SZÚ adds that, depending on severity, the mould can be removed by the occupant or by a company with the professional qualifications for mould removal. Where a company carries out the work, SZÚ also recommends a microbiological check to confirm that the treatment was effective; ask about this when choosing a company.

Do not tackle it yourself if the mould covers a large part of the wall, returns even after repeated treatment, is under wallpaper or tiles, or if someone in the flat has asthma or a weakened immune system.

What you will need

• a respirator (EPA specifies the N-95 type) to avoid inhaling spores,
• long rubber, nitrile or PVC gloves,
• fully enclosed protective goggles without ventilation holes,
• a mould product with a fungicidal effect, suitable for the surface in question,
• disposable cloths or sponges and a waste bag,
• a bucket of water with detergent for smooth surfaces.

Step-by-step removal

1. Prepare the room. Move furniture away from the wall, cover the floor beneath the affected area and open a window. Keep children and pets out of the room.
2. Put on protective equipment. Respirator, gloves and goggles, as recommended by the EPA.
3. Read the product label. Follow the dilution, contact time and ventilation instructions. Test the product on a small, inconspicuous spot to check that it does not damage the surface.
4. Treat the mould. SZÚ recommends always removing mould by disinfecting with a suitable product with a fungicidal effect on mould. Apply it according to the instructions and leave it to work.
5. Remove the residue. The EPA points out that even dead mould can cause an allergic reaction, so it is not enough merely to kill it; it has to be removed. For smooth surfaces (tiles, window frames, washable paint), the EPA recommends scrubbing with water and detergent and drying thoroughly.
6. Dispose of the materials. Put cloths and sponges straight into a bag and tie it. Wash your hands and face after finishing.
7. Ventilate and let it dry. The wall must dry completely before you apply any paint.

A chlorine product is not a necessity. The EPA does not recommend it as a routine method. If you use one anyway, follow the label and ventilate well. Never mix it with other cleaning products: the CDC, in its guidance on cleaning mould, warns that mixing bleach with ammonia or other cleaners produces toxic gas, and the Toxicology Information Centre, in its entry on toilet and sanitary cleaning products, warns that products containing hypochlorite and acids must not be mixed because chlorine gas may be released. A more detailed explanation is given in the article on dangerous combinations of cleaning products.

What to do about wallpaper, plaster and soaked items

Absorbent and porous materials are hard to clean. The EPA states that materials such as ceiling tiles or carpets may need to be thrown away after mould infestation, because the mould grows into them and may not be removable completely. In practice, for a household this means:

• it is usually better to strip mouldy wallpaper and treat the wall beneath it,
• if mould grows through a new coat of paint again, the problem lies deeper in the plaster and it is time to call a builder,
• upholstered furniture, mattresses and boxes that have stood for a long time against a mouldy wall should be inspected, and heavily affected items should be considered for disposal.

Similar principles apply to mould in shower corners and tile grout, only with different products; you will find the method in the article on how to clean bathroom grout.

Why mould returns and how to prevent it

In its position statement, SZÚ stresses that disinfection is only the first step and must be followed by removal of the causes, based on an assessment by a building specialist. The EPA sums it up by saying that moisture control is the key to mould control. At home, you can do the following:

• Ventilate briefly and intensively. In an interview on ventilation in winter, Bohumil Kotlík of SZÚ recommends airing for about five minutes with the windows wide open, in the bedroom at least twice a day and in the bathroom after every use. In his view, a tilted vent window does not air a room properly.
• Monitor humidity. In the same interview, Bohumil Kotlík gives roughly 30 to 60% as a suitable range of relative humidity. In its guide, the EPA recommends keeping it below 60%, ideally between 30 and 50%. A home hygrometer will show you where you stand.
• Take misted-up windows seriously. According to SZÚ, condensation on the glass is a signal that the room needs airing.
• Move furniture a few centimetres away from external walls so that air can circulate behind it.
• Limit drying laundry in the flat and use the cooker hood and bathroom fan, which extract air to the outside.
• Do not overdo humidifying. Among its measures, SZÚ lists limiting the excessive use of air humidifiers. Kotlík recommends them only where the health of sensitive persons requires it.
• Heat evenly. SZÚ lists inadequate or irregular heating among the causes.
• Repair leaks in the roof, windows and water pipes as soon as possible and dry wet areas within 24 to 48 hours, as the EPA recommends.

If you are dealing with mould when moving out or before handing over a rented flat, it also relates to the condition in which you hand the flat over; you will find general cleaning advice for handover in the article on cleaning before handing over a flat.