With the first autumn rains and the heating coming back on, many homes start showing the same signs again: windows steamed up first thing in the morning, a stuffy smell that will not go away and small dark patches in the corners of the ceiling. Mould on walls is one of the most common household problems at this time of year, and it rarely appears by chance. It is almost always the visible part of excess damp in the home that has been building up unnoticed.
In Portugal, this is far from a marginal issue. According to the Statistics Portugal (INE) Survey on Income and Living Conditions, almost a third of the population (32.1%) reported in 2025 that they lived in homes with damp or leaks. Understanding where that water comes from, how to clean mould without risk and which habits stop it coming back is the way to a more comfortable home with better indoor air quality.
What causes mould on walls
Mould appears when a surface stays damp long enough for fungi, whose spores occur naturally in the air, to grow. Without moisture there is no mould. This is why the United States Environmental Protection Agency (EPA) sums the matter up in a simple idea: controlling mould is, above all, controlling moisture.
The source of that moisture is not always the same. UK Government guidance on damp and mould in housing distinguishes several types. Identifying the right one avoids spending time and money on solutions that do not work.
Condensation: the most frequent cause
Condensation is the water that forms when warm, vapour-laden air touches a cold surface, such as a pane of glass, an external wall or the corner of a ceiling. The same UK guidance identifies it as the most common form of damp in homes. It is fuelled by activities as ordinary as bathing, cooking or drying clothes indoors.
The most typical signs are easy to recognise:
- wet glass and window frames when you wake up, especially in bedrooms;
- black spots in corners, around windows or behind furniture placed against external walls;
- mould on the bathroom ceiling, near the shower or bath.
Penetrating damp: water that comes in from outside
When water gets in through the façade, the roof, poorly sealed window frames or cracks, this is penetrating damp. You will usually notice it after days of heavy rain, as patches that spread out from one point and can leave the paint blistered or the plaster crumbling.
Rising damp and water leaks
Rising damp moves up from the ground through the walls and is more common on ground floors and in older buildings. It usually leaves a damp band along the lower part of the walls, sometimes with whitish salt marks. Leaks in pipes, washing machines or shower trays, on the other hand, cause localised patches that do not depend on the weather outside.
The role of ventilation
Whatever the source, a lack of ventilation almost always makes the problem worse. In its guidelines on dampness and mould in indoor air, the World Health Organization (WHO) recommends adequate control of temperature and ventilation to avoid excess humidity. It also advocates preventing persistent damp on indoor surfaces or keeping it to a minimum. Homes with new, well-sealed windows but no air renewal, and rooms that stay cold all day, create ideal conditions for condensation.
| Type of damp | Where it usually appears | Most common signs | When it gets worse |
|---|---|---|---|
| Condensation | Windows, ceiling corners, external walls, behind furniture | Wet glass, black spots, musty smell | Cold mornings, poorly ventilated rooms, clothes drying indoors |
| Penetrating damp | Façades, top-floor ceilings, around window frames | Spreading patches, blistered paint, damaged plaster | After heavy rain and wind |
| Rising damp | Lower part of ground-floor walls | Damp band, salt marks, damaged skirting boards | Continuously, with variations throughout the year |
| Water leak | Near pipes, appliances, baths and shower trays | Localised patch that never quite dries | Unrelated to the weather |
Mould, damp and health: who is most sensitive
Living in a damp or mouldy home is associated with an increased risk of respiratory symptoms, respiratory infections and worsening asthma, according to the WHO guidelines. The same document also reports some evidence of an association with allergic rhinitis.
The impact is not the same for everyone. UK Government guidance identifies the following groups as most vulnerable:
- people with allergies, asthma or other respiratory conditions;
- children and babies;
- older people;
- pregnant women and women who have recently given birth;
- people with a weakened immune system, for example due to medical treatment.
For anyone already living with respiratory allergies, damp in the home deserves extra attention. Filtrarte has dedicated articles on allergic rhinitis and allergic asthma, with information on symptoms and prevention. If symptoms persist, a doctor should always assess them.
How to remove mould from walls safely
Removing mould from walls deals with the visible part of the problem, but the result only lasts if you fix the source of the damp. Cleaning without tackling the cause is, in practice, simply postponing the return of the patches.
Before you start
The EPA guide on mould and moisture recommends protection during cleaning: gloves, goggles without ventilation holes and a respirator mask (the agency mentions the N95 model). During and after the work, keep the room well ventilated. The same agency advises anyone with health concerns to speak to a health professional before going ahead. Whenever possible, people with asthma or allergies would be wise to leave this task to someone else.
Step by step on hard surfaces
On tiles, glass, window frames and washable walls, the procedure set out by the EPA is simple:
- ventilate the room and move furniture, curtains and other textiles away from the affected area;
- scrub the mould with water and detergent, using a sponge or cloth that can be thrown away afterwards;
- dry the surface completely, because the moisture left behind is exactly what allows the fungi to come back;
- put used cloths and sponges in a sealed bag before throwing them away;
- keep an eye on the area over the following weeks, especially after rainy days.
What to avoid
If you choose chlorine-based products such as bleach, read the label carefully and never mix them with other cleaning products. The EPA warns that mixing bleach with products containing ammonia can release toxic gases.
Another common mistake is painting over the patches. The EPA is clear: you should not paint or caulk mouldy surfaces, and you should only paint once the area is clean and completely dry. In porous or absorbent materials, such as suspended ceilings, plasterboard or carpets, mould can penetrate inside. In these cases, the agency acknowledges that these materials may need replacing.
When to call in a professional
The EPA considers that, as a rule, occupants can clean a mouldy area smaller than about 1 m² themselves. Above that size, or in any of the following situations, a professional assessment is justified:
- the patches come back shortly after cleaning;
- there are signs of penetrating damp, cracks in the façade, broken roof tiles or deteriorated window frames;
- the damp seems to be coming from inside the walls or from under the floor;
- a leaking pipe is suspected;
- mould appears in ventilation or air-conditioning ducts, in which case the EPA advises against running the system until it has been checked.
In blocks of flats, when the source lies in a common area such as the façade or the roof, you should report the matter to the building management company or residents’ association.
Habits that reduce damp in the home
Damp in the home largely comes from everyday life. Small changes to daily routines reduce the amount of vapour left in the air and, with it, condensation on cold walls.
Drying clothes
Drying clothes indoors is one of the sources of vapour that UK guidance mentions most often. When there is no outdoor option, the best approach is to use a room with the door closed and the window ajar. Alternatively, place the drying rack next to a window, away from the bedrooms. Tumble dryers should vent to the outside, as the EPA recommends for appliances that produce moisture, or have a condensation tank that you empty regularly.
Bathing and the bathroom
The bathroom concentrates a lot of vapour in a small space. The EPA advises switching on the extractor fan while showering, and it pays to leave it running for a while afterwards. Squeegeeing the water off the shower walls, keeping the door closed while you wash and opening the window afterwards are quick habits with a visible effect. Filtrarte’s article on shower cleaning explains how to prevent limescale and mould in this part of the home.
Cooking
Soups, stews and boiling water release large amounts of vapour. Using the cooker hood or opening the window whenever you cook is the EPA’s recommendation, and covering pans also reduces the vapour that spreads through the home.
Ventilate every day
Opening the windows for a few minutes, preferably on opposite sides of the home to create a draught, refreshes the indoor air even on cold days. Keep window and bathroom air vents clear. Furniture placed against external walls also benefits from being moved a few centimetres away, so that air can circulate behind it.
Heat consistently
Rooms that are very cold during the day and only heated briefly end up with icy walls, where vapour condenses easily. UK guidance specifically links a lack of heating and poor energy efficiency in homes to the appearance of damp and mould.
Measure relative humidity
A hygrometer, a simple and easy-to-find device, shows the relative humidity of each room. The EPA recommends keeping it below 60% and, ideally, between 30% and 50%. Persistently high readings in a bedroom or living room indicate that ventilation or heating is not keeping pace with the moisture produced. In these cases, a dehumidifier can help bring the levels down.
Indoor air quality: what changes in a damp home
Indoor air quality is the set of characteristics of the air you breathe at home, from humidity to odours and airborne particles. It becomes more important when you realise how much time people spend indoors. According to the EPA, most people spend around 90% of their time in enclosed spaces, so a large part of exposure to air pollutants happens at home.
In a damp home, mould is often accompanied by a musty smell and the heavy air you notice when entering a room that has been shut up. The first step is always the same: deal with the source of the water and ventilate.
Ventilation first, purification as a complement
The EPA guide on air purifiers is explicit about the order of priorities. The most effective ways to improve indoor air are to reduce sources of pollution and to ventilate with outdoor air. Filtration is an effective complement to these two measures, without replacing them. The same agency stresses that portable air purifiers do not fix the cause of mould or of musty smells.
Once the damp is under control, an air purifier helps reduce particles and odours in the air of a room. The FilterQueen Defender, one of the purifiers sold by Filtrarte, is an example of this type of appliance. It combines a fine particle filter with an activated carbon wrap for odours and volatile organic compounds, and is suitable for spaces of up to 65 m².
Textiles, dust and regular cleaning
Sofas, rugs, curtains and bedding collect dust and absorb the odours of a damp home. Textiles with visible mould need a thorough wash or, if the stain will not come out, replacing. Otherwise, regular cleaning with equipment that traps dust instead of releasing it back into the air makes a difference to the atmosphere at home.
The FilterQueen Majestic is a cleaning system for floors, rugs, sofas and other textiles which, according to the manufacturer’s description, filters the air as it vacuums, through multi-stage filtration. Like any cleaning equipment, it does not replace fixing the damp, but it fits into a routine designed for a home with less dust. Filtrarte’s air purification solutions are gathered in the Air category.
Quick checklist for autumn and winter
Before the dampest months reach their peak, a simple check helps avoid surprises:
- check that air vents and extractor fans work and are clean;
- inspect gutters, valley gutters and window seals before the heaviest rain;
- move furniture that stands against external walls away from the wall;
- set aside a place to dry clothes with the window ajar;
- air the home every day, even when it is cold;
- put a hygrometer in the bedroom and living room and keep track of the readings;
- clean the first patches straight away, without forgetting to look for the source of the damp.
Frequently asked questions about mould and damp at home
Why does mould appear on bedroom walls?
In most cases, because of condensation. During the night, in a closed room, water vapour builds up in the air and condenses on the coldest areas, such as the corners of the ceiling, external walls and the area behind the headboard or wardrobes. Airing the bedroom every morning and moving furniture away from external walls are the first steps.
How to remove mould from walls without damaging the paintwork?
On washable walls, the EPA advises scrubbing with water and detergent and drying the surface completely. With more delicate paints, it is best to test first on an inconspicuous area. If the mould has penetrated the plaster or plasterboard, surface cleaning may not be enough and an assessment by a professional is justified.
Does bleach get rid of mould on walls?
Bleach can lighten the patches, but it does not fix the cause, and the mould will come back as long as the damp remains. Anyone using it should follow the label, ventilate the room and never mix it with other products, especially those containing ammonia, because of the risk of releasing toxic gases.
Does painting over mould solve the problem?
No. The EPA advises against painting or caulking mouldy surfaces. Clean the area and let it dry completely before any painting, and fix the source of the damp so that the patches do not reappear under the new paint.
What is the ideal humidity level indoors?
The EPA recommends keeping indoor relative humidity below 60%, ideally between 30% and 50%. A hygrometer lets you monitor these levels in each room and see where ventilation needs to be improved.
Does an air purifier get rid of mould?
No. According to the EPA, air purifiers do not fix the cause of mould or of musty smells, which always lies in the damp. Once you have corrected the source of the problem and cleaned the surfaces, a purifier works as a complement to ventilation in reducing particles and odours in the air.
Does it make sense to open the windows in winter?
Yes. Airing for a few minutes a day, preferably creating a draught, refreshes vapour-laden air without significantly cooling the home. It is one of the simplest and most effective measures against condensation.
Content developed by the Filtrarte well-being team.
This content is for information purposes only and does not replace the assessment or guidance of a doctor or other health professional, nor the diagnosis of a qualified technician.
