Why mould appears on walls and how to prevent it

Mould appears when damp stays trapped in a wall and there is little air moving. It needs very little: a cold surface, humid air and a few days. To prevent it, you have to remove the damp at its source. Cleaning the stain only works for a short while.

What mould is

Mould is a fungus. It feeds on damp and on the dust or dirt that builds up on walls. It shows up as dark, green, grey or even brownish patches. It often has a musty smell, like a closed-up house.

Mould spores float in the air of every home. It is not possible to get rid of them completely. What you can do is avoid giving them the conditions to grow.

The most common causes

In most homes, the cause is one of these three problems:

Condensation: water vapour in the air (from showers, cooking, drying laundry, breathing) hits a cold wall and turns into water.
Leaks and penetrating damp: water gets in from outside, through a crack, a roof, a blocked gutter, a poorly sealed window or a leaking pipe.
Rising damp: water rises from the ground up through the wall, especially in old houses and on ground floors.

Condensation is the most frequent cause of mould in corners, behind furniture and around windows. If you would like to learn to tell the three cases apart, see our article on condensation, leaks and rising damp.

Where it usually appears first

Mould likes cold places with no ventilation. Look out for these spots:

corners of bedrooms, especially on north-facing walls or walls that face the street;
behind wardrobes, beds and sofas pushed up against the wall;
around windows, where the glass and frame are colder;
on the ceiling and walls of the bathroom and kitchen;
on walls with a thermal bridge, meaning a concrete or pillar area that is colder than the rest of the wall.

How to stop it coming back

First, find the cause. If the stain always appears after rain, water is getting in from outside. If it appears in the cold months and in corners, it is probably condensation. Each cause has its own solution.

For condensation, these habits help a lot:

Open the windows every day, even if only for ten to fifteen minutes, preferably creating a through-draught between two windows.
Use the extractor fan or open a window during and after showers and when cooking.
Put lids on pans when cooking.
Avoid drying laundry indoors without ventilation.
Leave a gap between furniture and the wall so air can circulate.
Keep a steady temperature. Very cold walls attract condensation.

For leaks, the solution is to repair the source: a gutter, a roof, a crack in the façade or a pipe. Painting before fixing the water ingress is money wasted.

The climate in each area

In the Sintra area, the air is more humid and there is more fog, so walls take longer to dry. In Cascais and Oeiras, by the sea, sea spray brings salt and moisture into the air. In Lisbon and Amadora, many older buildings have thin or poorly insulated walls, where condensation is common in winter. In any of these areas, ventilating regularly makes a difference.

Tip: If the mould is extensive, always comes back after cleaning or has a strong smell, ask a professional to assess the wall before painting.

When to paint

Only paint when the wall is dry and the cause has been dealt with. After cleaning and drying, apply a coat of primer and then the paint. If you are not sure the wall is dry yet, wait and observe it for a few weeks.