Why does mould keep coming back?

You scrub the black spots off the bedroom corner. Three weeks later they are back. That is the most common story we hear, and it has a simple explanation: mould is a symptom of moisture, and the moisture is still there.

What mould needs to grow

Mould needs three things: moisture, a surface to feed on and still air. Paint, plaster, wallpaper paste and dust all count as food, so every home has plenty. The only part you can control is moisture.

The five usual causes in Irish homes

1. Condensation from everyday living

This is the cause in most homes we survey. Cooking, showering, drying clothes and breathing put litres of water into the air every day. When that warm, damp air touches a cold wall or window, it turns back into water. The wall stays slightly wet all winter, and mould moves in.

What stops it: taking damp air out where it is made. See our ventilation solutions.

2. A bathroom fan that does not do its job

Many fans are too weak for the room, switch off with the light, or blow into the attic instead of outside. The steam from one shower then hangs around for hours.

What stops it: a correctly sized fan, ducted outside, that keeps running until the air is dry.

3. Cold spots and furniture against outside walls

Corners, window reveals and the wall behind a wardrobe are the coldest surfaces in a room, and air does not move there. That is why mould appears in the same places every year.

What stops it: better air movement, a small gap behind furniture and, in some homes, insulation of the cold spot.

4. Drying clothes indoors

One load of washing releases around two litres of water as it dries. In a closed room that is enough to push humidity well past the level mould likes.

What stops it: drying in a ventilated room with the door closed, or using a vented or condenser dryer.

5. Leaks and penetrating damp

A leaking pipe, a blocked gutter or a cracked render lets water in from outside. This mould often has a tide mark or a brown stain, and it does not follow the seasons.

What stops it: repairing the leak first. No mould treatment lasts on a wall that is still getting wet.

Why bleach makes it look fixed

Bleach removes the colour from mould, so the wall looks clean. On porous surfaces such as plaster, the growth below the surface often survives and returns. We compare the options in bleach versus eco mould removal.

How to break the cycle

1. Find out where the moisture comes from. A humidity meter and a moisture reading on the wall tell you more than the stain does.
2. Remove the mould properly, including what has grown into the surface.
3. Fix the cause, which is ventilation in most cases.
4. Only then repaint. See can you paint over mould?

If the same patch has come back more than once, the cause has not been found yet. That is what our free survey is for: mould removal in Dublin.