Damp and mould explained

Damp is a symptom with three usual causes, and they need different fixes. Condensation forms inside, penetrating damp arrives through the fabric from outside, and rising damp comes up from the ground. Getting the diagnosis right matters more than the treatment, because the wrong fix wastes money while the water keeps arriving.

Three problems that look alike

All three end up as a mark on plaster, a smell and mould on a cold surface. The clues are in when the damp appears and where it sits. Condensation is worse in cold weather, on external walls and in bathrooms and bedrooms. Penetrating damp gets worse after driving rain and tends to be localised to one wall or a ceiling under a gutter. Rising damp appears low down, brings a tidemark, and leaves white salt deposits in the plaster.

Before accepting any diagnosis, look for the boring explanations. A leaking gutter, a crack in the render, a blocked air brick under a timber floor, a dripping pipe under a bath or a tumble dryer venting into a room will all produce damp that looks serious. Those fixes cost little and solve the problem permanently.

• Condensation - Worse in cold weather, spread over cold surfaces, often with black mould.
• Rising damp - Low level, tidemark, salts in the plaster, and rarer than the sales patter suggests.

Condensation: the most common cause

Every home produces water vapour. Cooking, showering, drying laundry indoors and breathing add litres of it to the air each day. When that air meets a surface colder than its dew point, the vapour turns back into liquid on the wall, the window or the corner behind a wardrobe. The mould that follows is a symptom of the moisture rather than a separate problem.

The fix is ventilation plus steady warmth. Run the extractor during and for at least ten minutes after a shower, and check it moves air by holding a sheet of tissue against the grille. Keep trickle vents open and clear of furniture and curtains. Dry washing outside or in a vented dryer. Keep furniture a few centimetres off external walls so air can move behind it, and aim for humidity between about forty and sixty percent, checked with an inexpensive hygrometer.

• Extractor fans - Run them longer than feels necessary. Twenty minutes after a shower.
• Consistent heating - Short bursts in one room push moisture onto the coldest walls elsewhere.

Penetrating damp and leaks

Water entering from outside leaves a mark that grows after storms and spreads from an edge of a wall or ceiling. Blocked or leaking gutters cause a large share of it, because water overflowing behind the fascia runs down inside the wall. Cracked render, failed pointing, perished mastic around windows and old flashing around a chimney do the rest.

Ceilings are a useful diagnostic. A brown ring that spreads after rain points upward to a roof, a flashing or a gutter above. Greyish staining with no definite edge, or black spotting at a cold corner, points at condensation. If a ceiling under a bathroom is damp and the plumbing above is sound, check the sealant around the bath before blaming the roof. Tracing a leak often means opening a small area and looking.

• Gutters first - The cheapest thing to check and a common cause of damp walls.
• Render and pointing - Cracks let water in slowly and become internal damp over a winter.

Rising damp: rarer than it is sold as

Rising damp happens when the damp proof course in a wall has failed, been bridged, or never existed. In houses built before the late Victorian period there may be no course at all, and water wicks up through the brickwork. The pattern is a stain and a tidemark low on the wall, often with white salts, and it is usually worse where soil or a raised path sits against the outside.

It is diagnosed far more often than it occurs, because chemical injection is easy to sell. Before accepting it, check that ground level outside has not been raised above the course, and that a leaking downpipe is not saturating the base of the wall. An independent surveyor with no stake in selling a treatment is the right first call. If the course has genuinely failed, the remedy is injection or a membrane, and both are disruptive.

• Check outside - Raised ground, soil against the wall and blocked gullies are common culprits.
• Independent survey - Use someone who does not sell damp proofing to diagnose it.

Cleaning mould and stopping it returning

Mould spores are always in the air. They grow where there is moisture and a food source, and household dust is food enough. Clean affected surfaces with a mild detergent solution or white vinegar, wear gloves, and ventilate the room while you work. Do not dry brush a large area of mould, because that flings spores into the air. Curtains, underlay and soft furnishings that have stayed wet need replacing rather than cleaning.

Painting over mould with an anti-mould paint on a damp wall is a short delay rather than a repair. Fix the moisture source first, then redecorate. Sealant around baths and sinks should be replaced every few years, since black spots behind it mean water is sitting under the seal. Where a cold bridge keeps one corner damp, insulating that wall is the only permanent answer.

• Clean safely - Damp cloth with detergent or vinegar, gloves, window open, no dry brushing.
• Dehumidifier - Useful while drying out after a leak, not a substitute for ventilation.