If you manage a block of flats or let residential property, the arrival of autumn usually brings the first reports of black mould on walls, ceilings and window reveals. The quick answer is to blame condensation and tell residents to open a window. Sometimes that is right. Often the picture is more complicated, and treating the wrong cause wastes money and leaves the problem in place.
How condensation happens
Warm air holds moisture. When it meets a cold surface, the moisture condenses. Cooking, bathing, drying clothes and even breathing all add moisture to the air in a flat. If that moisture cannot escape, and if parts of the walls or ceilings are cold, mould will follow.
Several things make it worse:
- cold bridges, such as concrete lintels, floor slabs and the corners of external walls
- missing or failed insulation, especially at roof level
- extract fans that are missing, switched off or not working
- windows with no trickle vents, or with vents kept closed
- heating that is used only for short periods
When it is not condensation
Damp patches can also come from rain getting in, through defective pointing, cracked render, blocked gutters or failed roof coverings. Plumbing leaks, failed bath seals and problems in the flat above are common culprits too. These need a repair, not a new fan.
Diagnosing properly
- Look at the pattern. Mould in cold corners and behind furniture suggests condensation. A tide mark or staining that follows a wall below a gutter suggests water getting in.
- Measure, do not guess. Surface temperatures and relative humidity, recorded over several weeks with data loggers, show what is really happening.
- Use moisture meters with care. They can give misleading readings on masonry and salts, so they support a diagnosis rather than make one.
- Check the outside. Inspect the external walls, gutters and roof above the affected rooms.
- Talk to the residents. How the flat is heated and ventilated matters, and should be discussed without blame.
Putting it right
The right remedy follows the diagnosis. That may be better extraction, insulation of a cold bridge, repairs to the outside of the building, or a combination. Mould should be cleaned off safely, but cleaning alone will not stop it returning.
Our Building Surveys and Defect Diagnosis service investigates damp and mould properly before any money is spent. See our damp and moisture investigation of a London block.