"Damp" isn't one problem — it's three, with different causes and different fixes. Getting the diagnosis right matters, because treating the wrong one wastes money and can leave the real cause untouched.
Condensation
By far the most common cause of damp and mould, especially in colder months. Warm, moist air from cooking, showering and breathing meets a cold surface and condenses. Usually shows as mould on cold spots — external wall corners, behind furniture, around window reveals — rather than a rising tide-mark.
Penetrating damp
Water getting in from outside — a failed gutter, cracked render, a missing roof tile, poor pointing. Shows up as a localised, often irregular patch, usually worse after rain, and often traceable to a specific defect on the outside of the building.
Rising damp
Groundwater drawn up through a wall by capillary action. Genuinely rare compared to how often it's diagnosed — many "rising damp" call-outs are actually condensation or penetrating damp misread by a surface moisture meter, which can't distinguish between the two. A proper diagnosis uses comparative readings, not a single number.
Why the distinction matters
- Condensation is fixed with ventilation and heat, not a damp-proof course.
- Penetrating damp is fixed by repairing the defect letting water in — treating the wall internally without fixing the source doesn't work.
- Genuine rising damp is uncommon enough that it should be confirmed by proper diagnosis, not assumed.