How do I tell rising damp from condensation and penetrating damp?

The three have completely different causes and completely different fixes, and misdiagnosis is expensive — most damp diagnosed and treated as rising damp is actually one of the other two.

Condensation is by far the most common. Warm moist air meets a cold surface and water forms. The signs: it appears on cold surfaces — external walls, window reveals, behind wardrobes, in corners — rather than at a particular height. It is worse in winter, worse in kitchens and bathrooms, and typically shows as black spotted mould rather than a stain. There is no water coming in from outside; the water was generated inside by cooking, washing, drying laundry and breathing. The fix is ventilation, heating and insulation, not chemicals.

Penetrating damp is water crossing the wall from outside through a specific defect. The signs: a defined patch that darkens after rain and dries afterwards, at any height including upper floors. Look for the cause directly outside — a blocked or leaking gutter, cracked render, a failed roof flashing, a bridged cavity, a blocked airbrick, or ground level raised above the damp-proof course. Fix the defect and the damp stops.

Rising damp is groundwater drawn up through masonry by capillary action. Its distinguishing feature is that it is confined to the base of ground-floor walls, typically no higher than about a metre, with a fairly level upper edge often marked by a tide line of salts. It cannot occur upstairs. It requires a failed, bridged or absent damp-proof course.

Rising damp is genuinely rare in houses built with a DPC. If a free survey from a company that sells damp-proofing diagnoses it, an independent surveyor's second opinion is worth the fee.