Damp has three common causes and they need different fixes

There is a settled way of talking about damp. It is worth asking how much of it survives contact with the detail.

Condensation is the usual answer

Warm moist air meeting a cold surface deposits water, which appears in corners, behind furniture and on external walls. The tell is black spot mould, worse in winter, worse in bedrooms and bathrooms, and worse where air does not circulate.

The fix is ventilation, heating and insulation rather than any chemical treatment. A substantial share of damp diagnosed as something else is this, and the treatments sold for the alternatives do nothing about it.

Penetrating damp comes from outside

Defective pointing, blocked gutters, cracked render, failed flashing and ground levels above the damp-proof course all let water in. It typically shows as a localised patch that worsens after rain and dries slowly. The repair is external, and any internal treatment before fixing the source is money spent on a symptom.

Underneath it, locating the entry point is the hard part, because water tracks along mortar beds, joists and wall ties before it appears, so the stain is frequently well below and sometimes metres sideways from the defect, and an inspection carried out during or immediately after rain shows what a dry-day visit cannot.

Rising damp is real and over-diagnosed

Genuine rising damp involves groundwater drawn up through masonry and is generally limited to around a metre above ground level with a tide mark. Many buildings diagnosed with it turn out to have bridged damp-proof courses, raised external ground levels or condensation.

Live with it a while and independent assessment from someone not selling the treatment is worth paying for. Part of the over-diagnosis is instrumental, since the handheld meters used in most surveys measure electrical conductivity at the surface and hygroscopic salts, foil-backed plasterboard and even recent paint can all read as wet, which makes a diagnosis resting on meter readings alone weak evidence.

Ventilation is usually the cheap fix

Extract fans in kitchens and bathrooms that actually run for long enough, trickle vents left open, and drying washing outside or in a ventilated space address most condensation. Modern airtight retrofits without matching ventilation are a common cause of new damp in previously dry houses.

Humidity meters are inexpensive and turn an argument into a measurement. Two conditions have to hold at once, because condensation needs moisture in the air and a surface cold enough for it to form on, which is why a house heated in short bursts to save money can run damper than the same house held at a lower steady temperature through the day.

Old buildings behave differently

Solid-walled buildings were designed to absorb and release moisture, and sealing them with impermeable materials traps it. Cement render and modern plastics over lime construction are a well-documented cause of damage. Breathable materials — lime plaster, mineral paints — are the appropriate approach and are frequently ignored.

The current version of the same mistake is internal wall insulation fitted to a solid wall without regard to where the dew point then falls, which can move condensation into the fabric behind the insulation where nobody sees it until embedded timber has suffered, and that is a build-up to have designed by a qualified professional rather than specified from a product page.

When to stop diagnosing it yourself

Mould is not only a building problem: damp housing is associated with respiratory symptoms, and anyone in the household with asthma, a compromised immune system or a persistent cough should raise it with a doctor rather than wait for the wall to be fixed. Get a building professional in when the patch keeps returning after ventilation has genuinely improved, when it appears on an internal wall away from any cold surface, or when plaster is blown or timber is soft.

Ask for a diagnosis in writing that names the cause and the evidence for it, and treat a survey from a firm that also sells the remedy as a quote rather than an assessment. A damp problem that has been treated twice without resolving was almost certainly diagnosed wrongly the first two times.

The takeaway

Diagnose before you treat. Most damp is ventilation, and most treatments do not address it.

The best rooms look like a decision, not a purchase.