Condensation, rising damp or penetrating damp: how to tell which one you have

"We've got damp" is like saying "the car's making a noise" — the diagnosis is everything, and the three most common damp diagnoses are treated with three completely different fixes. Condensation is a moisture-and-airflow problem; rising damp is ground water climbing the wall; penetrating damp is water leaking in from outside. Spend on the wrong one and nothing improves. Here is how the pros tell them apart, using nothing more than your eyes, the weather, and a square of kitchen foil.

Condensation: the cold-surface problem

The tell-tales: black mould and water droplets on the coldest surfaces — window glass and frames, external corners, the wall behind the wardrobe or bed where air never moves. It's worst in winter, when warm moist air meets cold walls, and it wipes off... and comes back within days. Condensation is produced by living in the house — cooking, bathing, breathing, drying laundry (that's its own mistake piece) — meeting surfaces too cold to hold the moisture. It is by far the most common of the three, it is not a building defect in itself, and its fix is ventilation and surface temperature, not builders (the ventilation guide has the method).

Rising damp: the ground-floor tide mark

The tell-tales: dampness confined to the ground floor, climbing the wall evenly to a roughly horizontal line — the "tide mark" — usually no higher than about a metre, often with white salt crystals blooming at its edge (ground water carries salts that crystallise as the wall dries). Skirting boards rot, paint peels in that band, and it barely changes with the weather or your lifestyle. Rising damp is a building-fabric problem — a failed or bridged damp-proof course — and it is the rarest of the three and the most often diagnosed, because treatments are lucrative. An honest word the industry rarely gives: get an independent surveyor's opinion before anyone sells you an injected damp-proof course, and treat "rising damp" diagnosed from a single photo as a sales claim, not a finding.

Penetrating damp: the leak that moves with the weather

The tell-tales: irregular patches — not a horizontal line, not corners — that grow noticeably during or after rain, on walls near obvious external defects: cracked render, faulty guttering, a downpipe joint, window seals, a roof tile, a chimney. It can appear at any height, including ceilings, and it doesn't respond to ventilation because it isn't moisture in the air — it's water in the building. The fix is external: find the defect, fix the defect, let the wall dry. That's roofer and gutter territory, and it's also exactly the disrepair that landlord repair duties cover (the duties guide has who-pays-what).

The foil test, and when to stop diagnosing

Tape a square of kitchen foil flat over the damp patch, seal the edges, leave it 48 hours. Water on the room side of the foil means moisture is arriving in the air and condensing — condensation. Water behind it, between foil and wall, means the wall itself is wet — rising or penetrating, and the location rules above pick between them. Stop diagnosing and call a professional when: the patch grows fast, ceilings are involved, timber is soft or crumbling, mould covers more than about a square metre (health territory — treat remediation as a job for specialists), or you're in a rental and the building is implicated — report it in writing and let the duties machinery work.

Sources: UK property-preservation industry diagnostic guidance (PCA-member firms' published methods); Glass and Glazing Federation condensation guidance. Last reviewed September 2026 · general information, not a substitute for a survey.