In brief
Condensation forms when humid air meets a cold surface. Penetrating damp involves moisture entering through a defect. Low-level wall moisture can involve a failed or bridged damp-proof course, ground levels, leaks, salts or other causes. The mark alone will not tell you which one it is.
Humidity + cold surfaces
- Water on glazing
- Mould at corners or behind furniture
- Worse overnight or during colder periods
- Improves when airflow increases
A water-entry path
- Changes after rainfall
- Localised around an external wall or opening
- Nearby gutter, roof, pointing or render defect
- Staining may be away from the entry point
Lower-wall moisture
- Ground-level or bridging issues
- Plumbing or drainage leaks
- Salt-contaminated plaster
- Damp-proof course condition
Give the surveyor a timeline.
- Photograph the same area in dry and wet weather where relevant.
- Note whether it changes overnight, after showers or cooking, or after rain.
- Record recent window, insulation, roof, plumbing or plaster work.
- Do not redecorate or bleach the area immediately before inspection if it is safe to leave.
- Make the wall and nearby outside area accessible where possible.
Use the whole pattern.
Does black mould prove the problem is condensation?
No. Condensation is a common reason that a surface stays damp enough for mould, but leaks or persistently damp fabric can also contribute. Check timing, location, humidity and building defects together.
Does a damp meter prove rising damp?
No. A conductivity meter can identify a pattern worth investigating, but salts, materials and surface conditions affect the readings. A diagnosis needs context and more than one consistent clue.
Why does a patch get worse after rain?
That timing raises the likelihood of external water entry, but the entry point may not sit directly behind the visible patch. Roof details, gutters, walls, openings and internal plumbing still need checking.