The short answer
The location is the diagnosis. Behind furniture on an external wall is condensation. A defined patch that worsens after rain is penetrating. A band along the bottom of a wall is worth a proper survey.
- Corners and behind wardrobes on external walls: condensation, caused by no air movement against a cold surface.
- Around window reveals: cold bridging, where the structure conducts cold through the wall at the frame.
- A patch on one wall that darkens after heavy rain: penetrating, and usually a gutter, a downpipe or the pointing.
- Ceiling, in the corner of an upstairs room: roof or flashing.
- Along the bottom of a wall with a tide line and white salts: possibly rising, and worth an independent survey rather than a free one.
What will ruin it
Mould growing on a north-facing external wall behind a sofa is not a building fault and no damp-proofing contractor can fix it. Moving the sofa four inches and running the heating low and steady will.
Why it works
Mould needs moisture and a surface, and it grows first wherever the surface is coldest and the air is stillest. That is why it maps the physics of the room so reliably.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.
More damp
Which kind of damp you haveHow to stop condensation on bedroom windowsHow to remove mould from a wall properlyIs a dehumidifier cheaper than heating the roomWhat to do about damp in a rented homeAppliance Running Cost CalculatorWhat any appliance costs per use, per week and per year, plus eighteen worked out for you at today's rate.Open the calculator →