Damp, condensation or mould — telling them apart before you spend money

Almost every damp problem we are called to has been treated before. It came back because the treatment addressed the mould rather than the water.

Mould is not a problem in itself — it is evidence. It grows where a surface stays damp for long enough, and the useful question is always why that surface is damp. There are three common answers, and they need entirely different work.

Penetrating damp

Water coming in from outside through a defect in the building fabric. It usually shows as a patch rather than a band, often worse after heavy or wind-driven rain, and frequently traces back to something obvious once you look outside: an overflowing gutter, a cracked downpipe, blown render, defective pointing, or a slipped tile above.

The tell is that it corresponds to something. A damp patch on an internal wall directly behind a leaking gutter joint is not a mystery.

Rising damp

Moisture drawn up from the ground through the wall, where a damp-proof course is missing, has failed, or has been bridged. It shows as a band along the bottom of a wall, typically up to about a metre, often with a visible tide mark and salt deposits in the plaster.

It is genuinely less common than it gets diagnosed as, and it is worth being sceptical of anyone who reaches for it first. A very frequent cause is bridging — raised external ground levels, a path or patio laid above the damp course, or render carried down past it — and that is a much cheaper fix than a chemical injection.

Condensation

Warm, moist air inside the building meeting a cold surface and depositing its moisture there. It shows up on cold spots — external corners, behind furniture against an external wall, around window reveals — and typically appears as black spot mould rather than a spreading stain. It is worse in winter and worse in rooms that generate moisture.

Condensation is a building performance problem as much as a behavioural one. Insufficient extraction, blocked or disconnected vents, cold bridging and poor insulation all cause it, and none of those are the occupier’s fault.

Why the distinction matters commercially

For landlords and agents, the diagnosis decides who is responsible and what needs doing. Attributing damp to tenant lifestyle without investigating is no longer a defensible position, and the expectation is that a report of damp or mould gets inspected and acted on rather than explained away.

What a proper diagnosis involves

Moisture readings at different heights, an inspection of the outside of the building at the corresponding location, a look at ground levels and drainage, and a check of the ventilation serving the room. It takes an hour or two and it tells you which of the three problems you have — which is the difference between fixing it once and fixing it repeatedly.