Damp, condensation or a leak? How to tell them apart
They look similar and they are not the same problem. Getting this wrong is the most expensive mistake in the whole of retrofit, because the fix for one does nothing at all for the others.
Almost everyone who calls us about damp says the same word. Almost nobody has the same problem. Before anyone quotes you for anything, it is worth ten minutes working out which of three things you are looking at.
Condensation — the common one
Where it is: corners of rooms, behind furniture pushed against an outside wall, around window reveals, on the ceiling above a bathroom. Often several places at once, and usually on the cold side of the house.
When it is worst: winter, and first thing in the morning. It eases off in summer.
What it looks like: black speckled mould rather than a stain. Water sitting on the glass. It wipes off and comes back.
This is warm moist air meeting a cold surface. Every household makes moisture — cooking, washing, drying clothes, breathing. If the air cannot get out and the surfaces are cold, it condenses on them and mould grows on what is left behind.
The counter-intuitive part. Condensation usually gets worse after a house is made more airtight — new windows, draught-proofing, insulation — because the moisture that used to leak out no longer can. That is not a fault in the work. It is the ventilation half of the job not having been done.
Penetrating damp — water getting in
Where it is: one patch, on one wall or ceiling. Not spread around the house.
When it is worst: during and after heavy rain, and it dries back between times.
What it looks like: a stain with a defined edge, often darker in the middle. Sometimes salts on the surface.
Something is letting water through: a cracked render, a failed gutter, a slipped tile, a blocked downpipe, perished pointing, or a window seal. The fix is to that thing. No amount of ventilation or insulation will touch it.
Rising damp — the rare one
Where it is: low down, along the bottom of a wall, at a fairly even height. Ground floor only.
When it is worst: fairly constant, all year.
What it looks like: a tide mark, sometimes with a white salty bloom, and skirting or plaster breaking down at the base.
Genuine rising damp is far less common than the number of people selling treatments for it would suggest. A great many diagnoses of rising damp turn out to be a bridged damp proof course, raised external ground levels, or a leak. Ask what was measured, and what the reading was.
The quick comparison
| Condensation | Penetrating | Rising | |
|---|---|---|---|
| Where | Corners, windows, cold walls | One defined patch | Low along a wall |
| When | Winter, mornings | After rain | All year |
| Looks like | Black speckled mould | Stain with an edge | Tide mark, salts |
| Spread | Several places | One place | One wall, evenly |
| The fix | Ventilation, then insulation | Repair the defect | Investigate first |
Why the diagnosis matters more than the fix
Each of these has a different answer, and they do not overlap:
- Condensation is a ventilation problem. Extract fans that actually work, positive input ventilation, or heat recovery — and then insulation, to raise the surface temperatures the moisture is condensing on.
- Penetrating damp is a building repair. Find the defect, fix the defect.
- Rising damp needs measuring before anyone specifies anything, because it is the one most often diagnosed wrongly.
Spend money on the wrong one and you have spent money. The problem is still there.
What we would do
A damp and mould survey takes moisture readings at depth rather than at the surface, records where and when the problem appears, and gives you a written diagnosis. You keep it whoever does the work.
We will tell you if it is a repair rather than a retrofit job, and that is frequently the answer.
Sources. This is a guide to telling three problems apart, not a diagnosis. Moisture has to be measured at the property before anybody specifies work.