Condensation, penetrating or rising damp — which have you got?
"Damp" isn't one thing. There are three quite different mechanisms, they have different causes and different fixes, and treating the wrong one is how people end up spending thousands and still having a wet wall. Here's how to tell them apart.
1. Condensation — by far the most common
Condensation happens when moist indoor air meets a cold surface and the water drops out of it — the same way a cold drink "sweats" in summer. It shows up as black mould in corners, behind furniture, around windows and on cold external walls, often worse in winter and in bathrooms and kitchens. The cause is a combination of moisture, cold surfaces and not enough ventilation — not a defect in the wall itself.
2. Penetrating damp — water getting in from outside
Penetrating damp is water finding its way through the building fabric from outside: a leaking gutter or downpipe, cracked render, failed pointing, a slipped tile, a leaking window, or driving rain on an exposed wall. It tends to appear as a localised patch that worsens after rain, often at a level that has nothing to do with the ground.
3. Rising damp — real, but rare
Rising damp is ground moisture drawn up through a wall by capillary action where there's no effective damp-proof course. It's characterised by a tide mark low down on the wall and ground salts left behind by evaporation. It genuinely exists — but it is diagnosed far more often than it occurs, because the firms doing the diagnosing are usually the ones selling the injected damp-proof course that "treats" it.
How a proper diagnosis tells them apart
You can't reliably tell these apart by eye — they can look similar and often occur together. Forensic diagnosis uses several lines of evidence together:
- Comparative moisture readings at the surface and at depth, to map where the water actually is.
- Thermal imaging to reveal cold bridges and trace where moisture is sitting.
- Dew-point and humidity analysis to confirm or rule out condensation.
- Salt analysis where rising damp is genuinely in question.
Get the diagnosis right and the fix is often far cheaper and less invasive than the treatment you'd have been sold. That's the whole point of an independent survey: the answer isn't decided before we arrive.