Where this starts
Damp on an internal wall is a symptom with three unrelated families of cause, and the reason the distinction matters is that each remedy does nothing for the other two. A dehumidifier does not stop rain getting through a wall, and a chemical damp-proof course does nothing about condensation.
CONDENSATION forms when moist room air meets a surface below its dew point. It is a SURFACE phenomenon, so it appears where surfaces are coldest — external corners, behind furniture against an outside wall, at lintels and reveals, at the junction with an unheated space — and it is worst in winter, in rooms that generate moisture, and where air movement is poor. Black mould is its signature.
PENETRATING damp comes through the wall from outside. It follows the WEATHER, appears in a patch rather than a pattern, and is usually traceable to a defect on the other side: a failed gutter or downpipe, cracked render, a bridged cavity, a blocked weep, defective pointing, or a sill throwing water back at the wall.
RISING damp comes up from the ground by capillarity and is confined to the bottom of the wall, with a fairly level upper limit, often with a salt band at that line. It is far less common than the other two and is diagnosed considerably more often than it occurs, which is worth knowing before anybody proposes an injected course.
The observations below separate them, and they are all things a reader can make themselves before anyone is called.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Condensation on a cold surface
Most commonWorst in winter and in rooms that produce moisture. Appears in corners, behind furniture, at reveals and lintels — the coldest spots — with black mould rather than a tide mark. Improves with ventilation and heating.
How to check: Whether it is seasonal, and whether it sits where the wall is coldest. Move the furniture away from an external wall and see whether the pattern follows it.
A thermal bridge concentrating the cold
CommonCondensation or mould in a repeating line or a specific spot rather than generally — along a lintel, at a floor junction, around a window, or tracing studs or joists.
How to check: Whether the affected areas form a geometric pattern. Mould that traces structure is following the cold, not the water.
Water coming through from outside
CommonAppears or worsens after rain, especially driving rain from one direction, and is a patch rather than a pattern. The wall may dry out between wet spells.
How to check: Note whether it tracks the weather, then look at the same spot from outside during rain. A failed gutter or downpipe shows itself immediately.
A leak from plumbing or a roof
CommonPresent regardless of the weather, often getting steadily worse, sometimes warm, and frequently below a bathroom, a pipe run or a roof junction.
How to check: Whether it is independent of rain. Damp that does not care about the weather and does not dry is usually a leak.
Something bridging the cavity or the damp-proof course
Less commonA patch at a consistent height, or damp above a raised external level — a path built or grown across the barrier rather than a failure of it.
How to check: Compare the external ground, paving or render level against the damp-proof course. Ground raised above it, or render carried down past it, is a bridge.
Rising damp
UncommonConfined to the bottom of the wall with a fairly level upper edge, often a salt band there, present year-round and not following the weather.
How to check: Whether the upper limit is level and low. Damp that extends well up the wall, or that varies with rain, is one of the causes above rather than this one.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Height and shape do most of the separation on their own: low with a level top, a patch at mid-height, or corners and cold spots each point at a different family.
- 2
Seasonal and worse in winter points at condensation; tied to rain points at penetration; constant and worsening points at a leak. This single observation eliminates whole branches.
- 3
Most penetrating damp has a visible external cause, and a downpipe overflowing onto a wall identifies itself in seconds.
- 4
Bridging is common, cheap to fix, and routinely misdiagnosed as a failed damp-proof course — which is an expensive thing to be wrong about.
- 5
If the answer is condensation, the remedy is moisture, ventilation and surface temperature. Nothing done to the wall's fabric will help.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where damp persists year-round, does not follow the weather, and does not respond to ventilation — something is delivering water and finding it matters more than treating the surface.
- Before agreeing to an injected damp-proof course. Rising damp is diagnosed far more often than it occurs, and the observations above eliminate it in most cases.
- Where timber in contact with the damp area is soft, discoloured or crumbling, which turns a damp problem into a structural one.
- Where mould keeps returning over a substantial area after cleaning and ventilation, particularly where anyone in the building has a respiratory condition.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.