Where this starts
A cold water pipe carries water at mains temperature, which in winter can be close to five degrees and in summer is still well below room temperature. Air touching a surface colder than its dew point gives up its moisture onto that surface, and that is all condensation is — the pipe is not losing water, it is collecting it from the room.
THE DRY-CLOTH TEST SETTLES IT. Wipe a metre of pipe completely dry, including underneath, and look again after twenty minutes. Condensation returns everywhere at once, as an even film or as beads along the whole length, and it returns on the underside as much as the top. A leak returns at ONE point — a joint, a fitting, a pinhole — and runs downhill from there, so the pipe above the source stays dry.
WHERE AND WHEN narrows it further. Condensation is worst where the air is warm and humid and the pipe is exposed: a bathroom, a utility room, a kitchen, a cellar in summer. It is worse in humid weather and often disappears in a dry cold spell, which is the opposite of what people expect. A leak does not care about the weather.
WHY SUMMER IS WORSE, which surprises people: the dew point depends on how much moisture the air holds, and warm summer air holds a great deal more than cold winter air. So a cellar or an underfloor void that is bone dry in January can run with water in July, with the same pipes and the same water in them.
THE DAMAGE IS REAL EITHER WAY. Condensation dripping onto a ceiling, into insulation, onto a floor or into a cupboard rots and stains exactly as a leak does, and because it is spread along a run rather than concentrated it can wet a much larger area. Deciding it is 'only condensation' and leaving it is how a joist under a bathroom fails.
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 uninsulated cold pipework
Most commonWater reappearing evenly along the whole run including the underside, worse in humid weather, on bare copper or plastic in a warm room.
How to check: Dry a length and watch. Even return along the whole pipe is the air, not the pipe.
High humidity in the space
Most commonCondensation on pipes together with damp walls, misted windows or a musty smell, often in a cellar, a utility room or an unventilated bathroom.
How to check: Whether other cold surfaces in the room are also wet. If the windows and the walls are doing it too, the room is the problem.
A leak at a joint or a fitting
CommonWater starting at one point and running downhill from it, with the pipe above dry, sometimes with a green or white deposit at the joint on copper.
How to check: Whether the pipe is dry above the wet part. A leak has a top edge; condensation does not.
Insulation fitted but not sealed
CommonLagged pipes still wet, with water inside the insulation, where the lagging has open seams, gaps at fittings, or is not sealed at the ends.
How to check: Whether the insulation is continuous and closed. Air reaching the pipe through a gap condenses inside the lagging where it cannot dry.
A cold surface other than the pipe
Less commonWater apparently from a pipe but actually forming on a cistern, a valve body, a meter or a cold tank nearby and running along the pipe.
How to check: Follow the water back uphill. A WC cistern full of cold water sweats as readily as a pipe and is often blamed on the pipe beneath it.
A pinhole in the pipe itself
Less commonA fine spray or a persistent wet point mid-run with no joint there, often on older copper in an aggressive water area, sometimes with a green stain building up.
How to check: Whether the wet point is at a joint or in plain pipe. Plain pipe leaking is corrosion and the rest of that run is the same age.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Even return along the whole run including the underside is condensation; return at one point with dry pipe above it is a leak. Nothing else separates them as quickly.
- 2
Misted windows, a sweating cistern and damp walls together mean the room's humidity is the cause, and insulating the pipe alone moves the condensation elsewhere.
- 3
Worse in humid weather and absent in a dry cold spell is condensation. A leak is indifferent to the forecast.
- 4
Insulation with open seams or gaps at fittings lets air reach the pipe and then traps the water it deposits, which is worse than no insulation at all.
- 5
Condensation spread along a run wets a larger area than a drip, and a joist, a ceiling or a cupboard floor under it rots exactly as it would under a leak.
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 the water is arriving at one point with dry pipe above it, which is a leak and will not improve.
- Where plain pipe rather than a joint is weeping, since that is corrosion and the rest of that run is the same age and the same water.
- Where condensation has been running onto a floor structure, a ceiling or into insulation long enough to stain or soften it, because the damage is now the problem rather than the water.
- Where the space is a cellar or a void with high humidity throughout, which is a ventilation and moisture question about the space rather than about the pipes in it.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.