How do you stop condensation on cold water pipes?
The short answer
Insulate the pipe. Condensation forms when warm, humid air meets a cold pipe surface, so foam pipe lagging — which keeps that air off the cold metal — stops it. Improving ventilation and reducing humidity in the room helps too. It isn't a leak, but left alone it drips, stains and eventually rots what's underneath.
People often ring about this convinced they have a leak, because there's water on the pipe and a damp patch below it. The giveaway is that the water is on the outside of the pipe along its whole length, rather than coming from one joint, and that it's worse in humid weather or in a room where washing is dried.
The fix is genuinely simple: closed-cell foam lagging, pushed on and taped at the joins, with no gaps. The gaps matter — a bare section will still sweat and drip.
It's worth dealing with rather than ignoring. Persistent condensation in a cupboard or under a floor keeps timber damp indefinitely, and damp timber is a slower, more expensive problem than the pipe ever was.
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