Is it damp or condensation?

Question

Accepted Answer

The patch itself will not tell you. The timing will. Condensation forms when warm, moist air cools against a cold surface. It tracks the cold and the way you use the room. Penetrating damp is water crossing the structure through a defect, so it tracks the rain. If the wall darkens in the days after heavy rain, something outside is letting water in. If it worsens in cold still weather, with the room in use and the windows shut, you have condensation. That one test beats any photograph. All three kinds of damp leave the same dark patch, at the same height, on the same wall. Appearance is what those comparison tables online are built on. Appearance is also the part that fools people. Mould does not need a wet wall. It needs a humid surface, and a surface turns humid long before it turns wet. The coldest wall in a room runs more humid than the air in the middle of it, which is where your hygrometer reads. So a wall can be dry to the touch and still grow mould. The condensation guide sets out the humidity threshold mould needs and how to measure against it. Someone runs a meter along your skirting and shows you a high number. On its own, that number does not tell you which kind of damp you have. An electrical or carbide moisture meter cannot separate hygroscopic moisture from capillary moisture. Those are two different problems with two very different bills attached. Salt is a big part of why. Salt makes a wall read damp with no water arriving at all. Those marks outlast the water by years. So a wall can carry the evidence of a leak you fixed years ago and still light up a meter today. None of that makes the person holding the meter dishonest. It makes the reading incomplete. It is a prompt to investigate, not a diagnosis. Aim for roughly 40 to 60% relative humidity indoors. A cheap hygrometer tells you where you sit today. Getting there takes three moves. Make less moisture. Remove what you do make, at the point you make it. Warm up the cold surfaces it would otherwise land on. Which of the three matters most depends on your house, and the full condensation guide works through each one. Some patterns fit neither test. If it does not track the rain and does not track the cold, that is your signal. Same if it clusters round one detail, such as a chimney breast. Get someone to look at it who has no interest in selling you the treatment. This page cannot see your wall, and neither can a photograph.

Is it damp or condensation?

Why there is mould when the wall feels dry

What a damp meter reading does and does not prove

What to aim for if it turns out to be condensation

When to stop guessing