Is it damp or condensation?
By The Damp GuyLast updated
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.
Why there is mould when the wall feels dry
This is what sends most people looking for a damp company, and it is usually not what they think. Mould does not need a wet wall. It needs a humid surface. A surface turns humid long before it turns wet.
What governs it is the relative humidity at the surface. Not whether that surface has hit the dew point and started running with water. BS EN ISO 13788 puts the risk of mould growth at monthly mean surface relative humidities above a critical value taken as 0.8, so 80%. A surface can sit above 80% and still be warmer than the dew point.
Dry to the touch. Growing mould anyway.
That is also why your hygrometer flatters you. It reads the air in the middle of the room. Mould grows on the coldest surface in that room, where the humidity runs higher. Keep the indoor air humid for long enough and those cold surfaces cross the line. How far over depends on how cold they run. The room can feel fine while the outside wall behind the wardrobe is already in mould territory.
What a damp meter reading does and does not prove
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. Deposits and tide marks stay put long after the water that caused them has gone. 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.
What to aim for if it turns out to be condensation
What you want is a number to work towards, not a vague instruction to open a window. UK guidance lands in much the same place: roughly 40 to 60% relative humidity indoors. CIBSE says about 40 to 60%. The World Health Organization sits around the same. The HSE allows up to 70% as acceptable. 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.
When to stop guessing
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.