Damp and Mould: How to Tell What You're Actually Dealing With

Key points

• Most damp in UK homes is condensation, not rising damp. Genuine rising damp is uncommon and only ever affects the lower part of a ground-floor wall.
• A damp meter reading on its own does not prove anything. It needs to be read alongside a proper inspection.
• Damp-proof injection is a specific remedy for a specific problem. Used on the wrong type of damp, it does nothing except cost money.
• Persistent mould is a health issue, not just a cosmetic one, and the fix is almost always about controlling moisture at its source.
• The biggest risk to a homeowner isn’t the damp itself, it’s getting the wrong diagnosis and paying for the wrong repair.

The three types of damp, and how each one actually behaves

Damp is not one problem. It’s three, and they need to be told apart before anything gets fixed, because the treatment for one does nothing for the others.

Rising damp is groundwater being drawn up through a wall by capillary action, the way a paper towel pulls water upward. It only happens in solid walls without an effective damp-proof course, it is confined to the ground floor, and it typically tapers off somewhere below a metre in height, often with a visible “tide mark” and salt staining as the moisture evaporates and leaves minerals behind. Genuine rising damp is much less common than the industry built around treating it would suggest.

Penetrating damp is water getting into the building from outside through a specific defect: a cracked render, a blocked or leaking gutter, missing pointing, a failed flashing around a chimney or window, or ground level that has been built up too high against a wall. It can appear at any height, indoors or up a wall, and it usually gets worse in wet weather and eases off when it’s dry. Trace the source and the pattern almost always makes sense once you know what to look for.

Condensation is moisture already inside the home, from breathing, cooking, showering and drying clothes, turning to liquid when it meets a cold surface. It’s overwhelmingly the most common type of damp in occupied homes, particularly in winter, and it shows up as beading on windows, black speckled mould in corners, on north-facing walls, behind furniture pushed against external walls, and in poorly ventilated bathrooms. It responds to changes in how a home is heated, ventilated and used, which is both the good news and the reason it gets missed as “just something to do with the weather.”

Why misdiagnosis is so common

A lot of damp diagnosis in the UK still leans on a single handheld meter reading and a quick look around, and that’s where things go wrong. The three types can produce overlapping symptoms. Staining, musty smells and visible mould can all show up whether the cause is rising, penetrating or condensation, so a rushed inspection can genuinely land on the wrong answer even without anyone acting in bad faith.

There’s also a commercial layer to this. Some companies offer a free damp survey and make their money from selling and installing the remedy afterwards, usually a chemical damp-proof course or a re-plastering system. That’s not automatically dishonest, but it is a structural incentive to find the problem the company happens to sell a fix for. An inspector who is paid for their time and their report, independent of whatever work follows, has far less reason to reach the same conclusion every time.

The result is a pattern that turns up again and again: a wall showing damp from a leaking gutter or bad pointing gets diagnosed as rising damp, injected with a chemical damp-proof course, and the wall is still damp a year later because nobody fixed the gutter.

What a damp meter does and doesn’t prove

A handheld electrical resistance meter measures how well a material conducts electricity, and dampness increases conductivity. That’s genuinely useful as a first pass, but it is not a diagnosis by itself, because plenty of other things push the reading up too: salts left behind by evaporating water, metal foil in some wallpapers and paints, certain plaster additives and even the mineral content of some building materials.

A meter reading also can’t tell you where the moisture is coming from. A high reading a metre up a wall could mean rising damp, or it could mean a leaking pipe behind the plaster, or condensation that’s saturated absorbent wallpaper. This is why a proper diagnosis treats the meter as one piece of evidence among several: the age and construction of the building, the pattern and height of the staining, what’s happening outside at that exact spot, the room’s ventilation and heating habits, and, where it’s genuinely in doubt, a small sample sent for a proper moisture-content and salts test in a lab.

When damp-proof injection is, and isn’t, appropriate

Chemical damp-proof course injection is designed to treat one specific problem: rising damp in a solid wall that has no effective barrier against groundwater, or where the existing barrier has failed. Used on a wall like that, it can be a legitimate part of a repair. Used on penetrating damp, it does nothing, because the water is coming in from outside, not up through the wall. Used on condensation, it does nothing at all, because there is no groundwater involved in the first place.

This is why the diagnosis has to come before the remedy, not the other way round. If a survey concludes with a recommendation for injection before anyone has ruled out penetrating damp and condensation, that’s worth questioning.

Mould and health

Damp and mould aren’t just a cosmetic problem. Persistent mould exposure is a recognised risk to respiratory health, and the effects can be worse for people with asthma or allergies, for those with weakened immune systems, and for young children and older people. Even where the health effects are mild for a given household, mould is a sign that moisture is going somewhere it shouldn’t, and that tends to get worse, not better, if it’s ignored. If you’re worried about symptoms that might be linked to mould in your home, that’s a conversation for a doctor, not a damp contractor.

What a proper diagnosis looks like

A trustworthy survey starts with questions, not a meter. When did the damp appear, and does it track with the seasons or the weather? What’s the building’s construction and age? Has anything changed recently, like a new bathroom fan not being used, furniture pushed against a wall, or building work outside that changed the ground level? Then comes a visual inspection both inside and out, because penetrating damp especially often has an obvious external cause once someone actually looks for it. The meter reading comes into this picture as supporting evidence, not the headline finding, and a good surveyor will explain the reasoning behind their conclusion rather than simply naming a type of damp.

Red flags when choosing a contractor

A few patterns are worth being wary of:

• A free survey immediately followed by a quote. There’s nothing wrong with free surveys as such, but if the visit and the sales pitch happen in the same twenty minutes, be sceptical.
• No explanation of the evidence. If a surveyor can’t or won’t explain why they’ve concluded it’s rising damp rather than penetrating damp or condensation, that’s a gap worth pressing on.
• Pressure to sign on the day. A genuine problem will still be there next week. Urgency is a sales tactic, not a technical necessity.
• No written report. You should get something in writing that sets out what was found, where, and why, not just a verbal conclusion and a quote.
• Reluctance to have you get a second opinion. If a contractor pushes back hard on you seeking an independent view, that tells you something.

Getting the diagnosis right is the single highest-leverage decision in the whole process. Everything after it, from cost to whether the work actually solves the problem, follows from that first, careful judgement call.