Three short reads to give you the mental model. Each one will expand into a longer article over time.
Every person, shower and saucepan adds water vapour to the air. Modern airtight homes give it nowhere to go, so it condenses on the coldest surface it can find.
Cold corners, behind wardrobes on outside walls, around windows and in unheated rooms. The pattern almost always tells you the cause.
Reduce moisture at source, ventilate properly, then improve thermal performance. Doing those three in order beats almost any single intervention.
"Damp" is a loose word that covers several very different problems. The fix for each is different, and getting the diagnosis right matters far more than the chosen remedy. Here is a plain-English summary of the five most common causes of moisture in UK homes.
Water vapour in warm indoor air meeting a cold surface and turning back into liquid water. Tell-tale signs include black spotty mould in corners and behind furniture, streaming windows in winter, and damp patches that come and go with the seasons. It is by far the most common cause of indoor damp in modern UK homes and is fixed by managing moisture and ventilation, not by treating the walls.
Genuine rising damp is groundwater drawn up through the base of a wall by capillary action, where a damp-proof course is missing, bridged or has failed. It typically shows as a tide mark roughly up to 1m above floor level, often with salt deposits in the plaster. It is much rarer than the damp-proofing industry suggests and should only be diagnosed after other causes have been ruled out by an independent surveyor.
Water finding its way through the building fabric from outside. Common causes include defective render, cracked pointing, failed flashings, blocked or leaking gutters, missing tiles, and driving rain on exposed elevations. Patches usually appear high up or around openings, get worse after heavy rain, and follow a path that points back to a specific external defect.
Concealed leaks from pipework, waste runs, radiator joints, shower trays and overflows. Often mistaken for rising or penetrating damp because the water travels along joists and behind plasterboard before showing on the wall. Suspect a leak whenever the damp patch is near a bathroom, kitchen, radiator or pipe run, and especially if it appears in dry weather.
Moisture left in the building fabric after recent works, flood, or aggressive cleaning. Newly plastered walls, screeds, and properties dried out after a leak can all read as damp for months. The fix is patience and ventilation rather than treatment.
External ground levels that bridge the damp-proof course, faulty cavity wall insulation, blocked sub-floor vents causing damp timbers, hygroscopic salt contamination in old plaster, and even tumble dryers vented internally can all create damp-like symptoms. The pattern of where the moisture appears, and when, is usually the key to identifying which.
Each cause has a different fix. A chemical damp-proof course injection will not solve a condensation problem, and a dehumidifier will not solve a leaking gutter. Always work from diagnosis to remedy, not the other way around. Where the cause is unclear or the moisture is persistent, instruct an independent Chartered Building Surveyor or specialist damp surveyor rather than a contractor who sells the remedy.
If you want to do something about it, these are the practical tools.
A detailed PDF toolkit covering causes, patterns, ventilation strategies and a step-by-step action plan with images.
Air-change rate guidance and extractor fan sizing for kitchens, bathrooms and utility rooms, referenced against Approved Document F.
Match the right fan rating (l/s) to the room volume and use type. Coming as a standalone calculator.
If mould is widespread, persistent after ventilation changes, or affecting children, elderly residents or anyone immunocompromised, instruct a Chartered Building Surveyor or specialist damp surveyor. Do not accept a damp-proof course injection as the answer unless rising damp has actually been diagnosed properly. The content on this page is educational and is not a substitute for an inspection of your specific property.