Condensation is the most common form of damp in British homes and the only one that is caused by how the building is used as much as by how it was built. Air holds moisture, cold surfaces make it give that moisture up, and where the two meet you get water on a wall. Every real remedy works on one of those three things.
Where the water comes from
An ordinary household puts a surprising volume of moisture into the air every day: cooking, showering, washing and drying clothes indoors, and simply breathing. Drying a load of washing on radiators is one of the largest single sources and one of the easiest to stop. In a house that leaked air through draughty windows and an open chimney, that moisture left on its own. In a house that has been sealed up with new windows and insulation and given no replacement ventilation, it stays.
Condensation on walls: why it appears where it appears
Moisture condenses on the coldest surface in the room. That is usually the glass, which is why windows stream first, and after that the cold spots in the walls: behind furniture pushed against an external wall, in corners, at the ceiling to wall junction, and at any cold bridge where the structure carries heat out. A wall with black mould in a corner and nowhere else is almost always telling you about a cold bridge rather than about a leak.
Telling it from the other damps
Condensation is seasonal, worse in winter, worst in the rooms that make moisture, and it affects surfaces rather than soaking the structure. Penetrating damp follows rain and has a source outside. Rising damp is confined to the bottom of ground floor walls and does not appear upstairs. A damp patch on a bedroom ceiling is not condensation even if it looks like it. Getting this wrong is expensive, because chemical damp proofing does nothing whatever for a condensation problem.
What condensation treatment actually fixes it
Reduce the moisture: cover pans, dry washing outside or in a vented tumble dryer, and use the extractor while cooking and showering and for a while afterwards. Increase the ventilation: working extract fans in the kitchen and bathroom, trickle vents open, and in a persistently damp house a continuous low rate system. Raise the surface temperature: insulation on cold walls and at cold bridges, and enough heating to keep the fabric above dew point rather than heating in bursts.
The condensation fan and other kit that gets sold, and what it does
A positive input ventilation unit in the loft pushes filtered air gently into the house and displaces moist air out through the gaps, which works well in many houses and is what most condensation control units are. Extract fans with humidity sensors run when they are needed rather than when someone remembers. Dehumidifiers treat the symptom and are useful as a stopgap. None of it substitutes for stopping the moisture at source, and a firm that proposes a unit without asking how you dry your washing has not diagnosed anything.
Other questions people ask
Is condensation the landlord's problem or the tenant's?
It is frequently both, and the law has moved towards the building. A landlord has repairing obligations and government guidance is explicit that damp and mould should not be blamed on lifestyle alone. Where ventilation or insulation is inadequate, that is a property issue.
Will opening windows fix it?
It helps a great deal and it is not a strategy in January. Short sharp airing after cooking and showering, plus working extract fans, does more than leaving a window ajar all day while the heating fights it.
Why did condensation start after I had work done?
New windows, insulation and draughtproofing all reduce uncontrolled ventilation. If nothing replaced it, the moisture that used to leak away now condenses. It is the commonest sequence there is.
Do I need condensation specialists?
For a straightforward case, no. Where mould keeps returning after the obvious measures, a survey that measures humidity and surface temperatures and finds the cold bridge is worth paying for, and it should be independent of anyone selling a remedy.