The three levers you have
Condensation happens when humid air meets a surface below its dew point, so there are only three things to change: how much moisture goes into the air, how quickly that moisture is removed, and how cold the surfaces are. Every effective measure is a version of one of those three, and every ineffective measure is usually an attempt to treat the mould rather than the conditions. Most households can make a substantial difference with the first two at almost no cost, and the third only needs attention where a specific surface is running too cold to be rescued by moisture control alone.
Reduce the moisture at source
Ordinary living produces a lot of water vapour. Cooking with pan lids on, and using the extract while cooking, cuts a large part of it. Showering with the bathroom door closed and the fan running, rather than letting steam roll into the landing and bedrooms, contains it to one room. Drying laundry outdoors whenever the weather allows is the single biggest change available to most households, because a full load released indoors can add several litres to the air. Where washing must be dried inside, do it in one room with the door shut and extraction running, ideally with a dehumidifier, rather than on radiators around the house. Avoid unflued portable heaters, which release water as they burn.
Remove it properly
Extraction only works if it clears air to outside. Check that the kitchen and bathroom fans actually move air by holding a sheet of paper to the grille, that the external grille is clear, that the duct is not kinked or discharging into a loft, and that the fan is not switching off after a minute or two. Run fans for fifteen to twenty minutes after cooking or bathing, with the internal door closed. Open trickle vents and keep them clear, and do not seal them up to stop draughts. Short, purposeful airing of a room, windows wide for a few minutes, exchanges the moist air far more efficiently than leaving a window on the latch all day.
Keep surfaces warm enough
Condensation forms on the coldest surface, so its temperature is the other half of the equation. A steady background temperature in occupied rooms is more effective than heating hard for an hour and letting everything cool, because surfaces have thermal mass and it is the surface, not the air, that matters. Heating the rooms that are currently unheated, even modestly, usually removes the worst mould locations. Leave a gap between furniture and external walls, avoid packing wardrobes tight against outside walls, and leave internal doors of unheated rooms open enough for warmth to reach them, while keeping doors closed when moisture is being produced elsewhere.
Where a fabric improvement is justified
If the measures above are in place and a particular surface still runs cold and damp, the fabric is the limiting factor. Common candidates are uninsulated window reveals and lintels, cold spots where cavity insulation is missing or has slumped, solid walls in exposed rooms, and the wall behind a built-in cupboard on an external elevation. Targeted insulation at the cold bridge, insulated reveals and closers, or in some properties a designed continuous low-volume extraction system will resolve what behaviour change cannot. These are worth doing on the basis of measured surface temperatures rather than guesswork, because the wrong intervention in the wrong place changes very little.
What rarely helps
Anti-mould paint on a cold surface delays the growth without changing the conditions. A dehumidifier is genuinely useful, but running one permanently to make up for a fan that does not work is an expensive way to avoid a cheap repair. Sealing a room up makes matters worse. Bleach removes the stain but not the cause, and dry brushing spreads spores. Heating a room briefly then letting it cool tends to produce more condensation, not less, because surfaces never warm up. And a chemical damp-proof course will not affect a condensation problem at all, which is why the mechanism has to be identified before any money is spent.
How to tell whether it is working
Measure rather than guess. A cheap hygrometer in the worst-affected room, checked over a few weeks, shows whether the changes have brought humidity down; below roughly 60 per cent in an occupied room through the colder months is a reasonable target for most homes. Watch whether condensation still appears on the glass in the morning, and whether cleaned surfaces stay clear for a full season. If humidity stays high despite proper extraction, or if mould returns on a surface that should now be warm enough, an assessment that measures surface temperatures and looks for a fabric or moisture defect is the next step.
Cost of running the measures that work
Households reasonably worry that dealing with condensation means heating rooms they cannot afford to heat. In practice the cheapest measures are the ones with the largest effect. Running a bathroom extract fan for twenty minutes costs a fraction of a penny. Drying one load of washing outdoors instead of indoors removes litres of moisture at no cost at all. A dehumidifier used while laundry dries in one closed room typically costs less to run than heating the whole house harder. Keeping a modest steady background temperature in the rooms that suffer, rather than heating hard in the evening and letting everything cool, often costs little more than the intermittent pattern it replaces, because the surfaces never have to be reheated from cold. Redecorating a mouldy bedroom every spring is more expensive than any of these.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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