Condensation is not really a humidity problem or a temperature problem — it is the meeting of the two. Warm air holds more moisture than cold air, so when the air in your flat touches a cold window or an outside wall, it cools, and at a certain point it can no longer hold what it is carrying. That point is the dew point, and it is a number you can work out.
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Enter your room temperature, humidity and the temperature of the coldest surface in the room. If that surface is below the dew point, condensation is forming on it right now. If it is above the dew point but the air against it is at 80% humidity or more, mould can still grow there even though the surface looks dry — which is why black spots appear in corners and behind wardrobes while the windows stay clear.
Editorial note: BetterFlat is an independent UK home advice website. This tool applies standard dew point physics to readings you supply. It describes the conditions that allow condensation and mould growth; it is not a survey, and it cannot tell you whether damp in your home is condensation, a leak or rising damp. For persistent damp, structural problems or any health concerns, speak to a qualified surveyor, your landlord or your GP.
A cheap hygrometer gives you the first two. For the third, a window pane or the wall behind a wardrobe on an outside corner is usually the coldest spot — an infrared thermometer reads it in seconds, or try 5–7°C below room temperature as a rough stand-in.
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What actually brings the risk down
Condensation forms when damp air meets cold glass, so the lever that matters is the humidity rather than the window. A dehumidifier takes the moisture out before it reaches the glass, and a cheap hygrometer tells you whether it is actually working in the room you care about.
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How this works
The dew point is calculated from your room temperature and relative humidity using the Magnus formula, the standard approximation used in meteorology and building physics. The tool then compares that dew point with the surface temperature you entered.
It also works out the humidity of the thin layer of air actually touching that cold surface, which is higher than the humidity in the middle of the room. This matters because mould does not need liquid water. Most common household moulds can germinate when the surface stays at roughly 80% relative humidity for long enough, which is the state a cold external wall reaches well before the window starts running.
That is why the tool gives you a target humidity as well as a verdict: it is the indoor humidity that would keep your coldest surface below mould-forming conditions at your current room temperature.
What to do about the result
There are only three levers, and most flats need a bit of all three.
Put less moisture into the air. Lids on pans, the extractor fan on while you shower and for a while afterwards, bathroom and kitchen doors shut while you use them, and washing dried somewhere ventilated rather than in a closed bedroom.
Take moisture back out. Ventilation does this when the outside air is drier than the inside air, which in a British winter it usually is. A dehumidifier does it regardless of the weather, and is the more reliable option in a flat where opening windows means losing all your heat.
Warm the cold surfaces. This is the lever people forget, and often the most effective. A wall that sits at 16°C instead of 12°C stops being a condensation target without you touching the humidity at all. Steady background heat, moving furniture a few centimetres off external walls, thermal linings and insulating paints all work on this.
Drying clothes indoors is the smallest of the three, but it is the one you have most control over. The other two come with living in the flat.
Figures: Lancaster City Council damp and condensation guidance. Dehumidifier capacities are rated in laboratory conditions (30°C, 80% humidity), so a cool UK flat will collect appreciably less than the rated figure.
Common questions
What humidity should a UK home be at?
Somewhere between 40% and 60% relative humidity suits most homes. Below 40% the air feels dry; above 60% you are moving into the range where dust mites thrive and cold surfaces start to reach mould-forming conditions. The right ceiling for your home depends on how cold your coldest surfaces get, which is what the calculator above works out.
Why do I get mould when there is no condensation on the windows?
Because mould does not need droplets. It needs a surface that stays humid, generally around 80% relative humidity at the surface itself. A cold external wall or the back of a wardrobe reaches that state long before a window pane starts streaming, so mould appears in those places first while the glass still looks clear.
Is it better to open a window or run a dehumidifier?
Opening a window is free and works whenever the outside air is drier than the inside air, which is most of the time in a British winter. It also loses heat, which cools your surfaces and can make things worse in a poorly heated flat. A dehumidifier costs money to run but removes moisture without the heat loss, and it works in damp mild weather when ventilation does very little.
Why is condensation worse in the bedroom?
Two people breathing overnight release a surprising amount of moisture, bedrooms are often heated less than living rooms, and the door tends to be shut. Lower temperature means colder surfaces and a lower dew point threshold, so the same moisture that is harmless in a warm living room condenses in the bedroom.
Does the calculator tell me if I have rising damp?
No. It only describes condensation risk from the air in the room. Rising damp, penetrating damp and leaks all behave differently — they tend to leave tide marks, damage low on the wall, or patches that stay wet regardless of ventilation. If drying the air does not change anything, the cause is probably not condensation and is worth having looked at properly.