Why are your windows and walls wet?
Check which surfaces in your room will get condensation or mould — in 30 seconds. Then find the real cause and compare fixes, with honest running costs.
- Free, no sign-up
- Leaks & rain checked first
- Published calculations
- We don't sell damp treatments
Built on the same physics surveyors use (dew point and surface temperature factors from BS EN ISO 13788), with every assumption shown.
Where is the water on your window?
This one question rules out half the possible causes. Each position means something completely different.
Free tools
Full room assessment
Six steps: rules out leaks and rain, finds the cold surfaces and compares fixes with costs.
🌡️Dew point calculator
The temperature at which your room air starts to condense — and the mould-risk threshold.
🪟Will opening a window help?
Compares the real water content of indoor and outdoor air, not the misleading % figure.
💧Dehumidifier running costs
Daily and winter costs on your tariff, and how much water it will really remove in a cold room.
Learn about condensation
Plain-English guides from our knowledge hub.
What causes condensation?
Moisture, cold surfaces and ventilation — the science made simple.
Ideal humidity levels
What's normal in a UK home, what's too high, and how to measure it.
Bedroom condensation
Why windows are wet every morning, and fixes that work.
Drying clothes indoors
Each load releases litres of water — how to dry clothes without damp.
Condensation, damp or a leak? Start here
Wet windows and black mould in corners are almost always condensation: warm, moist indoor air meeting a surface colder than its dew point. But not every damp patch is condensation, and treating the wrong problem is the most expensive mistake people make.
- Worse after rain? Suspect rain getting in — gutters, roof, pointing, render or window seals.
- Near pipes, a bath or appliance? Rule out a plumbing leak with a water-meter test.
- A band low on the wall? Several causes are possible; a damp-meter reading alone can't diagnose "rising damp".
- Corners, behind wardrobes, around windows, worst in the morning? That pattern fits condensation.
Our damp or condensation guide walks through each one, and the full assessment screens for them before giving any condensation advice.
The three things that control condensation
Condensation isn't caused by one thing, which is why single fixes often disappoint. It depends on three things at once:
- How much water vapour is in the air — from breathing, showers, cooking and especially drying laundry indoors (each load releases around 1.5–2.5 litres).
- How quickly it's removed — by extract fans, trickle vents, open windows and draughts, or a dehumidifier.
- How cold the coldest surface gets — single glazing, window edges, uninsulated solid walls, corners and the wall hidden behind a wardrobe.
Change one and the others still matter. Heating the air makes the humidity reading fall without removing any water; new airtight windows warm the glass but can push condensation onto the walls. That's why our tool recalculates every combination of fixes together rather than adding up separate savings.
If you rent, damp and mould is a repair issue your landlord may be legally required to deal with — see renters' rights for damp and mould.