Condensation Risk & Dew Point Calculator
Calculate dew point and whether condensation will form on cold surfaces in your home. Identify mould risk and find solutions.
Condensation and Mould Guide
How Condensation Forms
Air contains water vapour. Relative humidity (RH) is the percentage of the maximum water vapour air can hold at a given temperature. When air contacts a surface cooler than the dew point, the air immediately adjacent to the surface is cooled below its dew point — water condenses. Dew point (Magnus formula): T_dew ≈ T - (100-RH)/5. At 20°C and 65% RH: dew point ≈ 20 - (100-65)/5 = 20 - 7 = 13°C. Any surface below 13°C will have condensation in this air. Cold bridging (thermal bridges) creates loc
Mould Growth Conditions
Mould grows when: surface RH exceeds 80% for extended periods (not the same as air RH — surface RH depends on temperature). Temperature is between 0-35°C (optimum 20-25°C). Nutrients are available (most building materials qualify). The relationship: if air is at 20°C/65% RH and a surface is at 12.6°C, the surface RH = 100% — condensation and mould will form. NHS definition of 'Category 1 Hazard': visible mould is a health hazard under the Housing Health and Safety Rating System. Black mould (Sta
Sources of Moisture in Homes
Daily moisture generation in a typical 4-person household: breathing and perspiration: 1.5L/day. Cooking: 3L/day. Washing and bathing: 1L/day. Drying clothes indoors: 5L/day (highest single source — avoid if possible). Total: up to 10-12L per day of moisture is generated — without adequate ventilation, this drives humidity up and condensation risk increases. Structural sources: rising damp (ground water wicking up walls — typically visible as a 'tide mark' below 1m). Penetrating damp (rain enter
Reducing Condensation Risk
Increase ventilation: extractor fans in kitchen and bathroom (minimum 15 L/s during cooking/bathing). Trickle vents open on window frames. Consider mechanical ventilation with heat recovery (MVHR) for very airtight homes. Reduce moisture sources: dry clothes outside or use a vented tumble dryer. Put lids on pots when cooking. Vent tumble dryer directly outside. Heat more consistently: cold bedrooms in the morning are common sites of condensation — steady background heat prevents surfaces falling
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