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.
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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, or thermal bridges, creates localised cold spots — often around window lintels, wall ties, or uninsulated junctions — where condensation forms even when the rest of the room is comfortably above the dew point.
What's the key thing to understand about 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, commonly Stachybotrys or Aspergillus species, can pose a genuine health risk to those with asthma, allergies, or weakened immune systems, and should be treated seriously rather than just wiped away cosmetically.
What should I know about 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 entering through a defect in the building's exterior, such as cracked render or a damaged roof — is distinct from condensation and needs a different repair approach entirely.
What should I know about 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