# What is the Dew Point and Why Does It Matter at Home?
The dew point is the temperature to which air must cool for water vapour to condense into liquid water. In a home, this concept marks the boundary between a healthy house and one with structural moisture problems. When the temperature of a surface (such as a poorly insulated wall) falls below the dew point, water droplets appear — the ideal breeding ground for Aspergillus and other harmful fungi. > 5°C Safe Difference
40–60% Ideal Humidity
< 1°C Extreme Danger
The Magnus-Tetens Formula
To calculate the dew point with scientific accuracy we use the Magnus-Tetens approximation, with constants b=17.625 and c=243.04°C recommended by the World Meteorological Organization for temperatures between 0°C and 50°C.
- Scientific accuracy validated by the WMO
- Valid for residential temperature ranges
Thermal Bridges and Mould
Room corners, window frames, and poorly insulated exterior walls are the coldest points. If their temperature falls below the dew point, condensation and mould are inevitable.
- Corners are the most vulnerable spots
- Thermal insulation prevents condensation
# Risk Levels
Low Risk (difference > 5°C): The air is far from saturation. Your walls are safe. Medium Risk (3–5°C): Watch corners and thermal bridges. High Risk (1–3°C): Condensation likely on glass and cold zones — ventilate immediately. Extreme Danger (< 1°C): Active condensation with imminent risk of black mould growth.The Golden Rule
If your wall temperature is within 3°C of the dew point, you face an imminent condensation risk. Ventilate, use extractor fans in bathrooms and kitchens, and keep relative humidity between 40% and 60%.
How to Prevent Condensation
Ventilate daily, especially after showering or cooking.
Keep relative humidity between 40% and 60%.
Use extractor fans in bathrooms and kitchens to remove steam.
Never dry laundry indoors without ventilation.
Thermally insulate walls to prevent cold surfaces.