Dew Point

The dew point is a fundamental concept in understanding moisture and condensation within buildings. It is the temperature at which air becomes saturated and can no longer hold all its moisture in the form of water vapor. Upon reaching the dew point, some of the moisture will condense into liquid water. This phenomenon can lead to several issues within buildings, especially when cold surfaces cause the indoor air to reach its dew point.

The Science of Dew Point

  • Definition: The dew point is defined as the temperature at which air becomes saturated, leading to the condensation of water vapor into liquid water.
  • Relative Humidity: The amount of moisture in the air compared to the maximum amount it can hold at a particular temperature is termed as relative humidity (RH). As the temperature decreases, the relative humidity increases until it reaches 100% at the dew point.

Rule of Thumb: Temperature Drop and Relative Humidity

A useful rule of thumb is that for every 1°C drop in temperature, the relative humidity increases by roughly 5%RH until it reaches the dew point. For example, if the temperature is 20°C and 65%RH at the center of a room, it could drop to 14°C and 95%RH in a colder corner, particularly behind furniture, near poorly insulated areas, or in reduced airflow zones created by radiators.

Calculating Dew Point

  • Simple Formula: The simple formula for estimating dew point (Tdp) from the ambient temperature (T) and relative humidity (RH) is: Tdp=T−100−RH5Tdp​=T−5100−RH
  • Complex Formula: A more precise calculation involves the Magnus formula which is more complex and provides a more accurate dew point temperature.

Mould Point

The mould point is the temperature at which the relative humidity exceeds 85%RH, creating a conducive environment for mould growth. Unlike condensation which occurs immediately upon reaching the dew point, mould requires a prolonged period of high humidity to develop.

Hygroscopic Salts and Deliquescence Point

Hygroscopic salts, like calcium nitrate, have a critical deliquescence point, which can be considered a form of dew point. When the humidity exceeds the deliquescence humidity of about 50%, these salts absorb moisture from the air and dissolve into a liquid solution. More information can be found on Calcium Nitrate on SaltWiki.

Implications in Buildings

  • Condensation: Cold surfaces in buildings can cause air to reach its dew point, leading to condensation. This is often seen on windows, walls, and other cold surfaces.
  • Mould Growth: High humidity and temperatures conducive to reaching the mould point can lead to mould growth, which may have health implications and cause material damage.
  • Material Degradation: The presence of hygroscopic salts can lead to material degradation, especially when the humidity is high enough to reach the deliquescence point.


Understanding the concept of dew point, mould point, and deliquescence point, along with the behavior of hygroscopic salts, provides a foundation for managing moisture, condensation, and material degradation in buildings. Regular monitoring of temperature and humidity, improving insulation, and ensuring adequate ventilation are crucial steps in maintaining a healthy and durable indoor environment.

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