How to avoid mold growth – determining the dew point

Determining the dew point Having appropriate knowledge about the value of the dew point is very useful for those who deal with breeding, farming, drying, painting, construction works, as well as during making a choice of building materials. It is very important when it comes to planning actions which aim is to prevent mold or rust formation. Therefore, having the knowledge and understanding about the dew point temperature is crucial not only from an engineering or meteorological point of view.

What is the dew point temperature?

The dew point, or dew point temperature, is the temperature at which gas achieves maximum degree of saturation. If the temperature is below the dew point, the gas state is transformed into the liquid state (condensation), or the gas state is transformed directly into solid state (deposition). In order to determine the value of the dew point temperature precisely, it is required to determine gas pressure and chemical composition of gas. The definition described above concerns also water vapor. Evaporation, condensation, and boiling of water are common physical processes we encounter in everyday lives.

When we talk about a mixture of air and water vapor, the dew point temperature is the temperature below which water vapor in the air is condensed (dew, mist, frost). The influence of air temperature is very important. When the temperature grows, hot air includes more water than cold air, despite having the same level of relative humidity. It should be remembered that relative humidity also indicates the water (water vapor) contents in the air, however, it doesn’t determine the level of air humidity as accurately as the dew point.

Determining dew point temperature

Physical dependencies show that the dew point temperature is determined by relative humidity of air, air temperature, and atmospheric pressure. Relative humidity, expressed in percent values, usually changes from several to 100%. Widely recognized commercial range includes temperatures from -5°C to +70°C; in case of industrial range it is from -25°C to +85°C. Atmospheric pressure is characterized by the lowest fluctuation, that is why during determination of the dew point temperature, it is assumed that it is constant. The simplest relations and tables show only relations between the dew point, temperature, and humidity.

On the basis of physical dependencies we know that:

the dew point temperature increases when temperature of air with water contents increases;
the higher the level of relative humidity of air; the lower the dew point temperature;
the higher the atmospheric pressure, the lower the dew point temperature;
the higher the level of relative humidity, the smaller the difference between air temperature and the dew point temperature;
at temperature of 100°C and relative humidity of 100%, the dew point temperature is equal to air temperature; in any other case, the dew point temperature is lower than air temperature;
the dew point temperature may only take values that are lower than zero if air temperature and relative humidity are adequately low (e.g. when relative humidity level is below 50% and temperature is below 10°C, the dew point will be below 0°C; however, at room temperature of 22°C and relative humidity level of about 20%, the dew point will be equal to 0°C).

The dew point temperature for humid air is determined on the basis of:

widely available tables,
Mollier Diagrams,
calculations made on the basis of Magnus formula,
measurements made with the use of hygrometers.

Places where temperature may decrease below the dew point

The fact that dew appears or water vapor condensates on surfaces with lower temperature than the ambient air is a common and very undesirable phenomenon. Spontaneous condensation caused by a drop of temperature is especially common in building services. This situation affects mainly elements which can be exposed to different temperatures, e.g. windows, window and door frames, structural elements, parts of the facade and roofing materials. If aforementioned building elements are exposed to temperatures inside and outside the building, then, drops of water will appear in the areas with lower temperature. It is due to the fact that air temperature dropped locally below the dew point temperature.

The dew point and thermal bridges

Places exposed to temperature and characterized by a relatively good thermal conductivity are called thermal bridges. These are elements of building structures that are considered to be disadvantageous from the perspective of thermal insulation. Thermal bridges cause intense heat dissipation from the building. Temperature of such elements is significantly lower than average temperature inside the building. Apart from the fact that thermal bridges reduce insulating efficiency of the building, they are a cause of dew formation on the surfaces inside the building. Long-term and intensive dampness of the surface of thermal bridges leads to increased corrosion of metal parts and mold formation. The worst areas where thermal bridges are formed include multi-layered walls inside the buildings, attics, and inadequately insulated steel structures. Frequent differences of temperature result in hard to identify and eliminate corrosion and mold formation.

Bearing in mind that condensation is a result of a local decrease of temperature below the dew point, it is possible to identify places where water vapor is condensed as well as places that are natural thermal bridges. The higher the humidity level in such areas, the more intensive is the process of heat release. Thus, it is important to prevent creation of such places during the construction phase, and to eliminate them during the maintenance of the building. It allows reducing costs of thermal energy and eliminating any undesirable processes.

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