Condensation and dew point

Condensation and dew point

One of the most evocative images of autumn are steamed up windows. But what causes this phenomenon?

Condensation occurs when humid air comes in contact with a surface at a temperature below the dew point . The dew point is the temperature at which the moisture present in the atmosphere condenses, and depends in large part on the amount of vapor present . For example, a cubic meter of air at 20 ° C can contain up to 17.7 grams of water. If this limit is reached, the air has a 100 % relative humidity . Exceeding this limit , the excess water becomes visible in the form of fog. The dew point varies depending on humidity and temperature. For example , at a temperature of 20 ° C the dew point with a humidity of 50% is 9.3 ° C, while for 90% of humidity is only 18.3 ° C. At a temperature of 10 ° C , the atmosphere can contain up to 9.4 grams of water. The excess amount precipitates and deposits on the cold objects in the form of droplets.

There are many sources of moisture in a house : pots, laundry , plants , showering and even simple breathing together can generate up to 10 liters of water per day in the form of steam. Even greater amount of moisture may be made by works such as paintwork or the application of plasters and mortars. All of this steam tends to condense when the temperature falls below the dew point . The condensate will therefore be greater on the windows , on the thermal bridges and generally colder objects . On porous materials such as plaster, condensation is less visible because it is absorbed. The humidity stagnates and becomes the cause of mold , peeling paint and stains. The wall has a lower insulation power when wet; also part of the heat supplied by the heating is absorbed by the water to return to a gaseous state . High humidity is also harmful to people’s health and can cause rheumatism and allergies.

To solve this problem is undoubtedly necessary to eliminate thermal bridges where water vapor can condense. Decreasing the relative humidity is even more important. The best way is to open the windows to change the air quickly , possibly creating a draught. In this way the walls and objects will not have time to cool and the temperature will return in a short time to a comfortable level . Even during a rainy day, outdoor air contains less absolute humidity than the air inside the house because it is colder, and will heat up quickly as soon as the windows closed . Alternatively, you can install a forced ventilation system with a heat exchanger that must be provided in the design of the house to avoid costs and important work. Help can come from both salt-based and electric dehumidifiers .

Sometimes those who change the windows complains about the increase of condensation on the glazing ; This phenomenon can be explained because the old fixtures were not airproof, which ensured a constant air exchange, albeit at the expense of increased heating costs . The new windows and doors, with a greater insulation , prevent moisture to escape . If the condensation should occur within the double glazing , means that the glass spacer is damaged and the glazing must be replaced.