The primary purpose of the roof cladding is to act as a rain screen so that no water enters the building from the outside.  It is, however, equally important to ensure that the building is kept dry from within. Because metal roof and wall cladding are good heat conductors and are not absorbent, condensation forms on metal cladding under conditions of high humidity or changes in temperature.

Water is present in or on the surface of most building materials. Roofing materials are designed to tolerate a certain degree of dampness, and can also withstand greater wetness for short time spans. Excessive and prolonged wetness, however, can reduce the durability of most building materials, and cause the formation of health-threatening mould.

In gas form, the kinetic energy of the molecules overcome the bonds of attraction. This is why water vapour has a strong tendency to migrate upwards into the ceiling cavities, and why it is so important to manage the atmospheric conditions in attic spaces.

Water vapour tends to condense into liquid form when the concentration rises or the temperature drops. Condensation occurs readily when the humidity is high. At a relative humidity of 95% and a temperature of 20°C, only 1°C difference in temperature is required before dewpoint is reached; at 50% relative humidity this difference is 11°C.

The ratio of the mass of water suspended in vapour form compared to the value that saturated air could contain is known as 'Relative Humidity' and expressed as a percentage. The point at which air can hold no more water is called the 'Dew Point' or saturation point and equals 100 % relative humidity.


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