Condensation in Buildings - Australian Building Codes Board

Information Handbook: Condensation in Buildings
3.2 Water Vapour
Water vapour is invisible. Clouds and jet trails, fogs and mists are not water vapour but are
accumulations up of fine, floating water droplets or ice crystals. “Steam” noticed rising from
boiling water, cooking pots, a shower head or a bath is the same (without the ice). Water vapour
is “seen” mainly through its effects on comfort and its constant interaction with materials. A
sticky day and a sticky door are, of course, two different things but water vapour helps to cause
both of them.
Water vapour is nothing more than a swarm of single molecules that have broken free of the
surface of water. They need heat to energise their escape but they do not need air to “hold”
them or to absorb them. They simply mingle with the molecules of the many gases in air and
travel with them in the moist air mass that forms the atmosphere.
There is always some water vapour in the atmosphere, even in the world’s driest places. The
amount varies from just a trace in Antarctica (the driest continent) to as much as 3% by mass in
the tropics. In Australia, the water vapour content of the atmosphere can fall lower than 0.25%
in alpine areas during winter and reach nearly 2% in the tropical north during the summer “wet”.
The relatively small amount of water vapour, even in the wettest places, is not due to any
shortage of water but is limited by prevailing temperatures. The limits that temperature sets on
water vapour formation and concentration are discussed in Section 3.4.
Perhaps surprisingly, adding water vapour to air makes the increasingly moist air less dense
and more buoyant because water vapour molecules are lighter than the nitrogen and oxygen
molecules that dominate air. The small size of water vapour molecules also allows them to
travel where water and even air cannot.
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