Exploring dampness

Air moisture content in occupied houses is generally higher than outside air. Moisture in the home is generated by occupants' metabolisms, their activities (such as washing and cooking), by heating, and by the weather. It may also be generated as a result of plumbing leaks or from other building defects that result in penetrating damp or moisture entering the structure. Moisture generated in one area will disperse to other parts of the building or to the outside by air circulation caused by thermal currents or by diffusion.

Diffusion is the gradual mixing of particles in a gas or liquid. The various atoms and molecules of a gas such as air are constantly moving, bouncing off each other and other obstacles at random. As the temperature of the gas rises the mixing process speeds up. Over time, and with no external influences, an equilibrium state will be reached. Diffusion may play a part in how humidity inside a house reaches equilibrium from room to room but, according to BRE’s report Tackling Condensation, air circulation can carry far greater quantities of moisture. Opening doors and windows affects how moisture moves around a house and the design and layout of a building will influence patterns of moisture movement.

Condensation is when water vapour changes to a liquid. It occurs when warm, moist air comes into contact with a cold surface. (The 'dew point' is the temperature at which condensation occurs.) In the UK, condensation is most likely to take place in the colder months (from September to May, known as the ‘condensation season’). Since moisture travels around the building, the room in which condensation takes place may sometimes be far removed from where the moisture originated.

In warmer climates condensation can often occur on the outside of a building that is being overcooled. Studies in the USA have shown that in the warmer southern states, when the air conditioning inside a property falls below 70 degrees Fahrenheit, there is a risk of condensate forming in between the floor timbers within the ‘crawl space’, leading to timber decay.

Evaporation is the reverse of condensation, when liquid water changes to water vapour. This is the mechanism by which walls dry after rain wetting. Temperature and air movement influence the process; for example, a rain-soaked wall will dry faster on a warm windy day than on a warm still day. Evaporation is what dissipates moisture in voids under traditional timber floors.