The life and times of Condensation - Part One [or what causes it happening in your house]

The lifestyle of the building's occupants and the amount of heating and ventilation can influence the development of condensation, as well as structural cold breaks and maintenance problems.

The amount of water in the air varies with temperature, as warm air has a greater water-holding capacity than cold air. Air within a building is usually warm and carries a considerable quantity of water vapour, from breathing, cooking, kettles and bathing. 

The moisture in the air comes from a number of sources within the house. Water vapour is produced in relatively large quantities from normal day to day activities, such as cooking washing heating  especially gas heaters or paraffin during emergencies.

A five persons household puts about 10 kg of water into the air every day and pro-rata for smaller households (without taking into account any heating).  When it is cooled by contact with cold surfaces or when the heating shuts down at night, the excess water can no longer be supported in the air and condenses as liquid water on wall and window surfaces.

Condensed water on windows runs down and collects on the bottom rail and sill, often causing dark mould spots. Similar evidence of condensation can be seen on walls and ceilings, if cold. The use of double glazing has increased condensation on walls etc, as single glazing in older houses tended to ensure more moisture collected on the windows as their surfaces were so much colder.

The lack of condensation in older houses was a combined result of chimneys, open fires and draughts, so to some extent it is a result of the modern building which has reduced draughts and not having open fires, the former with sealed windows and doors, and the latter by central heating.
 
If properties have serious problems and large patches of black mould can damage decorations and creating an unhealthy atmosphere through the release of spores, which can affect people with respiratory complaints.   These mould patches are the particles of protein from skin shredding from people within the property and the reason it is normally black, is due to the water vapour picking up dust particles and depositing it on cold surfaces.

Quoin [the pseudonym of a Chartered Building Surveyor]