Condensate causes condensation

What Causes Condensation

The presence of condensate (sometimes called condensation) is often mistaken for a structural damp problem, but although condensate accumulates as water is different from penetrating damp. Condensate can accumulate on any cold surface including walls and ceilings and can go on to promote growth of mould.

Watch what your eating

Watch what your eating

The persistence of condensate build up has been the single largest complaint encountered by local authorities over the last twenty years. During the two decades condensate inspired dampness has become a serious challenge in dwelling houses. Condensate affects more than half of British buildings. And the accompanying condensate dampness increases mould growth. Contributors to the British Medical Journal have identified increased incidences of asthmas and ill health as originating in mould growth caused by condensate deposits. The critical element in mould development is the availability of water. Moulds are hydrophilic fungi meaning they tolerate high water environments. Surface condensate and the high relative humidity of the atmosphere causescondensate to persist provide the ideal environment for growth of the most commonly encountered moulds such as mucor, calosporium, penicillium, and rhizopus. Any numbers of moulds, some quite dangerous, are also supported by the presence of condensate. Some of the apparently innocuous spots that grow on bread with the assistance of the condensate in the bread bin can create poisons that are highly toxic to humans.

A Modern Curse

black spores concentrate at the cold parts of a room

black spores concentrate at the cold parts of a room

Condensate accumulation for the most part arises owing to modern heating, insulation and ventilation practices. Air contains water vapour and the amount it can hold is related to temperature. Warm air holds more water than cold air. When air becomes saturated and can hold no more the water vapour in the air ‘condenses’ into liquid water or ‘condensate’. It is the process itself that’s called ‘condensation’. This saturation point can be arrived at from two directions. When the warm air becomes so saturated that condensate forms spontaneously without a change in air temperature, the air is said to have reached 100% ‘relative humidity’. Alternatively condensate can form when air cools or touches a cold surface. When that happens the air is said to have cooled to its ‘dew’ point. The dew we see on plants in the morning is condensate.  Formation of condensate is therefore directly dependent upon the water vapour content of the air and its temperature. The air has to discard water vapour as condensate when it can’t hold it at its present temperature. Condensate in a building usually arises in the form ofsurface condensate when the dew point is reached as the air touches the surface causing the condensate to form. Condensate can also form in the fabric of a building owing to the internal air penetrating into the structure itself. The more humid air in the building causes the internal air containing the water vapour to exert more pressure than the drier air outside. Almost all construction materials (except steel etc) are to some extent permeable and as the air penetrates and cools condensate forms. This type of moisture deposit is called interstitial condensate. The damage that interstitial condensate can cause can be serious. The condensate can go unreported for long periods causing serious structural damage and decay on the timbers and can make the insulation present useless. Condensate can even affect the electrical fittings.

How condensation is formed

How condensation is formed

How condensate forms

Condensate mostly forms in houses in winter when the difference in temperature, water vapour content and pressure between the internal air and the external air is at its greatest. Water vapour is produced in the house from a variety of sources, not least the process of living and breathingbut houses conceal a further source of condensate unconnected with human habitation. Water vapour can rise from the damp ground under the building and penetrate into the cavities in the walls and into the roof space. The effect which the resultant condensate has on the building will depend on the construction type and whether any barriers have been built in to the structure to restrict the movement of the vapours which eventually condensate. The external cladding of timber framed buildings is liable to become wet with condensate as vapour passes through the walls. Condensate can therefore cause failure of the paint coatings on the outside of the building causing chemical change and blistering. Modern impermeable roofing, although good for excluding rain makes the problem of internal moist air passing through walls and forming condensate in the structure that much worse. The impermeable roof also causes condensate to form on the roof deck. Condensate can be a particular problem in low pitched roofs when the roof follows the same slope as the ceiling. Ceilings can become saturated with condensate owing to lack of provision for ventilation in the roof space. Condensate most often affects brick walls where the internal moist air and outside cold air circulate in a closely confined space. Condensate might even be seen dripping from surfaces. Condensate also affects suspended floors causing timber to rot in the wooden floor below.


The formation of condensate is aggravated by the way dwellings are ventilated nowadays. Adequate ventilation is the only effective means of avoiding condensate deposits appearing. Until the end of the 1950s most homes were adequately ventilated by their open fire places. Windows tended to be ill fitting and double glazing virtually unknown. Condensate was rarely seen anywhere other than the kitchen and even there it was limited in effect as 1950s mothers were typically at home with the doors open.  Virtually every advance in home heating, insulation, comfort and modern lifestyle has increased the opportunities for water vapour to condensate inside the home. Some of the worst affected areas are behind furniture where air doesn’t circulate properly. Often the first sign of the presence of condensate is the resultant mould growth. Modern houses are often left unoccupied all day and left to cool down. Moisture producing activities are concentrated into a relatively short proportion of the day before the building structure has warmed up making the walls and ceilings perfect surfaces for condensate to form. Even modern changes in roof design where overhanging eaves have gone out of fashion contribute to condensate deposits forming through lack of ventilation.

Heat exchangers

Domestic heat recovery

One way of preventing condensate whilst actually increasing energy efficiency is ‘Heat Recovery Ventilation’. Moist air in the loft is filtered and mixed with fresh air drawn in through the eves. When it circulated into the lower floors it forces out the stagnant air creating the opposite effect to a draft. Avoiding condensate through this process avoids the wasted heat that goes out of an extractor fan. This type of input ventilation system can save as much as 80% of the heat which would otherwise have been lost and at the same time remove the vapour laden air which would otherwise condense into condensate. Some units reliant on low wattage motors can provide significant net energy inputs into a home and are therefore highly energy efficient adding condensate to the long list of renewable energy sources.

This type of Heat Recovery Ventilation will be featured in a further article shortly with detailed technical explanations of how they work and how they are installed. Solving condensate  problems and cutting energy bills at the same time must be worth trying.