Apart from the many problems of dampness caused by actual structural defects or uncommon site circumstances such as an underground stream for example, there is one, which, under certain circumstances, can be common to all buildings.

This form of dampness is condensation, which may be associated with a general surface wetting of an isolated area, and which might be mistaken for external water penetration. It is easy to conclude that there is a leak on a window frame for example, when a morning water puddle on a windowsill is in fact moisture condensed from within the atmosphere of the room. Perhaps the most common form of condensation that we are familiar with is that of steamed up windows, which is experienced during winter conditions when internal temperatures and humidity are high, and external temperatures are low.

In order to deal effectively with this problem it is advisable to understand the basic cause of the phenomena, and then apply that theory to your own practical scenario to see what modifications you could make to achieve relative freedom from condensation.

Moisture in the Air

It is well known that our atmosphere contains moisture in vapour form, but it is not generally understood that the warmer the air the greater the quantity of moisture it can contain. However, should warm air containing a high moisture content come into contact with cold air or a cold surface, a dew point is reached and the reduced temperature causes some of the moisture to be condensed into water.

When would condensation occur ?

(1) Without alteration of the air temperature, or window and wall surface temperatures, but increasing the air moisture content i.e. filling a bath without first introducing sufficient heat into the room to warm the air and walls.

(2) The sudden lowering of the temperature of the wall or window surfaces, i.e., the effect of freezing external air on a window when internal temperatures and air moistures and air moisture content remain unchanged.

(3) The rise in air temperature and air moisture content without increasing the wall or window temperature, i.e., boiling saucepans or drying clothes.

Effects of Condensation

The above is the theory relating to the cause of condensation, but what of its effect? Generally when condensation is slight, a darkening of the wall surfaces, buckling of wall paper or 'steaming up' of windows will occur, and under such conditions it could be estimated that as soon as adequate warming of the affected surfaces occurs, a complete return to normal dry conditions will take place. Dense brick or masonry walls take some time to warm up, thus an immediate increase in air heat could result in absorption of more air moisture and may cause an immediate increase of moisture deposited but as the walls warm, evaporation will take place and soon the normal dry appearance will return.

More severe condensation may occur where surfaces are sufficiently wetted taking several days to dry out. In such instances some damage may occur. Stains on decorations can develop, timber may swell due to the moisture absorbed or a mould growth may be promoted with accompanying musty smell.

Any Cold Surfaces

Serious condensation where water actually runs does not occur very often in domestic property, but we are aware of the intermittent 'running water' experienced in such instances as bathrooms and kitchens with glazed wall tile surfaces, cold water service pipes, china cisterns and window or roof glazing. If such occurrences are infrequent, drying may take place without causing any damage, but where vertical surfaces are encountered the continuous formation of condensation may run down and cause such severe wetting of the structure receiving the water that deterioration may take place. A cold water pipe, for instance, passing though a floor could cause sufficient saturation to bring about timber decay. A cold-water storage tank in a roof may result in condensation dripping on to the ceiling and giving rise to a water stain the same way as a straightforward roof leak. Glazed tiles may shed moisture on to a timber floor and result in decay of the flooring. Heavy condensation on single glazed wood windows may percolate into the joints causing the paint to crack and eventually shell off.

Rising Dampness

There are more obscure reasons for condensation to form such as a layer of impervious paint under wallpaper and instances of 'rising dampness' have been reported which have defied considerable effort to eradicate. This 'rising dampness' after careful inspection has been revealed to be condensation caused by the encroachment of paint from the skirting on to the plaster wall. The natural absorbency of plaster at this point is shielded and condensation formed readily on the impervious layer of paint with the subsequent wetting of the paper layer above. Continuous formation of such condensation of absorbed by the paper and an area of several inches, as a strip along the top of the timber skirting will appear simulating rising dampness. Removal of the encroaching paint to restore the absorbency of the plaster layer has often saved much trouble in tracing a structural fault. Similarly, an area of paint beneath the paper where a brush has been 'rubbed out' prior to cleaning after painting could cause a small damp patch.

What to do to 'cure' condensation

We now have the cause and in some measure the effect of condensation on the structure. It now remains to see what a householder can do to overcome the problem. As we have seen, very mild attacks of condensation can be ignored or simple experiments in heating and control of kitchen and bathroom steam and increased ventilation can make. Perhaps the largest percentage of slight dampness due to condensation is due to leaving a kitchen door open when cooking, or not ventilating a bathroom following use. The heavily laden atmosphere is allowed to permeate the whole house and cause condensation on any cold surface with which it comes into contact. It is not always possible or even desirable to go into 'solitary confinement' in order to prepare a meal, or do the laundry or have a shower. Therefore adequate ventilation of the saturated areas is a necessary consideration to carry away the excess moisture to the outside to alleviate the possibility of self-induced high moisture contents within the home.


Window’s present a difficult problem and years ago there used to be only one practical means of dealing with the traditional domestic window. This usually took the form of collecting the water forming in a channel at the bottom of the window and leading it via 'weep holes' to the external face. These 'weep-holes' or condensation outlets are small holes about 1/4" in diameter drilled at an angle through the frame so that water will pass by normal gravitation to the outside. The insulation properties of both framework and glass is improving all the time as technology moves on, and as measures are introduced by government to improve energy efficiency in line with the Kyoto Protocol. Glass is still glass, and no matter how well insulated by double-glazing, the interior pane will always be susceptible to the formation of condensation in a situation where the Relative Humidity reaches impossible limits.


Many questions are asked as to whether increased ventilation would cure condensation, and in most cases this is the most important keyword. In most instances judicious ventilation will help to overcome the problem since it will encourage rapid evaporation of a condensate. However, with increased ventilation may come a tendency for surfaces to become cooled. This is brought about by the air stream passing across the surface of the wall and bringing about the process of cooling by evaporation. The introduction of mechanical ventilation in the form of electric extractor fans can achieve some benefit if they are sited in such a position that humid air will be extracted, but care should be taken to see that it does not extract too much heat from the room.