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Condensation and mould that results from high humidity has become one of the most common causes of dampness within buildings. A Report by the Building Research Establishment estimated that one and a half million homes in the UK are badly affected by condensation.

Condensation occurs when warm moist air is cooled that it no longer has the capacity to hold so much water in vapour form. Typically this can be seen to occur at cold surfaces such as windowpanes as mist or cold water pipes as droplets of water.

A brief summary of the terminology The amount of water (or moisture), which can be held by air, (the humidity ) depends on the air temperature. The warmer the air the more water (or moisture) it can hold. Cold air at 0 C is unable to hold any moisture. The temperature at which the air cannot hold any more moisture is described as the dew point. The dew point is the point at which condensation occurs (water or moisture that was in the air has condensed to liquid on a surface that was
at or below dew point) As the air temperature increases the dew point temperature will be determined by the amount of moisture in the air. The relative humidity is the amount of moisture in the air relative to how much moisture the air can
hold at that temperature. So, at any given temperature, increasing the moisture in the air will increase the dew point temperature. As the humidity increases the relative humidity increases and the risk increases that relative humidity will reach 100% Dew point temperature occurs when relative humidity reaches 100% (and vice-versa). The inter-related nature of temperature humidity relative humidity and dew point can be seen from examination of a “psychrometric chart”

Some points to consider

The amount of moisture within the air in a building varies continuously. Certain lifestyle activities generate moisture and these will increase the amount of moisture in the air. Examples of these are cooking, drying clothes, bathing and even breathing. Such activities need not always produce a condensation problem. The moisture can be dispersed out through
an open window or extractor fan, or it may condense on a nearby windowpane and be easily wiped away.

It is to where the moist air goes and the temperature of the places it goes that will determine whether a condensation problem is produced.

When the air is heated throughout a property the air is capable of holding the moisture as vapour to the extent that can be seen from the psychrometric chart. Most properties are not heated to the same temperature all the way through all the time. For example it is not unusual to heat a living room to a higher temperature than a bedroom. This means that the
air in the living room can hold more water vapour than the air in the bedroom.

Typically moisture generated in the kitchen and bathroom might be allowed to escape through the whole house. This
might typically result in some local visible condensation in the kitchen and bathroom, no signs of a problem in the living room, but some damp patches and mould growth on the external wall of a back bedroom.

Unless a study is made of the varying temperatures and humidities within a property it can be difficult to predict whether the property is likely to be affected by a condensation problem. The factors that cause the problem, vary all the time. It
is certainly true that the tendency in recent years to draughtproof houses and insulate to conserve energy has reduced
the amount of air (and therefore airborne moisture) escaping to the outside. This has had the effect of increasing the overall humidity levels.

The reduction in ventilation and increase in humidity can also encourage the growth of mould fungi. These are often the first visible sign that a problem is present. Relative humidity does not require to be as high as 100% for mould growth to occur. Mould growth can occur when the relative humidity is around 70%. This often happens in a part of a property that
is less well heated and where the ventilation is reduced. For example: a cupboard or wardrobe on the external wall of a back bedroom. Although there may not be visible wetness due to condensation The temperature in this location may not be as consistently high as that throughout the house, and as a result the relative humidity can be locally increased to above 70% at certain times of the day. That can be sufficient to support mould growth.

It is fairly easy to spot a condensation problem when there is visible evidence of moisture on a window or cold water pipe. Fungal growth (usually moulds) can occur before the problem is visible in that way.

The conditions, which produce condensation and related problems of mould growth depend also to a great extent on the temperature and humidity of the air outside the building. It is not uncommon therefore for a house to be affected by condensation or mould growth only at certain times of the year. In such cases it would be unlikely that altering the ventilation alone would be sufficient to reduce or eliminate the problem.

A successful strategy for combating condensation will usually depend on a number of factors. Since these will be most likely to produce a successful result.

New digital technology has facilitated studying how and when condensation problems are generated.

Where condensation is suspected as a source of dampness it is useful to monitor internal climate conditions. This is best done over a continuous period, and should be compared to external conditions, in order that a comparison of vapour pressure can be made. The methods are described in the British Standard for Condensation (BS5250, 2011)

Dataloggers provide a means of monitoring atmospheric temperature, dewpoint and moisture conditions continuously. This provides useful information about how temperature and relative humidity change on an hourly basis. By analysing the data and calculating vapour pressure, a comparison can be made between different locations at different temperatures. That comparison can be used to determine the source of condensation within a building.

Vapour pressure provides a measure of atmospheric moisture that is independent of temperature. Vapour pressure can be calculated from relative humidity and temperature measurements. Vapour pressure comparison is the method described in the British Standard (BS5250) for condensation.

Vapour pressure is that proportion of normal air pressure that is made up of water in vapour form. This is typically around 1% of atmospheric pressure, but varies continuously. The moisture within the air within a building is made up of the moisture in the air outside plus that which is generated within the building by normal living activity, such as cooking washing and bathing.

Vapour pressure provides a useful means of comparing airborne humidity between different locations within a building, and of determining the direction of moisture movement. The direction of moisture migration is from areas of higher vapour pressure to lower until equilibrium is reached.

A way of determining the source of condensation within a property is to track vapour pressure migration because vapour pressure will generally migrate from the source location to other parts of a building.


1) Reduce humidity (Extractions fans in kitchens and bathrooms help to do this)

2) Improve air circulation

3) Improve insulation

4) Improve overall background heating.

There are a number of ways of dealing with each factor and the most appropriate will depend on the individual property.
It is generally true that more than one factor will require modification.

It makes good sense to start with the simplest and least expensive option which typically might be the installation of extractor fans in kitchen and bathroom, the places where most of the airborne moisture is generated.

A monitoring period should be allowed after each course of action is taken so that the contribution of each factor can be evaluated.

Further details and advice can be found in BS 5250 (2011)

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