Air at a given temperature contains certain amounts of invisible water vapour. The higher the air temperature, the more water vapour it can hold. The lower the air temperature, the less water it can hold. When warm air hits a cold surface, the air looses some of its heat energy to the body of the surface causing the air temperature to fall. When the air temperature falls to a temperature known as the 'dew point' temperature for the air, it is unable to hold as much moisture and so it deposits some of the water vapour as droplets on the cold surface. The water that is formed is known as condensate and the whole process is called condensation.

General household activities like cooking, bathing, taking a shower, washing, drying and even just breathing produce warm moist air. Unless the warm moist air produced can escape to the outside through windows, extractor fans, or airbricks, it will remain inside the home. The warm moist air will continue moving around the home until it finds a cold surface where it can condense and deposit droplets of water on the surface.

There are two basic types of condensation:

• Surface condensation
• Interstitial condensation

Surface condensation takes place on the surface of a body when the temperature of the surface is less than the dew point temperature of the surrounding warm moist air.

Interstitial condensation: Warm air inside a building is known to travel outwards through certain building fabrics such as external walls from the warm interior surface to the cold exterior. When the air is moving through the wall, at some point inside the wall its temperature can be cooled below its dew point temperature. When this happens condensation occurs. This is known as interstitial condensation.

Is condensation a problem?
Condensation is a problem because it causes staining and mould growth and can severely damage wallpapers, wall surfaces, window frames, furniture and clothing. Black mould which grows when there is a severe condensation problem can cause ill health in some people. The mould produces tiny spores or seeds which when inhaled can irritate the lungs and throat.

Condensation occurs in cold areas of a house and is worst when humidity is high, and temperatures inside the house are low. The conditions for condensation exist in areas around the home with poor circulation and or high heat loss such as in the following areas:

• Cold surfaces (i.e. single glazed windows, mirror and metal window frames)
• Cold corners of rooms
• Kitchens
• Bathrooms
• Built in cupboards
• Behind furniture against an outside wall
• Walls in an unheated room

The main factors involved in the production of condensation are the amount of moisture in the air, the temperature of the air, and the presence of cold surfaces.

The following actions are what can be done to avoid or control condensation problems in the home:

• Reduce the amount of moisture produced in the home
• Correctly ventilate the home to remove moist stale air
• Improve heating to raise air temperature and surface temperatures
• Insulate the house to retain more heat, and reduce the risk associated with cold surfaces

It has been discussed that one of the main factors involved in the production of condensation is the quantity of moisture in the air. Hence one way that helps to reduce the risk of condensation is to keep moisture production to a minimum.

Some of the activities that produce moisture in the home cannot be avoided. For example we have to keep on breathing to stay alive, cooking food and taking a wash are also essential for our well being.

However, it is possible to reduce the effect of some of these activities by removing the moisture produced as effectively as possible when it is generated. For example, keeping windows open whilst cooking or having a wash, or using extractor fans in these area to remove the moisture produced, will help avoid condensation problems.

Dehumidifiers can also be used to remove moisture in areas with condensation problems. 

The following are a number of practical steps that people could take to reduce the quantity of moisture production:

• Keep covers on sauce pans when cooking
• Soak clothes no longer than is necessary
• Vent tumble driers to the outside air
• Avoid sing radiators to dry clothes
• Avoid non - vented bottle gas and paraffin heaters
• When carrying out activities that generate moisture keep the area well ventilated and stop moisture migration by closing off the rest of the house

Allowing moist air to remain in the home due to insufficient ventilation increases the risk of condensation taking place. Ventilation is essential in the home for other important reasons. For example, fuel burners like gas appliances need fresh air to work correctly, people need fresh air to keep healthy and fresh air is required to replace moisture-laden air in the home. To meet these needs a ventilation rate is required which does not incur excess heat loss from the building and discomfort to occupants due to draughts.

Achieving and maintaining a suitable level of ventilation for these needs can be quite difficult. It follows that too much ventilation leads to draughts which reduce temperatures, the low temperature leads to high humidity and increased condensation risk. Too little ventilation and the relative humidity rises and again increases the risk of condensation. The solution is usually to make provisions for an adequate amount of ventilation in the right places. Provisions for background ventilation in bedrooms is usually through airbricks or ventilators, which can be fixed or adjustable openings.

Areas of high levels of moisture generation like the kitchen and bathroom need higher ventilation rates. Opening windows or the use of extractor fans and keeping internal doors closed can achieve the required level of ventilation in these areas. The 1990 building regulations require that new homes be fitted with extractor fans in kitchens and bathrooms.

Too much ventilation in homes can arise from gaps and openings around windows, doors, floors etc. This unwanted air infiltration brings cold air in to the home and displaces warm heated air from the home. This has the effect of reducing temperatures in the home making it possible for condensation to occur. As much as 15% of heat energy required for adequate warmth in a home can be lost through air infiltration.

Draught proofing can successfully reduce heat loss through gaps in building fabrics such as windows, doors, letterboxes, floorboards etc. However, blocking of all the gaps and openings in a home will also reduce natural flow of fresh air into the home and moisture-laden air out from the home. This can make condensation problems worse. So clearly to avoid condensation problems after draught proofing a home requires some form of ventilation balance.

It follows that areas in the home with severe condensation problems should not be draught proofed unless some other provisions like airbricks and extractor fans are installed. Also when closing off disused chimneys air vents should be installed for natural flow of air through the chimneybreast. Provisions should exist in kitchen and bathrooms for the extraction of moist humid air during use. In areas with open fires, adequate provision should be made for a fresh air supply to the fire when draught proofing a home.

Under heated homes are very likely to be subjects for condensation problems. Under heating in homes can be due to a number of reasons,the following being the most probable:

• Inadequate provision for space heating systems
• Householder unable to purchase sufficient fuel for adequate warmth
• Excessive heat loss from the building due to poor insulation
• Over ventilation

Adequate heating in cold weather conditions is important in reducing the risk of condensation problems within homes. Heating increases the interior air and surface temperatures above that at which condensation can occur.

Unheated areas such as little used rooms and storage areas are often sites for condensation. Condensation risk is higher in these areas because warm moist air naturally migrates to cooler areas. Temperatures in these areas are significantly lower than in the rest of the home. Hence, moisture-laden air that enters these areas is likely to be cooled below the dew point temperature at which condensation will occur.

For protection against condensation problems, continuous low level of heating is better than intermittent higher levels of heating. This is because when heating systems turn off, and the air temperature falls, condensation is much more likely to occur. But, the continuous low level of heating may not be appropriate for the home occupants life styles or health needs.

For protection against condensation problems, it is important to adopt a heating regime that prevents the existence of conditions likely to give rise to condensation on surfaces within the home. There needs to be a balance between space heating and ventilation. Too much ventilation results in excess heat loss and high fuel consumption. Too little ventilation increases the potential for condensation to occur. Home insulation, controlled ventilation and a controlable heating regime can significantly reduce the potential for the occurrence of condensation in the home.

In homes where the occupants choose the use of individual heaters such paraffin heaters and bottle gas heaters, the risk of condensation problems are highly likely. This is because for every litre of fuel burned, such heaters, release an equivalent quantity of water vapour. This can make condensation problems worse. It follows that heating systems should provide adequate warmth and be affordable to run to prevent low-income households resorting to heaters such as is described above.

One of the important solutions to the treatment of condensation problems is insulation.

Insulation has three main effects when applied to the treatment of condensation problems:

• Insulation improves the heat retention ability of buildings. This improves their potential to maintain internal surface temperatures above that where condensation is likely to occur
• Insulation increases the general temperature of a home by minimising the heat loss and hence reduce condensation risk
• Insulation has the effect of reducing heating bills. This enables individuals to heat their homes to a higher standard without incurring excess costs and hence can maintain conditions less likely to aid condensation
  

Building regulations now ensure that homes are adequately insulated. However, many older homes including those built in the 1960s and 70s have design defects that make them susceptible to condensation problems.

Un-insulated walls, concrete floors and ceilings are some examples of cold bridges. Areas with cold bridging have high U-values, which loose heat quickly and provide cold surfaces ideal for condensation to occur on. In treating condensation problems, these areas are often insulated. A cold bridge is a continuous path for heat loss from the inside to the outside of a building.  It is provided by materials that conduct heat very well such as steel and dense concrete. Condensation can be rampant in areas where cold bridging exists. Insulating these areas can be complicated but can be essential in treating areas with severe condensation problems.

The term relative humidity refers to the fraction of water vapour in a given volume of air. Dry air is air that has no water content and has a relative humidity of 0%. Air at a given temperature can hold up to a specific volume of water vapour. Below this saturation temperature, the excess water vapour in the air will return to its liquid state (droplets of water) this is known as condensation. When the vapour pressure and temperature are such that the air cannot retain anymore water, it is classed as completely saturated air and has a relative humidity of 100%.

When two different gases such as air and water vapour exist as a mixture, they occupy the same volume at the same time as if they are each alone in it. When two such gases exist together in this way, their volume and temperature is the same for each. In practice air always has some amount of water vapour. The higher the temperature of a volume of air the more water vapour it can hold. As the air temperature falls the quantities of water vapour it can hold also falls. The term humidity refers to the amount of water vapour in a given amount of air.