Condensation

What is condensation

Condensation is the process where water vapour becomes liquid. It is the reverse of evaporation, where liquid water becomes a vapour.

Warm air can take up more water vapour than cold air and as such when it is cooled there comes a point where the air can no longer retain the levels of moisture present when it was warmer, and the air becomes fully saturated; any further cooling and the excess water begins to drop out as liquid water (condensate). This can lead to mould growth and increases in other biological agents.

Condensation happens in one of two ways:

• The air is cooled to its dew point
• It becomes so saturated with water vapour that it cannot hold any more water

Dew point is when relative humidity reaches or exceeds above 70% moisture content; at this point the atmospheric conditions for condensation to occur is ideal.

Condensation dampness depends on the relationship between heating, ventilation, insulation and the patterns of the occupiers’ activities.

The amount of water that can be held by the air depends on temperature. How saturated the air is with water is known as the relative humidity. Therefore with a constant amount of water in the air and change in temperature lead to a change in relative humidity. It is not uncommon to find a high relative humidity in problem properties not because of an excess water vapour production from life-style activities but simply due to the property being maintained at too low a temperature.

It is the above relationship between atmospheric moisture and temperature that can lead to a misunderstanding when considering relative humidity alone in that a high relative humidity does not necessarily reflect a high level of moisture vapour in the air-the air may simply be cold. For example, if say the lounge was at say an average of only 16°C and the average relative humidity was 80%, i.e., the risk of mould/dust mite increase was high, we should increase the air temperature to a more reasonable 20°C and assuming no extra internal moisture production, the relative humidity would drop to around 65-66%, a more respectable figure taking conditions out of the ‘risk zone’ for development of biological agents. If this cannot be done then there is often very little in the way of cost effective alternative action that can be implemented.

It has also been shown that the surface temperatures of walls are closely related to internal air temperature – raise the air temperature and the wall surface temperature is raised quite rapidly with it. If the air cools then so does the wall surface. Thus maintaining a constant warm indoor temperature raises surface temperatures of walls thereby maintaining a lower relative humidity in the boundary layer of air for a given amount of water vapour.

However, this appears not to be the case behind furniture and larger items in contact with walls. There is an absence of warm air flow behind these structures to warm up the surface and in these cases the overall wall temperature governs the temperature of the boundary layer of stagnant air behind these structures; this leads to colder stagnant air and an increase in relative humidity sufficient to cause mould to develop in some cases.

The presence of mould growth and associated odour not only creates unpleasant living conditions and damage to property but is associated with ill-health.

Some fungi produce toxic substances called Mycotoxins, as well as a large number of volatile organic compounds - which cause the familiar "mouldy" odour.

It is also important to appreciate that moulds can develop in some materials without condensation occurring. Some mould will readily develop on some cloths, leather, cardboard at relative humidities of 75-80%; this can lead to clothes and shoes in cupboards going mouldy without condensation occurring. Indeed, the musty odour that sometimes encountered is frequently mould growing in such situations but most not actually readily visible. What mould grows where and when depends on the material (and finish), relative humidity and probably air flow across a particular surface.

However, high relative humidities, irrespective of temperature, and condensation in the domestic environment are important in that they will significantly increase the risk of mould growth, dust mite numbers, and other biological agents which may, if conditions persist, may lead to some health problems for some people who are particularly sensitive to these agents. Tiny spores produced by the mould and the higher number of dust mites due to the moist conditions can increase the risk of asthma and respiratory illness on some people.

The most common signs of condensation dampness area:

• Mould (black or green) formation / growth on cold surfaces
• Water droplets collection on cold surfaces such as windows.
• Damp patches on cold surfaces.

Condensation is more prevalent during winters and seems to be on the rise as buildings become more air tight & insulated.

What causes increase in moisture in property

Moist air is created by many factors such as: cooking, bathing/showers, drying clothes inside the property, over occupancy.

What can be done to reduce condensation in the home

Produce less moisture by:

• always cook with lids on pan and turn the heat down once the water has boiled.
• always use minimum amount of water to cook vegetables.
• when filling the bath, run the cold water first then add hot water, this will reduce the steam which leads to condensation by 90%.
• never dry laundry on radiators.
• dry washing outdoor if possible or put it in the bathroom with the door shut and the window open or extractor fan on.
• extractor fans should be humidistat controlled not just operated when light switch in on.
• tumble dryer should be vented to the outside of the property or use a condensing dryer.
• do not use the gas cooker to heat the kitchen up as it produces moisture when burning gas.
• ventilate your home. Some ventilation is needed to get rid of the moisture produced at the time including people’s breathing. Keep trickle vents on the windows open at all time or open small windows or install passive vents.
• kitchen and bathroom produce more moisture than other rooms and should be fitted with humidistat controlled extractors which work automatically when humid air is detected.
• close bathroom and kitchen doors when these rooms are in use even if these rooms are fitted with extractors. This stops moisture reaching other rooms.
• allow space to circulate around furniture.
• the use of positive pressure unit (flat of loft mounted) can be considered whereby fresh, dry filtered air is introduced continually into the property diluting and displacing stale moist air and reducing the cause of condensation.

For more information on condensation in buildings visit: www.property-care.org