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Condensation issues in houses

Condensation creates beads of moisture which appear on non-absorbent surfaces and can also occur in other surfaces but may not be noticeable until the material is fully saturated.

From time to time, HIA receives reports from members about issues relating to condensation in houses. It appears that the problem may be becoming more frequent with the introduction of more stringent energy efficiency and bushfire construction requirements.

Condensation creates beads of moisture which appear on non-absorbent surfaces and can also occur in other surfaces but may not be noticeable until the material is fully saturated.

The current requirements for building in bushfire prone areas and 6-star energy efficiency requirements can make it difficult to achieve adequate ventilation through the home due to the need to have buildings more airtight, to increase thermal performance and reduce risk of ember attack. This has proven to impact on the airflow throughout the building. Therefore it is important that builders and designers consider the potential condensation problems that could arise in new homes and incorporate adequate ventilation to minimize the risk.

Some of the effects of condensation within buildings are:

  • Mould, mildew and fungus growth on walls, ceilings & floor surfaces
  • Blistering of paint
  • Water stains and the musty smell that eventuates
  • General degradation of building elements

Causes of condensation

We are surrounded by air and the air around us contains water or moisture. The amount of moisture in the air is dependent on the temperature. The warmer the temperature, the more moisture the air can absorb. The temperature in which the air can hold the maximum amount of moisture is called the “dew point”. Cooling below this “dew point” temperature causes condensation. At this temperature, water leaves the air and forms beads of moisture or a film of water on any cooler surfaces in a room such as windows, walls and ceilings.

Managing relative humidity and condensation risk indoors begins with understanding the local climate.

When the temperature is low (say, during winter), the air cannot hold high levels of moisture. A cold room in a house would therefore produce moisture.

Domestic activities also contribute to condensation as they generate warm moist air which is a significant factor to the high moisture levels within buildings. Some examples of these activities include, cooking, bathing/showering, clothes drying, high occupancy and uncontrolled moisture ingress. All of these factors contribute to raising the indoor relative humidity (RH)

An increase in RH increases the dew point temperature for the same air temperature. This increases the risk of condensation should the water vapour come into contact with a surface below dew point.

People in buildings also produce moisture in the air by breathing and perspiring. It is said that a family of four can produce about 12 litres of water vapour or moisture each day in a house.

Kerosene heaters and unflued gas fires also produce large amounts of water vapour and therefore these types of heaters should be avoided in condensation problem areas.

Usually moisture escapes from the interior of a house through the air vents or through the chimney. However, in modern homes natural ventilation has been reduced and more moisture is being retained inside the home, especially with closed windows in winter.

Ventilation is an essential safeguard for condensation from the above causes, although this may have an impact on the heating costs for a home if not well managed. If good thermal insulation is installed and the ventilation is not excessive, then condensation may not be a major issue in a house, and it can be readily heated and cooled all year round.

Problem areas


Steam from cooking and washing up is the main source of moisture in the kitchen. An extractor fan and a range hood ducted to outside air are effective means of removing steam. Any build-up of mould on the walls or ceilings should be treated with a bleach/water solution or commercial mould remover.


Pantries should be constructed to provide sufficient ventilation. It is good practice to allow 15 mm gap between the shelves and the wall to allow free circulation of air.


Bathrooms usually have high condensation during and after the use of showers and baths. The moist air can cause mould formation if ventilation is poor. Windows should be opened for ventilation during and after showering or bathing. Glossy paint on the walls and ceilings should assist the evaporation of moisture. If condensation is a major problem, then an extractor fan should be installed, preferably in an external wall.


Bedrooms can sometimes be the coldest rooms in the house. Water vapour from other warmer areas or rooms of the house may flow into bedrooms and condense on the walls, most commonly on the windows. It is best to keep bedroom doors closed and remove sources of water vapour in other rooms, like the kitchen and bathrooms with exhaust fans to minimize the water vapour in the home, thereby reducing the potential for the bedrooms to be affected.

Wardrobes and cupboards

If wardrobes and cupboards are built on external walls, then condensation can become a problem. Clothes, shoes and other items may be damaged by mildew and mould growth. The use of louvered doors or drilling holes in the doors at high and low levels is a way of ventilating these storage areas. It is a good idea to take clothes out and air them at regular intervals especially during humid weather in summer.

Ceilings and roofs

Ceilings can become cold enough for water vapour in the air to form directly on the underside. This is caused by the entry of cold air through metal roofing or tiling above the internal ceiling.

Roof condensation occurs when moisture becomes trapped between the roof and the ceiling insulation, this problem can be caused by insufficient ventilation or no ventilation to the roof cavity. This can be improved with the use of roof ventilators (whirly birds) or gable vents.

Windows and glass doors

Condensation commonly forms on large areas of glass. The walls below the glass can become permanently damp and pools of water can even form on the floor. The opening of the windows slightly to allow for ventilation is the simplest remedy for most problems like this. However, in bad cases, a condensation channel may be necessary with weep holes to the outside of the windows to ensure water can be drained.

Prevention is better than cure

It is always best to seek to prevent or reduce the potential effect of condensation at design and construction stage. Appropriate use of sarking and insulation can be effective in the control of condensation but it is sensible to discuss appropriate products and installation methods for the climate with manufacturers and suppliers. Seeking advice on the appropriate use of sarking materials including the correct placement and type to be installed for the location is recommended, for example using either breathable sarking or a sarking that will act as a moisture barrier.

One of the main problem areas is metal roofs. Metal roofs are generally one of the most exposed surfaces and can be highly subject to condensation. Therefore special consideration around the installation of sarking that can act as moisture barriers and bulk insulation blankets should be given.

A good source of information on condensation control in roofs is provided in the installation code for metal roofing and wall cladding HB-39 (published by SAI Global) which provides a detailed example of correct placement of sarking and insulation blanket to assist in condensation control.

Communicating a condensation control strategy at handover

Before a new home is handed over to the owner, it is appropriate that they be advised about the need for good management of the building to reduce the risk of condensation forming by understanding the significance of air movement and ventilation within their home.

Builders can alert the owner to the importance of maintaining good ventilation and air exchange through fixed ventilation options within the home during the colder months of the year or for those days that may create opportunities for condensation to form. Outside the building, pointing out ventilation openings to any sub-floor space, wall cavities or roof space and the importance of avoiding any obstruction to these openings is a useful part of the handover process.


When designing a home it is very important to consider potential condensation problems, especially in the problem areas mentioned above. Minimising moisture ingress is a significant factor along with ensuring that adequate ventilation and insulation is provided throughout the home to assist in condensation prevention over the life of the house.

Ultimately both the builder and the future home owner should understand the importance of condensation, how it is formed and the impacts it can pose to a new home.

The Australian Building Codes Board has published an updated Handbook Condensation in Buildings – Second Edition 2014 which provides useful information on managing and controlling condensation in buildings and is freely available online.

To find out more, contact HIA’s Building Services team.

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