Through the looking glass: Managing condensation in Australian homes
This article provides general information but is not intended to be taken as design advice. Consult an expert on your specific needs before undertaking any measures described in this article.
Preface
Canberra had a cold winter last year. Like many people in cold homes, I decided to take matters into my own hands. I added an additional layer of removable glass on the inside of some of my single glazed windows to better insulate them and to keep my home warmer.
On cold mornings I noticed one of the windows with two layers of glass turned foggy while nearby single-layer windows stayed dry. The second layer of glass keeps the inside of my home warmer, but it also keeps the outer layer of glass colder. Just like on an icy drink in hot weather, condensation has formed on the cold outer layer of glass.
I’ve accidentally made my windows wet, simply by insulating them. My windows are see-through, so fortunately I can see the water, take off the inner layer of glass and wipe away the water as it forms. However, this raises a tricky question - how do I know the other insulated parts of my home aren’t getting wet?
It turns out that many homes in Australia and all over the world have hidden moisture problems that can be made worse by well-meaning actions like insulation and draught-proofing. My windows, just like many buildings around Australia, have experienced condensation. In the worst cases, condensation causes buildings to decay and grow mould.
Condensation risks are a barrier to improved energy efficiency in buildings - a visible example being the Tasmanian government’s recent decision not to improve residential energy efficiency standards, citing condensation concerns. Managing condensation requires a new way of building energy efficient homes.
Left: Condensation occurs on the lower window that is kept cold by insulation, but not on the uninsulated upper window.
Right: Condensation damage inside an insulated wall in northwest Tasmania. (Source: M Dewsbury 2018)
Purpose of this article
This article simply and comprehensively explains how condensation can be managed in Australian homes. It discusses basic principles of condensation in Australia, explores the evidence and reasons for addressing condensation, and takes a deep dive into management measures. This article is intended to be readable by anyone, from construction professionals to individuals interested in managing condensation in their own homes.
Condensation management is a small yet growing discipline in Australia. Its significance will increase as we continue to improve the energy efficiency and standard of living achieved by our homes. Condensation management is essential to creating efficient, durable homes that support the health and comfort of the people who live in them.
Key opportunities
Some of the key opportunities discussed in this article, in addition to minimum legal requirements, are shown below.
Above: Key opportunities to manage condensation in Australian homes
These measures have significant benefits beyond reducing condensation risk, including:
Better energy efficiency and comfort, through improved airtightness, windows and insulation.
Better air quality through continuous ventilation.
Better weatherproofing through drained and ventilated cavities behind wall cladding.
1. Introduction
1.1. What is condensation?
We experience some form of condensation every day, whether it’s morning dew on grass, clouds in the sky, droplets on cold drinks or fog on the bathroom mirror.
When moist air contacts a cold surface, some of the moisture from the air condenses and forms liquid water. If condensation occurs inside a building and doesn’t dry out, it causes mould and water damage. Buildings with condensation problems can cause health issues for the people inside them and may require expensive repairs.
At the simplest level, moist air, cold surfaces and lack of drying are the three contributors that cause condensation problems in buildings. Condensation problems are managed by reducing the level of moisture in the air, keeping surfaces that are exposed to moist air warm, and allowing condensation to dry after it occurs.
Health impacts of condensation
Condensation can lead to mould which is unhealthy for building users. As noted by the Victorian Department of Health, mould in buildings can trigger nasal congestion, sneezing, cough, wheeze, respiratory infections and worsen asthma and allergic conditions. Individuals with weakened immune systems, allergies, severe asthma or lung conditions are particularly susceptible to negative health effects from being exposed to mould.
In 2018, the Commonwealth published the outcomes of a Parliamentary inquiry into biotoxin-related Illnesses in Australia. The inquiry took a particular interest in the incidence of Chronic Inflammatory Response Syndrome (CIRS), a term used to refer to severe symptoms associated with exposure to mould. Medical practitioners debate whether CIRS is a distinct disease caused by mould or a manifestation of other poorly understood disorders. The inquiry recommended that more research is undertaken to understand CIRS.
The inquiry heard a number of personal accounts of people with debilitating symptoms that they ascribe to CIRS from mouldy buildings. Personal accounts of severe health impacts from mould are regularly discussed in the media.
The government’s response to the parliamentary enquiry noted that work was being undertaken to improve the way condensation is managed in buildings through changes to Australia’s building code – the National Construction Code (NCC). Some condensation management provisions have since been included in the NCC.
Above: Three contributors to condensation problems
1.2 Types of condensation
The four types of condensation discussed in this article are:
Wet area condensation which is the most familiar and widely understood form of condensation. Wet areas of buildings (kitchens, bathrooms and laundries) can generate a large amount of steam and water vapour which will condense on almost any surface they touch. This form of condensation may be seen for instance in foggy mirrors or mouldy ceilings in bathrooms.
Winter condensation which occurs because indoor spaces naturally generate moist air (e.g. moisture is generated when people exhale) and this moisture can condense on parts of the building that get cold when the weather is cold. This can cause, for example, condensation on bedroom windows on a cold morning.
Summer condensation which occurs when humid outdoor air comes into contact with cold internal surfaces (e.g. due to a home being air-conditioned). An example of summer condensation may be mouldy or musty smelling buildings in tropical climates like Darwin or Cairns.
Night-sky condensation which occurs due to an effect called radiative cooling, where sky-facing surfaces (typically roofs) can get extremely cold on cloudless nights. This is the same effect that causes dew and frost on the ground after a clear night.
In addition, there are two ways that each these types of condensation can occur:
Surface condensation is condensation that occurs on the visible surfaces of building elements, like condensation on bathroom mirrors or bedroom windows. This form of condensation is familiar to many people.
Interstitial condensation is condensation that occurs in unseen locations inside building elements like walls or roofs. Interstitial condensation is often more challenging to manage than surface condensation, as it may have already caused significant damage over many years before it is noticed.
Understanding of condensation risk in buildings varies across the Australian construction industry. Generally, the causes-of and solutions-to wet area condensation are well understood, however the other forms of condensation are less well understood.
This article does not discuss condensation caused by ground moisture. The causes and solutions to ground moisture are already well understood. Common solutions include underfloor ventilation and ground vapour barriers. This article focuses on surface condensation and interstitial condensation in walls and roofs that separate indoor spaces from outdoor spaces (referred to as “building assemblies”). This article doesn't discuss interstitial condensation in floors for which ground moisture is a significant source of condensation risk.
Why focus on condensation in homes?
This article focuses on condensation in residential buildings (i.e. “homes” - houses, townhouses and apartments) because condensation risks tend to be higher in residential buildings than in other building types.
Residential buildings are more likely to have people in them overnight, exhaling humid air when it is coldest outside and condensation risks can be highest. Domestic activities like cooking, drying clothes and bathing further increase condensation risks. Australian homes do not typically have continuous, powered ventilation, while powered ventilation is present and reduces condensation risk in many non-residential buildings.
However, some non-residential buildings also have heightened condensation risks - particularly those that are designed and used similarly to residential buildings, like student accommodation buildings.
While residential buildings are the focus of this article, the principles that are discussed can generally apply to both residential and non-residential buildings.
Top left: Surface wet area condensation on a bathroom mirror
Top right: Surface winter condensation causing mould on a window frame
Bottom left: Interstitial summer condensation causing mould behind a wall covering (Source: Building Science Corporation)
Bottom right: Interstitial night-sky condensation in a roof space
1.3.Condensation and climate
Buildings in different climates around Australia experience different condensation risks. Condensation risks in Hobart are different to condensation risks in Darwin, although both climates are high risk.
Locations that are further south or higher in altitude are typically cooler, and have greater risks from winter condensation, while locations that are further north and coastal are often more humid and have a greater risk of summer condensation.
Homes can experience both summer and winter condensation at different times of year, or even different times of the day, depending on how temperature and humidity vary in that climate over time. Higher summer condensation risk climates do not necessarily have low winter condensation risk, and high winter condensation risk climates do not always have low summer condensation risk.
The National Construction Code (NCC) defines 8 different climate zones in Australia. Summer and winter condensation risks can be correlated with these climate zones, as shown in the table below. Note that this is only an estimated correlation and that climate zone is only one of many factors that influence condensation risk.
Top: Australian climate zone map, Bottom: Estimated relative risks of winter and summer condensation by climate zone
Research undertaken by the University of Wollongong for the Australian Building Codes Board (ABCB) notes that “In Climate Zones 2-8, the simulated risk of mould growth was primarily caused by vapour drive from the indoor environment outwards” (page iv). That is, based on the models used in the study, winter condensation risk may be greater than summer condensation risk in climate zones 2 to 8.
Summer condensation is likely to be the primary form of condensation damage in climate zone 1, which represents tropical Australia including Darwin, Cairns and Townsville. The tropical north of Australia is extremely humid during the wet season and has drastically higher summer condensation risk than the rest of the country. Summer condensation risks are not well studied in Australia - they are estimated, based on limited evidence, to also be significant in the other warm coastal climate zones 2 and 5.
What is humidity?
All air contains some amount of water. Water exists as an invisible gas (called vapour) in air in the same way that salt can be invisibly dissolved in water. The term humidity refers to measures of how much water vapour is present in air.
There are two common definitions of humidity:
Absolute humidity means the amount of water vapour in air (e.g. 10 grams of water per kilogram of air).
Relative humidity means how close air is to containing the maximum amount of water vapour it can hold (e.g. 50% of maximum water holding capacity).
Warm air can hold more water vapour than cold air. This means that if two air samples at different temperatures have the same absolute humidity, the hotter sample will have a lower relative humidity.
Above: Warmer air can hold more water vapour than cold air and therefore has a lower relative humidity at an equivalent absolute humidity.
Water vapour naturally moves from areas of high absolute humidity to areas of low absolute humidity, an effect known as “vapour drive”. Winter condensation is associated with outward vapour drive - water vapour moving from the humid inside of a building assembly to the drier outside, while summer condensation is associated with inward vapour drive.
High summer condensation risk is associated with locations that experience high outdoor absolute humidity, as occurs in Australia’s tropical climates.
Mould can grow on surfaces that are exposed to high relative humidities (above approximately 70%), while condensation occurs when relative humidity reaches 100%. That is to say - mould can grow without condensation occurring. The condensation management techniques discussed in this article can reduce the risk of condensation as well as the risk of mould growth from high relative humidity.
In general, where the term “humidity” is used in this article it refers to absolute humidity.
1.4. Condensation management in the NCC
The National Construction Code (NCC) is the primary set of legal design requirements for buildings in Australia - often referred to as the “building code”. Buildings typically should comply with the version of the NCC that was in force at the time the building was built. New updates to the NCC do not require existing buildings to be updated to meet new requirements.
Wet area condensation has been addressed by the NCC since the NCC’s inception in the 1990’s, while requirements that partially address winter condensation were included for the first time in 2019. Summer condensation and night-sky condensation are not significantly addressed by the NCC. The NCC includes a clarification that the measures that it prescribes may not eliminate condensation risks in homes.
Condensation risks vary depending on climate and method of building, leading to an unlimited number of ways that condensation can occur. Updating the NCC requires a rigorous evidence base to ensure that costs from new requirements are not greater than the benefits of those new requirements. Gathering evidence is difficult and it’s possible that the NCC can never address all condensation risks. In many cases it is prudent for building designers to consider measures to address condensation beyond what is required in the NCC.
NCC requirements
This article uses breakout boxes like this one to describe measures required by the NCC.
Technically, the NCC allows flexibility in how buildings must be designed and there are usually options available to get around having to meet any individual NCC “requirement”. When this article describes a measure as being “required” by the NCC, generally, the vast majority of buildings will need to be designed in accordance with that requirement.
Anyone who needs to comply with the NCC should read the relevant clauses directly to ensure they comply with the specific wording of the NCC.
Comparisons with international building codes
This article uses examples from overseas building codes to show how condensation may be managed differently in Australia. Particular reference is made to the USA’s International Residential Code (IRC) and England’s Approved Documents. These building codes were selected as a point of comparison because they come from large, developed countries and are written in English.
Comparing condensation measures between countries requires accounting for any differences in climate. The USA includes climates that are similar to Australian climates, while England includes climates that are comparable to, although generally cooler than, Australia’s cooler climates.
2. Why do condensation risks need to be managed?
2.1. Evidence of condensation problems
Some condensation risks are self-evident - many people have cleaned a mouldy bathroom ceiling or wiped off a wet window after a cool night. Other condensation risks are less clear - how many homes experience decay in invisible locations? What risks are introduced by new energy efficiency standards?
The key challenge for managing condensation in Australia is that there is not a comprehensive evidence base for all condensation problems. Evidence is difficult and expensive to gather and more work is needed to better understand condensation in Australia. Nevertheless, there are at least five different kinds of evidence which paint a picture of the extent of condensation problems in Australia:
Case studies and anecdotes – these generally include stories and pictures of condensation that people have experienced. While anecdotal evidence can help to tell a story of real-world experiences, it generally can’t give information on how widespread condensation problems are. There is plenty of anecdotal evidence of the existence of condensation in Australian homes, including case studies by Australian researchers.
Statistical evidence – respondents to the ABCB’s 2016 condensation survey estimate that up to 32% of Australian homes are impacted by condensation. This survey was online and voluntary, so while the result is suggestive of a problem, it does not precisely show the extent of condensation problems, nor does it specify the nature of the condensation problems that are being seen. More rigorous statistical evidence - for example randomised testing for condensation in different buildings in different climates - may be more compelling, but is likely to be too expensive to undertake.
Test-based evidence – a very good, but rare form of evidence comes from the construction and observation of “test huts”. Test huts are small buildings with controlled internal conditions that use a range of different construction styles. The huts are monitored over years or decades to see which construction styles experience condensation problems. International test huts show that condensation problems are likely to occur in some climates that are similar to Australian climates. However, these tests have not been conducted in Australia using common Australian construction methods.
Modelling – computational modelling can be used to estimate the likelihood of condensation occurring in different scenarios. Modelling is much cheaper and faster to undertake than other forms of evidence. Recent modelling commissioned by the Australian Building Codes Board estimates that problematic condensation is likely to occur in many, if not most, Australian homes that use common construction methods. Accuracy of modelled results depend on the accuracy of the model and input assumptions. It’s not always satisfying seeing computer modelled evidence without the support of convincing real-world evidence, and experts can disagree about the correct input assumptions to use in a model. Some experts have argued that modelling is useful to compare condensation risk between different scenarios, but not as a predictor of whether condensation will occur in a given situation.
International comparisons – other countries around the world have been researching condensation in buildings longer than Australia has. Colder climates and tighter energy efficiency requirements may have made it necessary for other countries to get on top of condensation risks earlier than Australia. Australia can learn from the experiences of these other countries, but different climates and construction practices mean that the learnings are not a simple cut and paste from standards imposed in other countries.
Top left: A test hut in Vancouver, Canada evaluating different wall types (Source: Building Science Corporation)
Top right: Modelling output from the popular condensation modelling software WUFI
Bottom left: The cover page of the results from the ABCB’s condensation survey
Bottom right: A single instance of condensation – i.e. an anecdote or case study (Source: M Dewsbury 2018)
2.2. Are condensation problems becoming more common?
The ways that we build and use homes in Australia change continually. Even a perfect understanding of condensation in existing homes might not translate into an understanding of condensation risks in homes that are being newly built. There are reasons to suspect that condensation problems are becoming more common, including due to:
Insulation – While insulation in walls and roofs keeps the interior of a home warm in cold weather, it also keeps the exterior structure colder by preventing indoor heat from reaching it. These colder surfaces can lead to more condensation and less opportunity for moisture to evaporate from heat. An interesting side effect of older, poorly insulated windows is that since they often experience condensation they can inadvertently provide dehumidification of indoor air and reduce condensation in other parts of a home. As regulations and homeowner preferences lead to homes being more insulated, condensation risks in walls and roofs may increase. The state of Tasmania reportedly declined to increase insulation standards in new homes due to perceived condensation risks. Nevertheless, when used in the right way insulation can be used to decrease condensation risks, as discussed later in this article.
Building sealing – Old buildings may be more likely to have small gaps in walls, roofs and floors, making the buildings leak air between the indoors and outdoors. No one wants gaps in their building or to feel draughts when the wind blows, and newer buildings are more likely to have fewer gaps, whether from improved construction practices or from having less time for the building to settle. A study by the CSIRO found that new homes were 50% more airtight in 2024 than in 2015. The poorly sealed buildings of the past may accidentally have the benefit of allowing moist air to escape from inside, reducing condensation risk. Well-sealed buildings that are common today may require continuous ventilation systems to alleviate increased condensation risks.
Reflective foils – Reflective foils, thin membranes that look similar to aluminium foil, are used as insulation in many homes. Like putting cling wrap over steaming hot food, foils can trap in moisture and cause condensation. Many foils are not vapour permeable (see the box on “Vapour permeance” below), so they can prevent drying in a structure after condensation occurs.
Increase in apartment living – According to the Australian Bureau of Statistics, 30% of new homes in Australia are apartments. Apartments are generally built to be more airtight than houses, leading to increased condensation risk.
Vapour permeance
Some materials used in buildings, like sheet metal, metal foils or polystyrene, will block the movement of both air and water vapour. Other materials, like plasterboard, will block the movement of air, but will not block the movement of water vapour (which occurs via a process called diffusion). This property of permitting the movement of water vapour is referred to as vapour permeance. Materials with a high vapour permeance are “vapour permeable” and materials with a low vapour permeance are “vapour impermeable”.
Vapour permeance means that water vapour can enter and exit building assemblies like walls and roofs even if those building assemblies are airtight and waterproof. If not managed properly, this water vapour can result in condensation.
Top left: Insulation in a wall, Top Right: Reflective foil insulation used in a wall, Bottom left: Inadvertent gaps in a building, Bottom Right: An apartment building
2.3. Condensation management in context
Our homes do many things at once. As a starting point, they must be structurally sound, keep the weather out and be unlikely to burn down. Many people expect their homes to achieve additional goals like being accessible, quiet, warm and healthy. Condensation management, like all goals of building design, needs to be considered within the full context of what a building does.
Condensation risks, like any risks, can be managed but not eliminated. Home designers are responsible for determining whether to undertake measures beyond legal building code minimums. Costs, risks and benefits need to be balanced, and the right balance will vary from building to building.
Counter-intuitively, even without a perfect understanding of risks, understanding which condensation measures to undertake in a home becomes clearer as we understand how these measures interact with other building design goals. Managing condensation has significant side-benefits for weatherproofing, energy efficiency and air quality that may exceed the benefits from condensation reduction alone.
This article explores ways that managing condensation results in better homes to live in across a range of building design goals. Managing condensation is an opportunity to create homes that are robust, efficient, healthy and comfortable - homes that are great places to live.
Condensation and other causes of water damage
Buildings commonly experience water damage that is not caused by condensation. These other causes of damage include failed weatherproofing, pipe leaks, flooding and construction moisture.
Design factors that increase condensation risk also increase the risk of these other types of water damage. Insulation, building sealing and vapour-impermeable materials all inhibit the ability of building structures to dry once they’re wet, which means water from any source is at a greater risk of causing damage.
In other words - old, draughty, poorly insulated homes can often tolerate getting wet better than comfortable modern homes can. Moisture that could once be considered incidental in old homes is no longer incidental in modern homes.
Good weatherproofing features are more important than ever - for example eaves should be adequately sized to shelter walls and flashing should be used to provide drainage from windows, doors and penetrations.
These other causes of water damage are not the focus of this article. However, effective management of condensation - particularly the creation of drying pathways in building assemblies - can reduce the risk from these other causes of water damage.
Most homes in Australia already have measures to manage wet area condensation. Compared to other forms of condensation, wet area condensation is well understood by building designers and occupants – for this reason, it is not discussed in detail in this article.
Wet area condensation is primarily managed by installing exhaust fans in bathrooms and laundries, and installing rangehoods over stovetops in kitchens. Condensation risk is reduced significantly by running these fans generously to extract moist air while moisture is being generated (e.g. while cooking or showering). Generally, continuing to run fans for a period of time after the moisture source has stopped will further reduce humidity and condensation risk. Using heat lamps in bathrooms can also help to dry condensation from surfaces after showering.
In the past, rangehoods over electric stoves have been allowed to expel exhaust air back into the kitchen. Since NCC 2022, these “recirculating rangehoods” are no longer allowed in new homes and exhaust air must be discharged outside of the building. Expelling exhaust air outdoors reduces condensation risk and improves air quality as particulates and other pollutants are removed from the building.
In any rooms with exhaust fans, new air is needed to replace the air that is being blown outside. This air may come from a nearby open door, open window or permanent vents in the building. If exhausted air is not replaced with this “make-up air”, exhaust fans will have trouble sucking moist air out of a room.
Wet areas of buildings also tend to use water-resistant materials or water repellent, mould inhibiting paint. Other less common actions taken by occupants post-construction, for example installing glass domes over showers, can further reduce condensation risk.
Regulatory requirements: Managing wet area condensation
The NCC was updated in 2019 and 2022 to better manage wet area condensation. These changes include:
Minimum exhaust fan flowrates: Where exhaust fans or range hoods are installed in wet areas, the NCC includes minimum airflow rates that must be achieved by the fans.
Exhaust fan interlocking with lightswitch: Exhaust fans in bathrooms that don’t have openable windows are required to be “interlocked” to the bathroom light switch to make the fan run while the bathroom light is on. The fan must be configured to continue running for at least 20 minutes after the light is turned off.
Exhaust fans ducted to outdoors: Kitchen range hoods and exhausts from bathrooms and laundries are required to expel exhaust air outdoors. Before NCC 2022, exhausts from bathrooms and laundries were allowed to expel air directly into roof spaces, while kitchen range hoods were allowed to expel exhaust air directly into the kitchen using a recirculating range hood. Expelling exhaust air inside the house or into roof spaces increases condensation risk in those spaces.
Bathroom make-up air: Since exhaust systems don’t work in rooms that are completely sealed from the outside and other rooms, bathrooms without windows are required to be provided with make-up air. A common way of providing make up air is having an undercut (a gap between the door and the floor) in the door to the bathroom.
Clothes dryers ducted to outdoors: Where clothes dryers that create moist air (venting clothes dryers) are installed, they must have a duct connected directly to the clothes dryer that expels moist air to outdoors. Alternatively, condensing-type clothes dryers that do not create moist air can be installed without any ducting to outdoors.
3. Managing wet area condensation
Wet area condensation occurs in areas of homes that regularly experience very high indoor humidity from the intense generation of steam and water vapour. Think of a steamy bathroom after a shower, a muggy laundry while the clothes dryer is running or the steam created by boiling water on a stovetop. While winter and summer condensation risks vary based on climate, wet area condensation can happen in any climate.
Wet area condensation is condensation that occurs due to the intense generation of steam and water vapour in three types of wet areas: kitchens, bathrooms and laundries.
4. Managing winter condensation
Winter condensation is caused by moisture that is generated inside of a building. These moisture sources are typically not as extreme as those that cause wet area condensation and might include wet laundry drying indoors or moisture generated simply by people breathing. When this moist air comes into contact with cold windows, walls or roofs during cold weather, condensation will occur.
Winter condensation can occur over the course of many years, as moisture and mould slowly accumulate as temperature and humidity conditions change. Managing winter condensation is more complex than managing wet area condensation, as moisture sources are less concentrated at single locations in a home.
Techniques to manage winter condensation will be discussed in depth in this chapter. In short, winter condensation can be prevented by:
Removing moist air from a building using continuous ventilation.
Preventing moist air from entering building assemblies using airtightness and vapour control.
Eliminating cold surfaces using appropriate heating and insulation.
Allowing building structures to dry out using drained and ventilated cavities and materials that are appropriately vapour permeable.
Winter condensation is condensation that occurs when moisture that is generated inside a home comes into contact with building elements that are cold due to cold weather. This excludes wet area condensation.
4.1. Continuous ventilation
The first and most important method of reducing winter condensation risk is to reduce the absolute humidity of indoor air before that humidity has a chance to cause condensation. The absolute humidity of indoor air can be reduced by diluting the indoor air with dry outdoor air – i.e. by ventilating.
Most homes in Australia are designed to be ventilated using openable windows. Left to our own devices, most people do not consistently ventilate by opening windows due to:
Not knowing that we should ventilate by frequently opening windows
The weather outside being too hot, cold or wet
Security concerns
Noise concerns
A ventilation strategy that does not rely on opening windows can be highly beneficial to controlling condensation risk and improving air quality. This can be accomplished using continuous ventilation, which means providing fresh air to a home by running a fan continuously. The author of this article has also written a separate detailed article about continuous ventilation here.
There are many ways to continuously ventilate a home to manage winter condensation, which include:
Exhaust ventilation – Installing a bathroom exhaust fan that is capable of running at a low level continuously can be the simplest and cheapest way to continuously ventilate a home.
Heat recovery ventilation – These systems simultaneously supply outdoor air into a home while exhausting air from the home. The two airstreams are passed through a heat exchanger to ensure the supplied air is at a more comfortable temperature. These systems (and their energy recovery ventilation counterparts) are the most energy efficient methods of providing continuous ventilation.
Decentralised “push-pull” ventilation – Push-pull ventilation systems use fans that alternate between supplying (pushing) air into a home and exhausting (pulling) air from a home. These systems can incorporate heat exchangers to improve their energy efficiency performance. Push-pull ventilation systems can be good options for retrofitting ventilation into existing homes.
Above: A schematic of a heat recovery ventilator
Controlling condensation improves indoor air quality, as there is a lower likelihood of indoor mould spores. Continuous ventilation can further improve indoor air quality by:
Diluting indoor pollutants such as volatile organic compounds from furniture or from cooking. Poorly ventilated spaces are correlated with impaired cognitive performance, headaches and other health problems.
Reducing odours, which makes a home feel less “stuffy”.
Filtering outdoor air pollutants and allergens (e.g. pollen). However, this is only the case for ventilation systems that include air filters for supplied air (i.e. not applicable for exhaust-only systems).
In locations with high outdoor relative humidity in winter, indoor space heaters may need to be used alongside continuous ventilation, as heating air reduces its relative humidity (see “What is humidity?" in section 1.3). In locations with a high risk of summer condensation, alternative ventilation strategies are needed (see section 5.4).
Continuous ventilation and pressurisation
Continuous ventilation can cause a home to become pressurised, which means air will either want to push out of a home like an inflated balloon (positive pressurisation), or push into a home (negative pressurisation). Pulling air out of a home using an exhaust fan can cause negative pressurisation, while pushing air into a home using a supply fan can cause positive pressurisation.
In locations that are susceptible to winter condensation, ventilation systems should be designed to slightly negatively pressurise a home by exhausting more air with fans than is supplied with fans. If there are any gaps in a building assembly, positively pressurised homes will push humid air from inside a home into those gaps, increasing winter condensation risk. Negatively pressurised homes pull dry outdoor air into building assembly gaps, resulting in lower winter condensation risk.
Conversely, in locations that are highly susceptible to summer condensation, it’s often better to positively pressurise a home to prevent humid outdoor air from entering building assemblies (see section 5.4).
Above: Air movement through building assemblies in pressurised homes
Continuous ventilation and energy efficiency
The idea of continuous ventilation surprises some people. They may have spent effort to try to reduce the air-leakiness of their home to improve energy efficiency, and now they are expected to deliberately bring outdoor air into their home using fans that run 24 hours a day, 7 days per week. However, when running a fan at a low level to bring in the small amount of air needed for ventilation, the energy use is small compared to the energy benefit of being airtight, along with building durability benefits and occupant health benefits.
The NCC includes an optional method for including continuous ventilation in a home that specifies how much air should be provided based on the size of the home and the number of occupants (see section 4.3). It’s important not to accidentally over-ventilate by providing much more air than this, as too much ventilation air can unnecessarily increase the heating load, cooling load and energy bill of a home.
Standalone dehumidifiers
Humidity inside a home can be reduced with standalone dehumidifiers, which can plug into a normal power point and sit on the floor of any room. These devices are sometimes purchased by occupants after they have noticed condensation problems in their homes. Split-system air conditioners with “dry mode” similarly can be used to dehumidify air.
Standalone dehumidifiers can reduce condensation risk in homes and are easy to install after a building has finished being built. Compared to continuous ventilation systems, standalone dehumidifiers have the disadvantages of not being able to pressurise a home and in some cases being less energy efficient. Standalone dehumidifiers do not improve air quality as much as ventilation systems which bring in fresh outdoor air.
Standalone dehumidifiers are generally installed post-construction after condensation problems have been noticed by occupants. Ideally, buildings should be designed so that condensation problems do not occur in the first place and do not need to be remedied by occupants.
Nevertheless, standalone dehumidifiers may be necessary in some buildings - in particular to address summer condensation risk, as discussed in section 5.4 of this article.
Above: A standalone, plug-in dehumidifier
4.2. Example assembly: Timber clad wall
Figure 1 provides an example of a common timber-clad wall assembly that will be used in this section to discuss principles of managing winter condensation.
Figure 1: An example cross-section of a timber-clad wall
Typical occurrence of winter condensation in a timber clad wall:
In winter, internal air is usually more humid than outdoor air. The water vapour from the more humid indoor air can pass through the innermost layer of the wall (the plasterboard), either through small gaps that allow air to get past the plasterboard (e.g. through power points or gaps in skirting boards), or else water vapour will permeate through the plasterboard directly without need for gaps due to vapour permeance.
Materials used for insulation between studs are typically porous, with many small air gaps, so water vapour will also pass through the insulation layer.
In winter, the water control membrane on the outside of the insulation will typically be colder than the materials on the inside of the insulation. If the water control membrane is cold enough, some of the water vapour will condense on the inside of the membrane. This water will accumulate over time unless it is drained away or evaporated by heat.
If the membrane is vapour permeable (which it typically should be) some of the water vapour will pass through the membrane into the ventilated cavity, where it will then leave the wall assembly through ventilation holes to the outside.
When the weather heats up, such as when the sun comes out during the day, or even when the seasons change and become warmer, the outside of the insulation will become warm and the condensation that accumulated behind the membrane when the weather was cold will evaporate and pass through the water control membrane to the outside of the building, or even pass back into the interior of the building through the insulation and plasterboard. A condensation problem will be present if the building assembly stays wet long enough for mould and decay to occur. Since heat from the sun can help to dry out a wall, walls that are south-facing or significantly shaded may need extra care to reduce condensation risk.
This timber-clad wall is an example of a flow-through assembly. Flow-through assemblies manage condensation risk by allowing water vapour to pass between indoors and outdoors without being blocked by vapour-impermeable materials. Whether or not a flow-though assembly successfully manages condensation risk depends on the details of its design, as discussed in the following sections of this article.
Types of building assemblies
This article uses the term “building assembly” to refer to walls and roofs that separate indoor spaces from outdoor spaces. This article considers three different ways to design building assemblies to manage condensation risk:
Flow-through assemblies use materials that are vapour permeable to allow moisture to leave an assembly. Most walls and roofs in Australian houses are flow-through assemblies. The concepts discussed in sections 4.3 to 4.8 are particularly applicable to flow-through assemblies.
Impermeable assemblies use materials that are vapour impermeable to block water vapour from getting to a location where it can condense inside an assembly. Impermeable assemblies are discussed in section 4.9.
Hybrid assemblies use a combination of vapour impermeable and permeable insulation in a deliberate ratio to ensure that water vapour does not contact a cold surface within an assembly and condense. Hybrid assemblies are discussed in section 4.10.
Any of the three assembly types can work to control condensation in a building, and conversely, mistakes in the design of any of these assembly types can lead to condensation problems.
The treatment of “warm sides” and “cold sides” of building assemblies
The concepts of “warm side” and “cold side” of building assemblies are useful when discussing condensation risk. In cold weather, insulation keeps the inside layers of the assembly warm and the outside layers of the assembly cold. In hot weather, the outside will be warm and the inside cold. In figure 1, during cold weather the plasterboard will be warm (if an internal heater is turned on) while the water control membrane and cladding will be cold.
Cold surfaces are at a greater risk of having condensation form on them, so warm and cold sides of assemblies need to be treated differently. Generally, assemblies are designed to inhibit the movement of moisture through the side of the assembly most likely to be warm, while encouraging the movement of moisture through the side of the assembly most likely to be cold.
In most Australian climates, the warm and cold sides of assemblies will regularly switch at different times of year and different times of day. For this reason, care should usually be taken not to make either side of an assembly too restrictive of moisture movement (i.e. vapour impermeable) to reduce the risk of trapping moisture.
4.3. Airtightness
Airtight layers on the warm internal side of insulation help to prevent moist internal air from passing into a wall or roof assembly where it may condense when it reaches the cold side. In the example wall assembly in figure 1, this could mean making the plasterboard layer airtight, for instance by ensuring that any joints at the tops or bottoms of the plasterboard are well sealed.
While all homes can benefit from continuous ventilation, continuous ventilation is particularly necessary in airtight homes. Drafty, air-leaky homes can have a lower risk of condensation because air leaks can inadvertently reduce indoor moisture. However, homes that are both airtight and have appropriate continuous ventilation can have the lowest condensation risks of any homes.
Beyond condensation reduction, other benefits of airtight homes include:
Improved air quality: When combined with continuous ventilation, airtight homes can achieve better air quality by having less uncontrolled leakage of unclean air from interstitial spaces, like wall, roof or floor cavities.
Comfort and energy efficiency: Lower air leakage means fewer draughts and a home that is easier to keep warm in winter and cool in summer.
Reduced noise: Airtight homes have better soundproofing from outside noise, making the indoor spaces quieter.
Fewer creepy crawlies: Through ensuring there are fewer gaps in the building, airtight homes can reduce the number of insects that enter a home.
Regulatory requirement: Airtightness and continuous ventilation
The airtightness of a home can be measured using a blower door test, where a large fan is fitted to the front door of a home and a technician measures how much air is needed to pressurise the home to 50 pascals of air pressure. Airtightness results can be expressed in terms of air permeability - the air leakage rate per unit of surface area of the building exterior, measured in m³/hr/m² (another common metric is air changes per hour (ACH)). A lower air permeability means a more airtight home.
The NCC includes an airtightness target of 10 m³/hr/m². If a home is found to achieve less than 5 m³/hr/m², the home is required by the NCC to include continuous ventilation. However, under the NCC it is optional to undertake a blower door test or meet these targets, so in practice there is no requirement to achieve any level of airtightness or provide continuous ventilation to a home.
Internationally - the IRC in the USA requires all new homes to achieve an air permeability of at most 5 m³/hr/m². England’s building regulations require an air permeability of at most 8 m³/hr/m² and are incentivised to achieve a permeability of at most 5 m3/hr/m². Both the IRC and England require all homes to be provided with continuous ventilation.
Unknown airtightness levels and condensation risk
Homes can be made airtight accidentally, without using any special methods of design or building. A recent study by the CSIRO found that typical new homes in Australia achieve an airtightness of about 7 m³/hr/m², with a wide range from 0.8 m³/hr/m² to 17 m³/hr/m².
Homes that are relatively airtight (below the NCC threshold of 5 m³/hr/m²) are likely to need continuous ventilation to maintain air quality and reduce condensation risk. The CSIRO data shows that many homes in Australia already reach this level of airtightness, although they may not know it because airtightness testing is uncommon.
Continuous ventilation, which is almost never used in Australia, is likely to be necessary in these airtight homes to maintain air quality and reduce condensation risk. Apartments, which the CSIRO found to be about 20% more airtight than houses, are particularly likely to benefit from continuous ventilation.
In new homes that have not been tested for airtightness, continuous ventilation should be considered as a potential low-cost, high-impact method of reducing condensation risk.
4.4. Vapour control layers
A vapour control layer is a material with relatively low vapour permeance that is placed on the cold side of insulation in a wall or roof assembly. Like most walls in Australia, the example wall in Figure 1 doesn’t have a vapour control layer. If it did, there would either be a plastic membrane installed between the plasterboard and insulation, or else the plasterboard would be painted with a low vapour permeance paint.
Vapour control layers are used in some wall and roof assemblies because without them, even plasterboard that is perfectly airtight will allow some internal water vapour to pass into the assembly due to vapour permeance of the plasterboard.
Simple vapour control layers include polyethylene membranes with extremely low vapour permeance (used in some very cold international climates, but not suitable for Australia), or latex-based paint applied to the plasterboard.
Since water vapour moves more readily through air movement than vapour permeance, there is little point in including a vapour control layer unless that layer is also airtight.
Above: Air leakage is a much more significant cause of water vapour movement than vapour permeance (Source: Building Science Corporation)
Vapour control layers should be used very carefully in Australia. While vapour control layers decrease the rate that water vapour enters into a building assembly, they can also decrease the rate that water vapour exits an assembly. That is, if used incorrectly vapour control layers can increase the risk of condensation in a home.
A more sophisticated type of vapour control layer that is sometimes used in highly energy efficient Australian homes is what is called an intelligent vapour control membrane. Intelligent vapour control membranes vary their vapour permeance based on the relative humidity of the air. Permeance is higher when nearby air is humid and lower when nearby air is dry. The effect of this is to restrict the entry of water vapour into a wall while allowing moisture to escape from the wall when necessary.
Vapour control layers are one of the last measures that should be considered to manage condensation in flow-through assemblies. In many Australian homes the other mitigation measures discussed in this article are sufficient to manage condensation risk. Buildings that don’t or can’t use the other mitigation measures, for example due to bushfire protection requirements (see section 4.8), may benefit from the use of vapour control layers. Intelligent vapour control layers are often used for a side-benefit that they provide – the ability to have much higher airtightness to improve comfort and energy efficiency.
Above: An intelligent vapour control membrane installed in a wall and ceiling (Source: Pro Clima)
Terminology: Vapour control layers, vapour retarders and vapour barriers
Vapour control layers are layers of a building assembly that are designed to slow down, but generally not completely block, the movement of water vapour – they are commonly referred to as vapour retarders in North America.
In contrast, vapour barriers are a type of vapour control layer with extremely low vapour permeance that are used to severely inhibit or block the movement of water vapour. Vapour barriers need to be used carefully because they can prevent the drying of building assemblies that have become wet from condensation or rain.
4.5. Insulation and thermal bridging
Insulation is necessary in modern homes. It helps to reduce energy bills and keep occupants comfortable and healthy.
Insulation prevents heat from moving through building elements. In cold weather, insulation keeps surfaces on the inside of the insulation warmer and keeps surfaces to the outside of the insulation cooler. All else being equal, insulation makes condensation problems less likely to occur on internal surfaces and more likely to occur on external surfaces.
To keep a home comfortable, insulation should be used in all building elements that separate habitable indoor spaces from outdoor spaces. Condensation risks should be managed by the other measures discussed in this article.
Thermal bridging and continuous insulation
Insulation doesn’t work well if it is penetrated (i.e. “thermally bridged”) by a material that conducts heat. In walls and roofs, the most common elements that act as thermal bridges are the timber or metal framing members that insulation batts typically sit in-between. Metal is more than 100 times more thermally conductive than timber, so metal framing poses a much larger thermal bridging challenge than timber.
Severe thermal bridges can create cold zones that cause condensation. These cold zones may be in interstitial locations inside the wall or roof (e.g. the metal frame itself) or on internal surfaces inside a building. Metal frames that act as thermal bridges can cause an effect referred to as “ghosting”, where the shape of the metal frame can be seen on the indoor plasterboard of a wall as condensation occurs on the plasterboard where it contacts the metal frame.
Above: A steel framed wall showing ghosting
Figure 2: Example wall assembly with continuous insulation on the outside of the framing
In the United States, the IRC requires metal framed roofs and floors to be provided with continuous insulation that is not penetrated by the frame. Australia’s NCC does not require continuous insulation for metal framed buildings. This video from the Build Show provides a sobering assessment of this difference in American and Australian construction practice.
While framing is a major cause of thermal bridging in Australia, it is not the only cause. Thermal bridging occurs anywhere where insulation is non-continuous, such as where it is penetrated or there is a junction with another building element (for example between a wall and a floor).
If continuous insulation is vapour impermeable, the assembly becomes an “impermeable assembly” or “hybrid assembly” (as opposed to the more commonly used “flow-through assembly”). Additional design considerations need to be taken into account for impermeable and hybrid assemblies as discussed in sections 4.9 and 4.10 of this article.
Continuous insulation that is external to a building’s structural framing can keep the structure at a more constant temperature, reducing damage from expansion, contraction and temperature. In doing this, continuous insulation can help to extend the functional life of a building.
Window insulation
Windows are usually the parts of a home that are the least insulated, which often makes them the coldest indoor surfaces. This is why condensation is often seen on bedroom windows after a cold night.
Single glazed windows with metal frames provide particularly low insulation, and therefore have a particularly high risk of condensation. Improved windows, like double or triple glazed windows with more insulative frames (e.g. timber, PVC or thermally broken aluminium frames) decrease condensation risk significantly.
While curtains and blinds help to keep people warm in their homes, they can also increase condensation. By insulating the window from the home's interior heat, these coverings cause the glass surface to become even colder. The more insulative the blind or curtain (and the poorer the window’s insulation) the higher the likelihood of moisture forming on the window behind the covering.
Risk of condensation on windows should be managed by installing good windows – for example good quality double glazed windows with insulative frames, and by reducing indoor humidity by providing continuous ventilation to the home. Cooler climates require higher performance windows than warmer climates to manage condensation risk.
Regulatory requirement: Window insulation
The NCC does not specify minimum requirements for window insulation. Instead, the NCC requires a minimum level of energy efficiency for a home overall. This means that the minimum window insulation to comply with the NCC varies depending on home orientation, wall, roof and floor insulation levels, shading of windows and many other factors.
The NCC uses the average insulation value of a window, which means it doesn’t differentiate between the performance of the frame and the performance of the glass. Often in Australia, double glazed windows will be installed with aluminium frames, and these conductive frames will be more likely to have condensation occur on them.
Window insulation is measured in terms of “U-value”, where a high U-value has lower insulation than a low U-value. While there is no maximum window U-value permitted by the NCC, the median window U-value used in the state of Victoria under the current NCC is 3.8, with 10% of windows having a U-value of greater than 5.0.
In the United States, the IRC specifies a maximum allowed window U-value of 1.7 (when converted for metric units, for similar climates to Victoria), and in England the maximum allowed U-value is 1.2 (for climates slightly colder than Victoria). That is, the median window under the NCC in Victoria has less than half the insulation required by the IRC, and less than a third of the insulation required in England – leading to increased window condensation risks in Australia.
Thermal bridging from framing can be mitigated by using a continuous layer of insulation on the outside of the framing, for example as shown in figure 2 below.
4.6. Heating
Turning on a heater to warm a home in winter has both positive and negative impacts on condensation risk:
Warmer surfaces - warmer internal surfaces (including walls, windows, floors and ceilings) are less likely to have condensation form on them.
Lower relative humidity - heating reduces the relative humidity of indoor air, leading to lower risk of condensation and mould growth (noting that mould growth can occur at as low as 70% relative humidity).
Increased evaporation rate - water evaporates more quickly in hot air (including increased evaporation from the human body) leading to higher absolute humidity of indoor air, and increased winter condensation risk.
Heating is necessary to make homes comfortable and healthy for occupants. Heaters should not be turned off to decrease condensation risk. Instead, the other measures discussed in this article, particularly continuous ventilation to reduce indoor absolute humidity, may be needed to reduce condensation risk.
Solar passive heating and weatherproofing
Heat from the sun can be used to heat a home in cold weather, referred to as passive solar heating. Passive solar heating can help make a home comfortable and efficient and is encouraged by the NCC.
Solar heating of external surfaces of walls and roofs can also reduce condensation risk, as condensation that may have accumulated on the outside of the insulation layer is given a chance to evaporate.
However, in some cases the NCC encourages passive solar heating by encouraging homes to have small or no eaves shading walls or windows. Eaves help to protect walls and windows from becoming excessively wet from rain, so reducing eave depth can increase the risk of water ingress into a wall or window. Water ingress can cause damage on its own, or can exacerbate condensation problems.
Passive solar heating is worthwhile, however care should be taken not to inadvertently impede weatherproofing by artificially reducing eave depth or removing other structures that beneficially shelter walls from rain.
4.7. Vapour permeance
As discussed in section 4.4, it can be beneficial to prevent the entry of water vapour into building assemblies using materials with low vapour permeance on the warm side of insulation. In contrast, this section of the article discusses the benefit of using materials with high vapour permeance, especially on the cold side of insulation.
Building assemblies often need opportunities to dry out. Some moisture inevitably enters into a wall or roof assembly, whether due to water vapour from air leakage or vapour permeance, or simply due to the entry of rain. To enable moisture to leave an assembly, most building assemblies (in particular, flow-through assemblies) need to use materials that are vapour permeable.
Vapour permeance is particularly important in the parts of the assembly that are airtight and on the cold side of insulation. Water vapour tends to move from warm spaces to cold spaces, so it is important to let water vapour leave the assembly towards the colder outdoors, or else it will be trapped and will accumulate inside the assembly.
Figure 3: Water control membrane in a timber-clad wall
In the example timber-clad wall, ensuring that the water control membrane is vapour permeable is likely to decrease winter condensation risk. In contrast, the materials on the warm side of the insulation layer can be made partially vapour impermeable (as discussed in section 4.4). Low permeance on the warm side of an assembly helps to prevent water vapour entry into the assembly, while high permeance on the cold side of an assembly helps water vapour to leave the assembly.
Figure 4: Vapour permeance in a flow-through assembly
Many water control membranes that have been used in Australia, particularly those made of reflective metallic foil, are not vapour permeable. These impermeable membranes can trap water vapour inside an assembly and increase condensation risk.
Precast concrete walls, which are used in many apartment buildings, often include impermeable materials on the cold side of insulation (see figure 5) and may have an increased risk of condensation. Water vapour can pass through the plasterboard and insulation and then condense on the internal side of the concrete. The impermeable concrete does not allow the water vapour to escape through to the outdoor air.
Continuous ventilation (section 4.1) and internal vapour control membranes (section 4.4) can help to reduce condensation risk in precast concrete walls. However, the best solution is to externally insulate the wall (section 4.5) so the concrete is on the inside (warm side) of the insulation and doesn’t trap moisture inside the assembly.
Above: Condensation damage behind an impermeable foil membrane (Source: M Dewsbury 2018)
Figure 5: An example precast concrete wall
4.8. Drained and ventilated cavities
In most building assemblies used in Australia, the outermost layer of the assembly (e.g. the brick, weatherboard or steel outer layer of a wall) can't be made vapour permeable enough to reduce condensation risk. Instead, water vapour is allowed to exit the assembly by installing an air cavity with openings to the outside. Openings for ventilation and drainage to the outdoors allow water vapour and liquid water to leave the building assembly.
Figure 6: Drained and ventilated cavity in a timber-clad wall
Drained and ventilated cavities also assist assemblies to dry out after rain. No cladding is perfectly waterproof and some water will seep through the cladding after rain. In general, weatherproofing is more important to get right than condensation management, so the improved weatherproofing that cavities provide is likely to be even more important than the improvement to condensation control.
Drainage of liquid water from a cavity is more important than ventilation openings for air. Internationally, sometimes very narrow drainage pathways can be used to provide an effective solution without the need for a traditional cavity.
Roof ventilation
Moist air is lighter than dry air, which causes moist air to rise. Roof structures can have high condensation risk due to air movement and vapour drive pushing moisture upwards from the interior of a home into a roof structure.
Water vapour from indoors often enters roofs by moving upwards with air through gaps in the plasterboard ceiling – often through penetrations in the ceiling for downlights. Water vapour may also pass directly through the plasterboard through vapour diffusion. In most roofs, the water vapour will pass through ceiling insulation and create a high risk of condensation inside the roof.
Ventilated roof spaces are analogous to ventilated cavities in walls. Ventilated roof spaces allow water vapour that has entered from internal spaces to leave the assembly. Roof spaces can be provided with ventilation by installing openings at both the top of the roof (near the ridge), and at the bottom of the roof (e.g. the eaves).
Figure 7: An example ventilated roof space (Source: Bluescope)
Above: An example of ventilation openings at roof eaves
Regulatory requirement: Vapour permeance and ventilated cavities
As of the 2025 version of the NCC, walls are required to have drained and ventilated cavities in climate zones 6, 7 and 8 (e.g. Melbourne, Western Sydney, Canberra and Hobart). Roofs have been required to have ventilated airspaces in climate zones 6, 7 and 8 since NCC 2022.
Water control membranes used in walls are required to have a minimum level of vapour permeance in climate zones 3 - 8, with increasing permeance required in walls in cooler climate zones and in walls without drained and ventilated cavities.
In bushfire-prone areas the NCC requires ventilation openings in roofs to be treated to prevent ember entry, for example by installing a mesh over the opening. In very high risk bushfire areas ventilation openings are not allowed at all.
Roofs without ventilation will have to use other methods of managing condensation risk. This may include using vapour control membranes installed immediately above the ceiling plasterboard (section 4.4), or changing construction style entirely (e.g. using warm roof construction as discussed in section 4.9).
Bushfire zones
4.9. Special case: Impermeable assemblies
Impermeable assemblies manage condensation by keeping all water vapour on the warm side of insulation so there is little opportunity for condensation to occur. This contrasts with flow-through assemblies which allow some water vapour to pass through an assembly, as discussed in sections 4.4 to 4.8. Impermeable assemblies need to be designed and constructed with care because if they accidentally admit moisture, the impermeable nature of the assembly can inhibit drying. Impermeable assemblies are much less common in Australia than flow-through assemblies.
Figure 8: An example vapour-impermeable wall using SIP panels with an EPS core (Source: Building Science Corporation)
A structural insulated panel (SIP) wall with a vapour-impermeable expanded polystyrene (EPS) core is an example of an impermeable wall assembly, as shown in figure 8. The foam core of the SIP wall is an effective air and vapour barrier while also providing for effective continuous insulation. No water vapour can pass through the impermeable foam insulation and therefore internal water vapour never encounters cold surfaces on the outside of the insulation, and doesn’t condense.
SIP panels are manufactured in a factory, so they can be very consistent with small tolerances and few gaps. If the joints between SIP panels are sealed effectively, SIP assemblies can achieve very high airtightness levels, which is necessary to ensure condensation risk is low in impermeable assemblies.
Figure 9: Example warm roof construction
Another form of impermeable assembly is what is referred to as a “warm roof”. Warm roofs are roofs where the insulation is installed on the outside of the roof frame, thereby keeping the roof frame warmer in the winter time. Warm roofs are uncommon in Australia.
Like the SIP wall, warm roofs use vapour-impermeable insulation and a vapour control layer to keep water vapour on the cold side of the insulation and prevent condensation from occurring. Like SIP walls, airtightness and vapour control are crucial in a warm roof.
Most roofs need to be pitched at a moderate angle to encourage rainwater to run-off, improve roof ventilation effectiveness and ensure that any internal condensation that occurs on the roof can be drained at the eaves. Warm roofs do not need roof ventilation and have a lower risk of internal condensation, so they have less need to be as steeply pitched as other roofs - this allows more internal living space to be provided for the same height building.
4.10. Special case: Hybrid assemblies
A hybrid building assembly is an assembly that uses two layers of insulation with different vapour permeances - an internal insulation layer that is vapour permeable and an external insulation layer that is vapour impermeable.
Figure 10: Example wall assembly with continuous vapour-impermeable external insulation alongside vapour-permeable insulation between framing members
Some building assemblies use continuous vapour-impermeable insulation on the outside of framing in combination with vapour-permeable insulation in between framing (as in figure 10). In these assemblies, if the internal surface of the vapour-impermeable insulation is allowed to get too cold, winter condensation risks increase.
The temperature of the vapour-impermeable insulation depends on the ratio of the insulation provided by the two insulation layers. The more insulative the internal layer is compared to the external layer, the colder the internal surface of the vapour-impermeable insulation will be.
There is no public research in Australia that calculates the necessary ratio of insulation of the two insulation layers to limit condensation risk. However, the ratios can be estimated based on requirements in the USA’s International Residential Code and calculations by the Building Science Corporation. Note these ratios have been estimated for Australian climates for the purpose of this article and should not be taken as design advice.
Table 1: Impermeable insulation ratios adapted from the Building Science Corporation
Regulatory requirement: Hybrid assemblies
The NCC requires second layers of insulation to be vapour permeable in climate zones 4-8. That is, hybrid assemblies are generally not allowed in these cooler climates.
The exception is where hybrid assemblies have been modelled and shown to be low risk as discussed in section 4.11.
4.11. Modelling
Computer-based simulations can be used to estimate condensation and mould growth risk in a building assembly. These simulations are referred to as hygrothermal models.
A large amount of information is needed to undertake a hygrothermal model, including:
Hourly weather data including temperature, humidity, solar radiation and rain.
Material properties including vapour permeances, thermal conductivities, density and initial moisture content.
Assembly details including orientation, material thicknesses and airtightness.
Occupant activity including ventilation, heating, cooling and cooking and showering patterns.
The accuracy of a hygrothermal model is dependent on the accuracy of the input data. As with any model, inaccurate inputs lead to inaccurate results. The models necessarily make assumptions about occupant behaviour and quality of workmanship, which may not always reflect what occurs in reality. While hygrothermal models cannot be perfect, they are a useful tool for understanding whether a building assembly is more or less likely to experience condensation problems than another assembly.
Most common hygrothermal modelling tools (e.g. WUFI Pro) focus on the 1-dimensional heat and moisture flow through a cross-section of a wall or roof assembly. They are less effective at capturing complex 2D or 3D moisture issues at junctions or around thermal bridges - these require more complex modelling.
Regulatory requirement: Modelling
In the NCC, mould growth risk as measured by the “mould growth index” is the key measure of whether an assembly has a sufficiently low risk of condensation.
The NCC includes an optional method for demonstrating compliance of a building assembly, where the assembly is considered compliant if it is modelled in accordance with an industry standard and shown to have a mould growth index of less than 3.
Figure 11: Example hygrothermal modelling result (Source: M Dewsbury 2022)
Figure 11 shows an example result of a hygrothermal model of an assembly. The chart shows that mould growth risk in this assembly increases every winter as winter condensation occurs, and then decreases every summer as extra heat helps to dry the assembly. However, since the assembly does not dry as readily as it gets wet, the mould growth risk increases over time. The assembly has a mould growth index of more than 3 after 4 years and so does not pass the modelling requirements specified in the NCC.
Hygrothermal models can say whether risks of condensation are relatively high or low. However, due to model limitations, including uncertainty of input assumptions, these models don’t definitively say whether condensation problems will occur.
Hygrothermal models are useful tools but are not a complete substitute for a carefully thought-through condensation management strategy in a home.
5. Managing summer condensation
Summer condensation occurs when humid outdoor air comes into contact with a cold surface of a building - in summer, cold building surfaces often occur due to air conditioning. This contrasts with winter condensation, where moist air is generated indoors and cold temperatures come from cold outdoor conditions.
Buildings are at a greater risk of summer condensation in Australia’s more humid climates, particularly tropical locations like Darwin, Cairns or Townsville (NCC climate zone 1). Ironically, the term “summer condensation” is a misnomer in these tropical locations which experience wet seasons and dry seasons rather than summers and winters. This article continues to use the term “summer condensation” for the sake of consistency with established international terminology.
There is limited public documentation on the nature and extent of summer condensation in Australia. This lack of public research may mean that the mitigation measures discussed in this section are not comprehensive. The NCC only includes minor measures to address summer condensation.
Strategies discussed in this section to address summer condensation include:
Limiting the occurrence of cold surfaces by minimising the need for air conditioning and installing air conditioning ductwork in conditioned spaces.
Reducing the humidity of air using dehumidification and continuous ventilation.
Preventing moist air from entering building assemblies using airtightness and vapour control.
Allowing building structures to dry out using materials that are appropriately vapour permeable.
Summer condensation is condensation that occurs when humid air from outdoors comes into contact with a cold building surface.
5.1. Minimise the need for air conditioning
Condensation risks can be reduced by decreasing the need for air conditioning, which reduces the occurrence of cold surfaces. Homes can be designed to achieve comfortable conditions with less air conditioning using passive energy efficiency principles - taking advantage of cross ventilation, window shading, light coloured walls and roofs, thermal mass, insulation and ceiling fans.
5.2. Promote air movement
Circulating air within a home, for example with ceiling fans, can help to reduce summer condensation risk. Air circulation helps to equalise surface temperatures, keeping higher risk areas like corners and inside cupboards warmer and less likely to experience condensation or mould growth. At times when indoor humidity is low (for example if the weather is less humid or indoor dehumidification is used), air circulation helps to dry out condensation that may have previously occurred.
Because air circulation reduces summer condensation risk, people in humid climates are often advised to leave ceiling fans running and cupboard doors open when leaving a home unattended for long periods.
5.3. Install air conditioning ductwork in a conditioned space
In Australia, ducted air conditioning systems are often installed in unconditioned and uninsulated spaces, like roof spaces (figure 12). Air conditioning ductwork gets cold when it is transporting cold air, and that ductwork becomes a surface that condensation can occur on.
Figure 12: An uninsulated and unconditioned roof space has a higher risk of condensation on air conditioning ductwork
The best way to decrease the risk of condensation on air conditioning ductwork is to only install these ducts in spaces that are insulated and conditioned. For ducted air conditioning systems installed in roof spaces, this means insulating at the roof line, rather than the ceiling line, and installing an air conditioning outlet within the roof space (figure 13). This turns the roof space itself into a conditioned space.
Figure 13: An insulated and conditioned roof space has a lower risk of condensation on air conditioning ductwork
When a roof or floor space is conditioned, the air in the space is less humid, particularly if combined with dehumidification and positive pressurisation as discussed in section 5.4. Less humid air is less likely to condense on cold surfaces like air conditioning ductwork.
Conditioned roof or floor spaces are much more pleasant when someone needs to enter them and can be used as storage space. However, they are uncommon practice in Australia today, and are likely to be more expensive to build.
Ductwork insulation
A common Australian practice for managing condensation on air conditioning ductwork is to insulate the ducts themselves. Ducts can be insulated either internally or externally.
Internal ductwork insulation lines the inside of the metal duct, while external ductwork insulation wraps around the outside of the metal duct.
Metal foil vapour barriers are wrapped around externally insulated ductwork to prevent water vapour from passing through the insulation to contact the cold metal duct. In internally insulated ducts, the metal duct itself provides the role of the vapour barrier.
Relying on ductwork insulation alone does not reduce condensation risk as much as installing ductwork in a conditioned space. Most ducts leak air, which creates cold spots where condensation can occur on the outside of insulation. Vapour barriers on externally insulated ductwork can be ineffective if they are imperfectly sealed, damaged or degraded.
Above: Externally insulated ductwork (left) and internally insulated ductwork (right)
5.4. Dehumidification and continuous ventilation
Managing condensation in humid climates is a difficult problem: How can moisture be managed when the atmosphere itself is moist?
The solution often requires removing moisture from the air with dehumidification and then controlling how that air interacts with a building with continuous ventilation. This reduces the humidity of air that is present inside of homes and building assemblies.
Dehumidification combined with continuous ventilation is rare in tropical Australian homes because it is relatively expensive and energy intensive. It is more common in commercial or public Australian buildings and is sometimes used in homes in humid American climates.
Dehumidification
Dehumidification lowers the absolute humidity of indoor air. Dehumidification can be provided by standalone plug-in dehumidifiers, air conditioners on “dry mode” or dedicated whole-home dehumidification systems connected to central heating and cooling systems. Some benefits and challenges of standalone dehumidifiers specifically have been discussed in section 4.1 of this article.
Left: A standalone plug-in dehumidifier, Right: A simplified schematic of a whole-home ducted dehumidification system
By reducing humidity, dehumidification reduces condensation risk within a home. When paired with continuous ventilation and positive pressurisation as discussed below, dehumidification also reduces interstitial condensation risk within building assemblies.
Above: A low-cost method of providing whole-home dehumidification using a dehumidifier installed at the air intake of a ducted air conditioning system (Source: Building Science Corporation)
Continuous ventilation
Continuous ventilation means continuously bringing outdoor air into a home, usually with a fan. It is discussed in the context of winter condensation in section 4.1. The author of this article has also written a separate detailed article about continuous ventilation here.
Continuous ventilation with dehumidification can reduce condensation risks by positively pressurising a home and ensuring the only air that infiltrates building assemblies is moving from indoors to outdoors. For homes in humid climates, continuous ventilation should be provided by one of the following:
supply-only ventilation - using a continuously running fan to supply outdoor air into a home.
balanced ventilation - using a continuously running fan to supply outdoor air into a home and simultaneously using a continuously running fan to exhaust air out of a home.
energy recovery ventilation - a balanced ventilation system that passes the exhaust and supply airstreams through a heat and humidity exchanger. This results in the air from outdoors passing its heat and humidity to the air being exhausted. By contrast, heat recovery ventilation (discussed in section 4.1) exchanges heat but not humidity. Energy recovery ventilation decreases the load on air conditioners and dehumidifiers compared to other continuous ventilation methods. While this system uses less energy than other methods of ventilation, it also has the highest initial cost.
In humid climates, continuous ventilation should be designed to positively pressurise the home. That is, more air should be pushed into a home with a supply fan than is pulled out of a home using an exhaust fan. Positive pressurisation means that where there are gaps in the building structure, the air that passes into those gaps will be from the dry indoors rather than the humid outdoors. Positively pressurising a home prevents humid air from outside from infiltrating into building assemblies.
Alternative methods of continuous ventilation, like running exhaust fans, do the opposite of this – the home becomes negatively pressurised and draws in humid outdoor air through gaps in the building structure, increasing interstitial condensation risk. For this reason, continuous ventilation of a home using exhaust-only systems should be avoided in humid climates.
Above: Dry indoor air passes through building assemblies in positively pressurised homes
Dehumidification is needed alongside all of the continuous ventilation options in humid climates. Without dehumidification, humid air brought in from outdoors increases condensation risk. Dehumidification is energy intensive, and therefore expensive to operate - this is an unfortunate trade-off that must be made to reduce condensation risks for many buildings in humid climates.
Ideally, outdoor air should be dehumidified before it is brought into a home. This can be achieved with a ducted outdoor air dehumidifier or an energy recovery ventilation system.
Above: A ducted outdoor air dehumidification system dehumidifies outdoor air before it is supplied into a home
When air-conditioning, continuous ventilation and dehumidification are all present they can be controlled separately to achieve optimum indoor conditions. For example:
The continuous ventilation system can be controlled to maintain indoor air quality. By increasing or decreasing ventilation in response to markers of indoor air quality (e.g. carbon dioxide levels), air quality can be maintained while conditioning no more outdoor air than necessary. However, a minimum constant level of outdoor air may be needed to ensure positive pressurisation is maintained.
The dehumidification system can be controlled to maintain indoor humidity.
The air conditioning system can be controlled to maintain indoor temperature.
Controlling these systems to achieve separate goals allows them to work together for optimal indoor conditions for comfort, health and condensation risk.
5.5. Airtightness and vapour control applied to external layers of assemblies
Airtightness and vapour control layers have previously been discussed in sections 4.3 and 4.4 of this article. These measures help to prevent moisture entry into an assembly when they are undertaken on the warm side of insulation, which in humid climates is the outside of the insulation. For example, in the flow-through timber clad assembly in figure 14, water vapour can be inhibited from entering the assembly using a vapour-impermeable water control membrane. Making the membrane airtight further helps to reduce moisture infiltration into the wall.
Figure 14: In humid climates, a vapour-impermeable water control membrane on the outside of insulation can prevent moisture entry into an assembly
In climates that are prone to winter condensation, vapour-impermeable layers on the outside of insulation can increase condensation risk (as discussed in section 4.7), and therefore care should be taken with this measure in these climates.
Limiting roof ventilation
While roof ventilation can decrease winter condensation risk in cooler climates, roof ventilation may increase summer condensation risk in humid climates. Roof ventilation can introduce humid air into a roof cavity and should be used carefully, if at all, in humid climates.
Tropical, humid climates are often also cyclone-prone. Designing roofs without roof ventilation may reduce the risk of structural damage from extreme winds associated with cyclones.
Regulatory requirement: Vapour control layers, airtightness and continuous ventilation
The NCC does not allow the use of highly vapour-permeable membranes on the outside of insulation in walls in humid climates.
The NCC includes an airtightness target that can be used with a voluntary blower door test. If a home is found to be particularly airtight through this voluntary test, the home is required by the NCC to include continuous ventilation. However, since it is optional to undertake this test, in practice there is no requirement to achieve any level of airtightness or provide continuous ventilation to a home.
Impermeable assemblies
Impermeable building assemblies, which are discussed in section 4.9 of this article, can be highly effective at managing summer condensation risks.
Impermeable assemblies use air- and vapour-impermeable materials to keep water vapour on the outside of the assembly. They do not give water vapour the opportunity to enter building assemblies or the home.
Care must be taken to design and build impermeable assemblies correctly. An impermeable assembly that inadvertently admits moisture may be poor at drying out.
Figure 15: An example vapour-impermeable wall using SIP panels with an EPS core (Source: Building Science Corporation)
5.6. Vapour permeance of internal assembly layers
As discussed in section 4.7, materials on the cold side of insulation in a flow-through assembly should be vapour permeable so that moisture can pass out of the assembly. In humid climates, the cold side of an assembly is everything to the inside of the insulation. If materials on the inside of the insulation (for example wallpaper) are not vapour permeable, water vapour can be trapped and condensation and mould can form. Making these materials vapour permeable allows the assembly to dry towards the inside.
Above: Vapour-impermeable internal wall coverings, like this vinyl wallpaper, can increase summer condensation risk in humid climates (Source: Building Science Corporation)
6. Managing night sky condensation
Night sky condensation is caused by an effect called radiative cooling, which often causes the external surfaces of roofs and walls to become extremely cold overnight and experience condensation.
During the day, the sun radiates heat down to the earth and heats up exposed surfaces. At night, those same surfaces radiate heat back to the sky. Counterintuitively, this can cause surfaces facing the sky to cool to a lower temperature than the ambient air. This radiation of heat back to the sky is particularly severe on clear nights with little cloud cover. This is the same effect that causes dew or frost on grass after a clear night.
Radiative cooling can cause surfaces that face the sky, including the undersides of those surfaces, to experience condensation. This condensation – night sky condensation – is a particular risk in roofs.
In roof spaces, humid air can enter either from outdoors (especially if the roof space is ventilated) or from indoors from air leakage or vapour diffusion through the ceiling. When the surface of the roof gets cold from radiative cooling, that humid air can condense and create liquid water inside the roof.
Above: Night sky condensation causing frost in a roof space
Night sky condensation is condensation that occurs due to the radiative cooling of surfaces that face the sky.
6.1. Flow-through roof assemblies
Foil-backed insulation blankets
In Australian homes night sky condensation is commonly managed by installing a foil-backed insulation blanket under metal roof cladding.
A foil-backed insulation blanket is a layer of soft fibreglass insulation with metal foil adhered to one side (commonly referred to by the brand name “anticon”). The non-foil side of the blanket is installed directly underneath the metal face of the roof, with the foil side facing into the roof space. The foil acts as a vapour barrier to prevent moisture from coming into contact with the underside of the cold metal roof surface. The fibreglass insulation keeps the foil warmer so that condensation does not occur on it.
Figure 16: A ventilated roof with a foil-backed insulation blanket (Source: Bluescope)
Above: A foil-backed insulation blanket and metal roofing
Drained sarking
Another method of reducing night sky condensation risk inside a metal roof is to install a waterproof membrane (often referred to as sarking) underneath the metal surface of the roof. Installing the membrane with a small space between it and the surface of the roof allows the membrane to catch any condensation that drips from the underside of the metal surface of the roof, and then drain that condensation to a gutter. In this design the sarking should be highly vapour permeable to allow moisture inside the roofspace to pass through the sarking and condense on the metal roof, rather than condensing on any other surface in the roof.
Figure 17: A ventilated roof design with a drained sarking (Source: Bluescope)
6.2. Impermeable roof assemblies: Warm roofs
Warm roofs are a type of vapour-impermeable roof assembly that are currently uncommon in Australia, but can be highly effective at managing condensation risks. Warm roofs are roofs that exclusively use continuous external insulation on the outside of the roof frame (as discussed in section 4.5), thereby keeping the roof frame warmer in the winter time.
Warm roofs use vapour-impermeable insulation to keep water vapour on the cold side of the insulation, which prevents the water vapour from contacting the cold roof surfaces that would result in night-sky condensation. A high degree of airtightness and vapour control beneath the insulation is crucial to manage condensation in a warm roof.
Most roofs need to be pitched at a moderate angle to encourage rainwater to run-off, to improve roof ventilation effectiveness and to ensure that any internal condensation that occurs on the roof can be drained down to the eaves. Warm roofs do not need to be ventilated or to drain condensation, and therefore can often be less steeply pitched, which can gain more internal living space for the same height building.
Figure 18: Example warm roof assembly
7. Key opportunities for Australian homes
This article documents how condensation can be considered and managed in Australian homes. The measures described are opportunities to not only reduce condensation risk, but also to improve the comfort, resilience and health of Australian housing. Some of the key opportunities, beyond minimum legal requirements, are shown below:
Above: Key opportunities to manage condensation in Australian homes
These measures have significant benefits beyond reducing condensation risk, including:
Better energy efficiency and comfort, through improved airtightness, windows and insulation.
Better air quality through continuous ventilation.
Better weatherproofing through drained and ventilated cavities behind wall cladding.
Apartments, which are more airtight than houses, will particularly benefit from continuous ventilation.
Beware unintended consequences
Many design decisions have unintended consequences. For example, some buildings have accidentally increased fire risk with inappropriate cladding, while others have accidentally increased condensation risk through energy efficiency measures.
When considering condensation management measures, building practitioners need to be careful that they don't cause unintended consequences. Particularly careful thought is needed when undertaking measures that are unfamiliar to the design or construction teams.
The measures presented in this article are not a final position on condensation management in Australia. This article documents a perspective on the current state of condensation science in Australia, which is still a work in progress.
Effective condensation management is challenging because it forces consideration of many different goals that homes need to achieve - energy efficiency, indoor air quality, weatherproofing and more. Getting condensation management right often means that a home design has been well thought out and that the home will be a great place to live.
Note from the author:
Feel free to contact me using the contact form or by emailing alex@modernefficienthomes.com. I'm interested in opportunities to collaborate or in seeing examples and photos of condensation that can be shared on this website.
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