Key passive
design principle
#1 Orientation
Key passive
design principle
#2 Materials
Key passive
design principle
#3 Windows
Key passive
design principle
#4 Solar passive
Cross ventilation
Key passive
design principle
#5 Cross ventilation
Key passive
design principle
#6 Utilities

The following key areas are the most important elements in an eco-effective home. This is just an introduction, please refer to the links at the bottom for more information.

Understand the climate and get the orientation right

You can’t design a more thermally efficient home until you know what the climate is like – when is it hot? When is it cold? How hot or cold does it get? What’s the humidity and rainfall like? Where does the sun come from? These are all important questions we need to answer to understand what are going to be the best design strategies for a particular climate.

In Australia there are eight distinct climate zones identified under the National Construction Code of Australia:

Thus, the design strategies that would work in Perth would be different to Darwin and Canberra. Many towns and cities around Australia are located in climate zone 5, which has the following general characteristics:

  • Warm to hot summer days and nights
  • Cool to cold winter days; cold winter nights
  • Moderate to high humidity

This means we have to design in response to both hot and cold temperature conditions throughout the year, and not give precedence to one over the other. The building materials and windows we select will have a big impact on how the building will perform (see design principles No. 2 and 3). Moderate to high levels of humidity also mean we need to ensure there is adequate ventilation in the building to help remove excessive moisture from the air (see design principle No. 5).

The next most important thing is to understand where the sun is coming from. The sun’s relative position in the sky changes throughout the day and year, and also varies with latitude and longitude. We can use the principles of solar geometry to determine the exact position of the sun for every date and time of the year, though the following are the key things you need to know about the position of the sun in climate zone 5:

  • In the southern hemisphere, as the sun moves from east to west, it tends to follow a northerly trajectory through the sky.
  • The sun is higher in the sky in summer; lower in winter.
  • The arc of the sun is longer in summer; shorter in winter.
  • Sun from due east and west will hit a building almost perpendicularly.

This means we should following design principles when working out the layout of rooms in a new building:

  • Make the building rectangular in shape, stretching from east to west. This means the longest sides of rectangular prism face north, which is the easiest side of the building where we can control the impact of the sun.
  • The short sides of the rectangle will face east and west – these are the sides where it is hardest to control the impact of the sun. By making sure these sides have the smallest possible surface area, we can minimise the impact of the sun’s radiation.
  • Day spaces in the building such as living rooms, dining rooms, kitchens, studies and activity areas should be located on the north side of the building – these spaces will benefit the most from facing north, both in summer and winter.
  • Night spaces such as bedrooms should be ideally located on the south side of the building – these spaces are less frequently occupied during the day, and so don’t generally need to benefit as much from the sun during the day.
  • Utility spaces such as bathrooms, ensuites, laundries, toilets and storerooms can be situated on the east and west sides of the building. These rooms are only occupied for short periods of time, and can be used to buffer and protect the habitable rooms from the worst of the summer sun at sunrise and sunset.

Of course, the layout and shape of a building will also be influenced by the shape and orientation of the site, and the location of existing buildings and other structures. A skilled architect or building designer will be able to account for the impact of these other factors AND still achieve these orientation and room layout principles.


Choose appropriate building materials

Improving the thermal performance of a building is about controlling the heat flow that occurs into and out of it – we want to minimise and remove heat gain during hotter months, while increasing heat gain and retention during winter. The following diagram shows the approximate percentage breakdown of how heat is gained and lost in a typical residential building:


If you add up the total of the floors, walls and roof, this equals around three-quarters of total heat transfer. Thus, the materials from which we construct a new building or renovation can have a major impact on the overall thermal performance!

When we are evaluating what makes a building material good in terms of its performance, there are three key characteristics we need to consider:

  • Insulation: refers to a material’s ability to resist the flow of heat from inside to outside, or vice versa. Generally speaking, we want floor, wall and roof materials to have a high level of insulation, as this helps to keep heat out during summer, and warmth in during winter. Insulation is also known as R-Value – the higher the r-value number, the greater a material’s resistance to heat flow (but you also have to check if you are comparing metric units to imperial units, which need to then be converted to be compared). The National Construction Code gives the following recommended R-value ranges for a residential building in climate zone 5:

  • Floors above ground level
  • 1.0Recommended R-value range
  • External Walls
  • 2.4-2.8Recommended R-value range
  • Roofs and Ceilings
  • 4.1-5.1Recommended R-value range

  • Thermal mass: refers to the ability of a material to store and retain heat. While this may sound very similar to insulation, it actually refers to very different physical properties. Insulation is about thermal resistance, while thermal mass is about capacity. Thermal mass helps to improve the thermal performance of a building in both summer and winter. In summer, the thermal mass is able to store and retain heat, helping to maintain a lower and more stable internal temperature; in winter, if exposed to solar radiation from the sun, the thermal mass absorbs this heat, and can help to maintain warmer room temperatures even at night. The formula for calculating the right about of thermal mass in a building is quite complex, but as a very rough rule of thumb, you want a “goldilocks” amount – not too little, not too much!
  • Colour: last but not least, the colour of a surface or building material will also influence the thermal performance of a building. You probably already know this intuitively, if you have ever worn dark coloured clothing or parked a black car out in the summer sun. Colour influences a material property known as solar absorptance – a value between 0 and 1, where the higher the number, the higher the solar absorptance. Generally speaking, darker colours have a higher solar absorptance ie. they absorb a greater amount of solar radiation and heat up when exposed to the sun. For a building located in climate zone 5, I would recommend the following solar absorptance values for different surfaces of a building:

  • External walls
  • 0.3-0.5Recommended solar absorptance ranges
  • Roof
  • 0.1-0.3Recommended solar absorptance ranges
  • Internal Floors
  • 0.7-0.9Recommended solar absorptance ranges

There are a few other things to be aware when selecting construction materials for your new build or renovation:

  • Insulation and thermal mass tend to be mutually exclusive properties ie. if a material has a higher level of insulation, it will have a low level of thermal mass, and vice versa.
  • The trick is to use a combination of different materials to get the best of both worlds – materials with level levels of thermal mass should be exposed on the inside of a building, while materials with a high level of insulation should be on the outside. This way, the internal thermal mass helps to regulate temperatures inside the building, while the insulation on the outside protects the thermal mass from the extremes of the external temperature.
  • Materials with a high level of insulation include polyester, polystyrene, fibreglass and wool batts, and also blow-in insulation products such as those made from old newspaper. Generally speaking, the thicker the insulation material, the higher the r-value (though different materials will have different r-values for the same thickness). Make sure the insulation doesn’t get squashed or compressed when being installed – it’s actually the tiny pockets of air trapped within the insulation that provides the thermal resistance – if you squeeze all the air out, it won’t work anymore!
  • Materials with a high level of thermal mass include concrete, brick, plaster, stone, tiles, sand and clay. Even water has a high level of thermal mass, though it can be very difficult to build with! Thermal mass works best when dark in colour (ie. high solar absorptance), exposed to solar radiation during winter (but not summer), and not covered over the insulative materials such as carpet or floorboards.

You should also give consideration to other factors such as cost, availability, ease of construction, warranties, durability, waste by-products and toxicity of the building materials you choose for your new build or renovation. Some of these topics are explored in more detail in the Thoughts section of my website.


Select the best windows for summer and winter

Windows are an essential part of any building – they let in natural light, provide views and can be opened to provide fresh air and natural ventilation. Yet they can make or break the thermal performance of a building if we don’t position, size and specify them properly in a new build or renovation.

Firstly, it’s really important to size and position windows on a building to take advantage of the solar orientation covered in the first design principle. For a building located in climate zone 5, the smallest windows should be on the east and west facades, or avoid them altogether if not absolutely essential – this is because the sun in the early morning and late afternoon in summer will hit the window directly, making it very difficult to shade (see design principle no. 4 for more details). The largest windows should face towards north – this will allow optimum solar gain in winter, which can be used to help naturally heat your home both during the day and at night (see design principle no. 2). North facing windows are also the easiest to shade in summer (see design principle no. 4). However, don’t make them too large, otherwise they will lose too much heat at night during winter, and can gain excess heat from the air in summer even if shaded. The optimum window to floor area ratio is around 20-25% ie. for 100m2 of floor area, you should have around 20-25m2 of north facing windows.

As with selecting the right building materials, the materials that a window is made from is also very important. Most windows are generally made up of two components – the glazing and the frame, and in combination they will affect the thermal performance of a window in the following three ways:

  • Conduction: this refers to how heat flows through the materials of a window, whenever there is a difference in temperature between inside and outside. Glass is a highly conductive material, meaning that heat flows very easily through it. Frame materials such as aluminium are also very conductive, while timber is not. Conduction is the opposite of insulation, and is measured using a U-value (which is the opposite, or reciprocal, of R-value). The lower the U-value, the less conduction that occurs through the window. Generally, you want a window to have as low a U-value as possible, however this can make a window more expensive.
  • Radiation: refers to how solar radiation will pass through the window when exposed to the sun. Solar radiation can work both for and against us when it comes to thermal performance – increasing solar radiation is good during winter, as it helps to heat our home for free. However, solar radiation isn’t such a a good thing in summer, as it can cause a home to overheat and can create unpleasant glare. We measure the radiation performance of a window using a number between 0 and 1 called the Solar Heat Gain Coefficient (SHGC) – the higher the number, the more solar radiation that can pass through the window. Generally, we want north facing windows to have a high SHGC (07-0.9), south facing windows a medium SHGC (0.4-0.6), while east and west facing windows should have a low SHGC (0.1-0.3). The way we shade our building will also affect how it is exposed to solar radiation – see design principle No 4.
  • Convection or Infiltration: windows also need to be operable, so that we can open them to let in fresh air and cooling breezes, but close them when it’s cold outside. However, making a window operable means that it will also leak air to some degree – no window is 100% air-tight. Infiltration refers to how much air can leak through a window even when closed, and there are a number of different ways it can be quantified. Generally speaking you want a window to have as low an infiltration rate as possible. Ways we can make windows more airtight include using quality hinges and opening mechanisms, rubber or mastic seals, and weather- and brush-strips. Don’t forget we also want the ability for a window to open up when required – see design principle No 5.


Shade the building properly throughout the year

The sun can be a building’s best friend or worst enemy, depending on the time of year. In summer, we generally want to keep sun off as much of the building as possible, as excess solar radiation can cause the building to overheat; in winter, we want the sun to enter the windows of a building as much as possible, as it provides passive heating for the spaces and room within.

An effective shading strategy for a building is about finding the simplest, low-cost solution to keep the sun out in summer and let the sun enter in winter. While it’s possible to install things like awnings, louvres, roller blinds and screens, one of the most effective shading strategies is to build the roof with an eaves overhang. When the roof is slightly bigger than the external walls below, the overhang can help to shade the walls and windows of the building in summer, while not being so deep that they block winter sun from entering the building. They can also help to keep rain out of the building and prevent leaks!

For most houses in climate zone 5, an eaves overhang of around 450-600mm is generally sufficient to provide the optimum amount of shading. If you have a large area of external wall facing east or west, a vertical shading strategy might be more appropriate – for example, you could plant deciduous trees, build a lattice screen or locate rainwater tanks adjacent to the building to help prevent the sun from hitting the wall and windows. You can also position structures such as sheds and carports on these east and west sides of the building – the roofs of these structures function like a large overhang.

On the north side of the building, make sure the eaves overhang isn’t too deep, as this can affect the amount of winter sun that can enter the building. If you want to have a covered outdoor area or alfresco coming off the north of a building, you might want to build a solar pergola (link and article to come).

The roof is the largest surface area of a building that is exposed to the sun. As discussed in design principle No 2, make sure you have a low solar absorptance, light coloured roof to prevent overheating in summer. In hotter areas such as climate zones 1-4, you may even consider building a shading structure over the roof – this is called a parasol roof.


Encourage air movement when it’s hot

Natural breezes and winds are just like free air-conditioning! They provide a cooling effect as the air passes over our bodies, by increasing the rate at which sweat evaporates from our skin. They can also help to remove humidity, contaminants and stale air, so it’s a really good idea to design our buildings to naturally cross-ventilate as much as possible.

It can help to understand what time and direction the breezes come from in your area. Most towns and cities that are located in climate zone 5 are also located near the coast, which generally means there will be afternoon sea breezes that can bring welcome relief on a hot summer’s day. For example, in Perth, afternoon sea breezes in summer tend to come from a southwesterly direction, while in Sydney they tend to be north-northeasterly winds. It’s a good idea to check the wind-rose diagram for your town or city on the Bureau of Meteorology website.

Wherever possible, each room should have windows and external openings on opposite sides of the room – this helps to encourage air to flow from one side to the other. Smaller window openings on the windward side tend to work best – this helps to increase the wind speed as it enters the room. This can then be complemented by larger openings on the leeward side of the room – the different window sizes can help to create higher and lower pressure zones that can improve the airflow through the room.

If the overall building layout makes it difficult to locate windows on both sides of every room, the next best thing to do is to have windows in adjacent walls, or multiple windows in the same wall. You can also install a vent in the door – even when the door is closed, if a window is open an adjacent part of the house, air can still flow through and into the adjacent room.

The operation of the window or door opening can also influence the airflow in a room. Sliding and sash windows provide large opening areas for airflow. Casement windows or hinged doors also provide a large opening, and the window can even be positioned to help direct the flow of air into the room – however, it can be challenging to add fly and insects screens to these kind of windows. Louvres windows are also really good for ventilation, however can be expensive and difficult to keep airtight when closed. In my humble opinion, the least favourable type of opening is an awning window – they only open at the bottom of the window, and it is often a very small opening. It’s difficult to get air to flow through these windows, as the wind tends to deflect off the glass.

Ceiling fans are a low-cost, energy efficient way to improve air circulation within a room or space – when the increased air speed from a fan passes over your skin, it creates an enhanced cooling effect on the body. They can be useful in winter on low speeds, as they help to push warmer air down towards the floor. Using ceiling fans in conjunction with air conditioning can also improve the cooling efficiency of the air conditioner.

Needless to say, during the colder months of the year, the goal is to generally make the building as airtight as possible when it’s cold and windy outside. This means the floors, walls and roofs need to be built to minimise air leakage, while external doors and windows need good quality gaskets and seals around them to prevent unpleasant drafts. An airtight building is also efficient to heat in winter.


Want to know more?

That’s it! As you have discovered, the fundamental principles behind designing a more thermally efficient building are easy to understand. You have now gained some incredibly valuable skills and knowledge – the challenge is how to apply them along with everything we need to consider when designing a building a new home or renovation! This is where engaging an experienced architect or building designer can really help you prioritize your ideas and turn them into a floor plan or design.

There are also a number of useful websites and resources that provide more information and go into further detail on how to make your new home or renovation more energy efficient and sustainable:

  • www.yourhome.gov.au this invaluable website and book is published by the Australian Government, and contains a wealth of detailed information on how to design a more sustainable, comfortable, energy and water efficient home.
  • www.sustainablehouseday.com every year around September, the Alternative Technology Association (ATA) where people open up their homes around Australia to showcase more sustainable designs and living. It’s free event and a great way to get inspired about your own sustainable build or renovation.
  • www.sustainability.vic.gov.au contrary to what some people say about Victorians, the Sustainability Victoria website contains lots of useful tips, design advice and information on how to make your home and household more energy efficient and sustainable.
  • www.nathers.gov.au The Nationwide House Energy Rating Scheme (NatHERS) is an Australian Government initiative that helps you to rate the thermal performance of a new home or renovation. Using specialist software developed by CSIRO, house designs can be rated between 0 and 10 stars all around Australia. Under most jurisdictions, new homes need to achieve a minimum 6 star rating.
  • www.wers.net an industry initiative of the Australian Window Association (AWA), the Window Energy Rating Scheme (WERS) helps you to select the most energy efficient windows for summer and winter in your home.
  • www.sanctuarymagazine.org.au this is a fabulous magazine published by the ATA that features sustainable homes all around Australia, and useful articles on how to create a beautiful, healthy sustainable home.