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:
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:
This means we should following design principles when working out the layout of rooms in a new building:
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.
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:
There are a few other things to be aware when selecting construction materials for your new build or renovation:
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.
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:
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.
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.
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: