Sustainable Design – Super Energy Efficient Extension

Sustainable Design Tips

Energy Efficient Builders. Welcome to part three of this great series on Sustainable Building and Green Living. In previous posts, we have looked at an introduction to sustainable home design and provided you with some great information regarding the significant changes that you can make to improving the energy efficiency of your existing home. In this part, we are going to look at how you can build a super energy efficient home extension starting with the most crucial element to achieve this – passive design.

What is Passive Design?

Passive design is an approach to design that takes advantage of the climate and the sun’s orientation to create a thermally comfortable home; maximising heat retention in colder months and keeping the house cooler in the hotter summer months.

The importance of passive design cannot be overstated as an extension designed using it requires little maintenance and can significantly reduce, and sometimes eliminate, the need for mechanical devices to regulate indoor lighting and temperature.

Passive design can be broken down into a number of design components:

  • orientation
  • cooling and heating
  • insulation
  • glazing and sky lighting.

Of these design components, we’ll start with the most important, orientation.


The orientation of your extension refers to the position that it sits on your site. In passive design, the correct orientation is vital to take advantage of the climatic conditions throughout the year, such as direct sunlight and cool breezes. The best time to consider orientation is when building a new home, but by considering it when doing substantial renovations, such as building a new extension, you can significantly increase the thermal comfort of your home whilst reducing your energy bills. It’s crazy not to take advantage of orientation.

So, how you orient your new building is driven by the climate. For example, here in Melbourne, it is common for cool winds to come in from the south-west, so having good ventilation to take advantage of these winds will help cool your home down in summer. We also know that hot winds come in from the north, so in Melbourne, it is best to avoid large northerly facing verandahs and to use some cover, such as shade cloths or natural vines.

Talking globally, we know that the sun rises in the east and sets in the west, so to minimise the amount of solar heat penetrating your home it is advisable to have small windows to the east and west.

In summer, there is a balance that’s required between having natural daylight in the house and preventing too much solar energy from heating your home. In winter, however, it is desirable to have the winter sun shining through your windows to heat your home, so some northerly facing windows will catch the low winter sun and provide some extra heat during the daytime.

Direct summer sun can generate the same amount of heat as a single bar radiator. By effectively shading from the sun you can reduce up to 90% of this heat. For example, consider installing eaves. Eaves protect your windows from the sun whilst also permitting daylight to penetrate. The general formula for the size of an eave is approximately 40% of the window height.

Cooling and Heating

Taking advantage of passive solar heating and passive cooling design is the least expensive way to keep your house thermally comfortable.

Passive heating uses the energy of the sun to heat your home, therefore, reducing the need to use mechanical devices, for example, ducted heating. There are a number of techniques that are used such as the effective placement of windows, use of thermal mass (materials that retain and slowly release heat), good insulation and sealed windows, doors, chimneys and wall vents.

Conversely, during the warmer months, good passive design is effective at keeping both your house and its occupants cool. Such design techniques include the strategic placement of windows, effective coverings and shades, ensuring good air movement throughout, the use of evaporative cooling and the use of thermal mass (in this case materials that do not retain heat).

Like orientation, passive heating and cooling are dependent on your climate. In Melbourne, both summer and winter design elements need to be considered.


Insulation is another important feature of passive design, effectively keeping your home warm in winter and cool in summer. Get your insulation right and you can cut up to 50% off your energy bills, whilst also reducing your greenhouse gas emissions.

Climate is again very important and your type of insulation will be determined by how hot and cool temperatures get throughout the year. It is recommended to use both bulk and reflective insulation in ceilings and/ or walls.

Bulk insulation is one of the most common forms in Australia and works by trapping air within its fibres, therefore, slowing down the transfer of heat. This can help your home retain heat in winter and cool air in summer. Fibreglass, or glass wool, is a common type of bulk insulator, as are aerated panels.

Reflective insulation creates a radiant barrier, usually made of metallic foil*, to reflect heat away from your home. This type of insulation is more effective in warmer climates.

If you are building an extension or new home then consider a large roof cavity e.g. a pitched roof, as the air within will act as an insulator. Also, sealing against draughts is a basic thing you can do to improve your home’s insulation.

* Foil insulation in roofs should only be installed on the top of rafters and before the roofing is installed, and never within the ceiling cavity or near electrical wiring.

Glazing and Sky Lighting

Choosing the right glazing can reduce winter heat loss from your home by up to 40% and reduce the summer heat gain by up to 87%.  The window products you choose should be determined by your climate and orientation of your building. As we discussed in Orientation above, the passive design of your extension should consider the location and sizes of window openings.

There are a variety of window products out there. To help you select the right ones refer to the Window Energy Rating Scheme (WERS) which provides the energy-related performance of different glazing products.

Skylights are used in rooms where daylight is minimal or non-existent. As part of passive design, they are generally not recommended as they are a big inductor of solar heat, especially during summer. There are solutions to the problem of low natural light, such as have south-facing windows or covered roof lights i.e. lantern lights, or the use of Sola tubes, which reflect natural daylight into rooms where windows are not possible to install.

Physical Design

Once you have implemented the concept of passive design to your new home extension it’s time to look at the physical components of your design:

  • construction materials
  • heating and cooling systems
  • renewable energy
  • lighting
  • electrical appliances – Star Rating.

Construction Materials

When looking at sustainable design it is important to consider embodied energy when choosing your construction materials. Embodied energy is:

the energy consumed by all of the processes associated with the production of a building, from the mining and processing of natural resources to manufacturing, transport and product delivery.

Simply, you can think of it as a material’s overall environmental impact. For example, standard bricks have a high embodied energy whereas a light material such as weatherboard has a lower embodied energy. However, a balance needs to be struck between the energy required to make a material and the amount of energy that material will save once your extension/ home has been constructed.

When choosing a material consider ones that are durable, locally made, recycled, standard sized, low-maintenance and made using renewable energy sources.

Standard bricks have a high embodied energy and are therefore not a good material to choose if you want to be energy efficient. If you do want to use bricks you can always use recycled bricks. Concrete, however, is increasingly being recognised for its strong environmental benefits in support of creative and effective sustainable development. It does have a relatively high embodied energy, however, with greener versions now available e.g. recycled fly ash, and its ability to regulate temperature through thermal mass it is becoming more popular.

Lighter materials such as weatherboard are great for the Melbourne climate. If they are painted well and protected by eaves they are low maintenance. Ensure that the timber you use is carbon neutral i.e. from suppliers that replant trees to offset the trees they use as timber. As alternatives, fibre cement sheeting and weatherboards also have a low embodied energy and are more durable than timber.

Heating/ Cooling Systems

In Melbourne, there is a need for both cooling and heating.

In summer, temperatures can reach upwards of 40 deg C. If you have designed your home efficiently then you will have minimised the effect of solar energy in heating up your home. In this case, a system such as reverse cycle air-conditioning is a good choice. These systems can be energy hungry but are efficient at cooling your home. If used in synergy with passive design your use of the system will be limited.

Evaporative cooling, which uses less energy, is an alternative but it is not as efficient at cooling your home. When using evaporative cooling it is essential to have good ventilation and air flow through your home.

Melbourne does get cool enough to warrant a heating system. Again, in synergy with passive design a gas-powered unit will be the most efficient at heating your home on those chilly mornings. Hydronic heating is another option but is not necessary for Melbourne’s climate.

Renewable Energy

Renewable energy is concerned with converting energy sources, such as solar and wind, into electrical energy. Some households (mostly rural) use standalone systems which are used only for personal use, and others connect to a grid where any excess electrical energy is sold back to an electricity company. With the prevalence of sunshine in Melbourne photovoltaic systems, more commonly referred to as solar panels, are the preferred source of renewable energy. They can be expensive to set up but the running costs are low. The benefit is determined by the size of your house, your usage and, importantly, having the panels northerly oriented.

Other renewable energy systems include batteries and inverters, wind and smart meters.


On average the electricity used by lighting accounts for up to 12% of your home’s energy budget.  Efficient and well-designed lighting, including maximising daylight throughout, can reduce the amount of electrical energy used – down to as low as 3% –  saving you money.

The biggest improvement you can make is to switch to energy efficient globes. Instead of incandescent and halogen, down-lights use LEDs. LED bulbs use only 9 to 14 Watts of power each, which is on average one fifth that of an incandescent or halogen light globe.

Electrical Appliances

When choosing electrical appliances look for those with Energy Rating and Water Efficiency (if appropriate) Labelling schemes. These allow you to compare the energy efficiency of appliances via a star rating. Also, consider the potential long-term maintenance of your appliances and not just the cost of purchase, as the initially more expensive ones can have lower maintenance costs in the long run.

Water Consumption

Finally, efficient water consumption is something that can be incorporated into your passive design. Apart from using water efficient appliances such as toilets, washing machines, dishwashers and showers, there are systems for recycling water:

Grey water system: this system collects water from activities like showering and clothes washing, filters it and then makes it available to use for watering the garden and toilet flushing, for example. Such systems are expensive and require a lot of maintenance.

If you have a big garden you can, however, use your shower water from time to time as a subterranean irrigation system. Such systems must be set up by an experienced plumber to be effective.

Stormwater collection system: stormwater can be collected in tanks and re-used, particularly as garden water. The system is less expensive than a grey water system and requires less maintenance. However, it is necessary to frequently change or clean filters.


A new extension should improve the overall energy efficiency and thermal comfort of your home, particularly when the principles of passive design are employed. There are also basic things that can be done before you renovate or build.

These tips will cover what to look for when hiring a ‘Green Living‘ certified builder, some guidance on energy-compliant homes and some great tips for environmentally preferred outcomes.

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