Passive Design

Passive Design

Good passive design is a critical element in sustainable, low carbon buildings.

It responds to local climate and site conditions to maximise building users’ comfort and health while minimising energy use.

It achieves this by using free, renewable sources of energy such as sun and wind to provide household heating, cooling, ventilation and lighting, thereby reducing or removing the need for mechanical heating or cooling. Using passive design can reduce temperature fluctuations, improve indoor air quality and make a home drier and more enjoyable to live in.

It can also reduce energy use and environmental impacts such as greenhouse gas emissions. For most parts of New Zealand, well integrated passive solar home design results in comfortable internal temperatures of between 18°C and 25°C with minimal (or no) costs year round.

While there are limited statistics on resale benefits of these houses in New Zealand, there is strong evidence internationally that better-performing homes sell for a premium price and often sell faster. Many overseas studies that show that homes with green labels – a rough proxy for better-performing homes – sell for a premium of around 4–10%.

Interest in passive design has grown, particularly in the last decade or so, as part of a movement towards more comfortable and resource-efficient buildings.

There are big benefits to be gained. BRANZ examined 210 randomly-selected new detached houses that met Building Code minimum requirements but did not take advantage of passive solar design. Using modelling, BRANZ found that the average house required two to three times the amount of heating energy to maintain comfortable conditions compared to a house of similar price that incorporated passive solar design.

Key features of passive design

The key elements of passive design are: building location and orientation on the site; building layout; window design; insulation (including window insulation); thermal mass; shading; and ventilation. Each of these elements works with others to achieve comfortable temperatures and good indoor air quality.

The first step is to achieve the right amount of solar access – enough to provide warmth during cooler months but prevent overheating in summer. This is done through a combination of location and orientation, room layout, window design and shading.

Insulation and thermal mass help to maintain even temperatures, while ventilation provides passive cooling as well as improving indoor air quality.

All of these elements work alongside each other and therefore should be considered holistically. For example, large windows that admit high levels of natural light might also result in excessive heat gain, especially if they cast light on an area of thermal mass. Similarly, opening windows that provide ventilation will also let in noise.

Alongside passive design features, designers should also consider other factors such as views, covenants and local authority restrictions, and building owners’ preferences.

Passive design in new and existing buildings

It costs little or nothing to incorporate passive design into a new building. The benefits are greatest when passive design principles are incorporated into the entire design and build process, from site selection onwards. Simulation tools can provide crucial help in assessing the impact of different design decisions and maximising the benefits of passive design.

Once a building is completed, some passive design features can be incorporated during later upgrades – for example, insulation can be improved, and it may be possible to alter room layout to improve orientation and solar access.

But it may be difficult to achieve the full benefits. For example, it will not be practical to turn a completed house around on the site to take better advantage of sun or cooling breezes.

Building size and passive design

The trend to larger houses in New Zealand is having a big impact on energy efficiency, University of Otago research published in 2017 has found.

Our homes have become significantly bigger over the last 50 years – from an average of around 112 m² in 1976 to a peak of around 200 m² in 2010 and even bigger in Auckland and some other areas. The increase largely cancelled out the energy efficiency gains from improved insulation requirements introduced since the 1970s. (The average new home has become smaller in recent years, perhaps reflecting the larger amount of infill housing in urban areas.)

With larger homes it is even more important to take full advantage of passive design and to specify better insulation than that which simply meets the minimum requirements.

Updated: 22 February 2022