Passive Design

Passive design is a key element of sustainable building. It aims to maximise comfort for people living in a home while minimising energy use and other impacts on the environment. This means making the most of free, natural sources of energy, such as the sun and the wind, to provide heating, cooling, ventilation and lighting and to contribute to responsible energy use.

The importance of design

People’s comfort and health depend on indoor environmental conditions including temperature, humidity, air quality, lighting, and noise levels. These can be managed by carefully considered passive design in a new home, and influenced by changing design features in an existing home. A badly designed new home can cause discomfort and poor health. Designers must also integrate passive design with the requirements of the New Zealand Building Code, aesthetics, client requirements etc.

All aspects of home design are interlinked, so it is important to approach passive design as a total process. Sometimes it is not possible to make changes to one area without affecting others. For example, good natural light must be balanced against possible heat gains from large windows, and natural ventilation needs can clash with efforts to control noise from external sources. No single factor should be considered without assessing its effect on all the others – a holistic approach is essential.

What does passive design cost?

Many passive design features cost nothing – they’re about more effective use of features that are common in most or all homes. Other passive design features do add initial cost but provide significant benefits to the owner by making the home warmer/cooler, healthier and drier, and by reducing ongoing costs such as energy bills.

No cost or low extra cost features include:

• orientation to the sun to provide natural heating and daylighting
• building shape – compact building plans have less external wall area and therefore less potential heat loss while long, thin buildings are better for daylighting and cross-flow passive ventilation
• harnessing of natural breezes for ventilation – to cool internal spaces and remove contaminants
• zoning of rooms and occupant activities to take account of the sun’s movement, daylighting and potential sources of noise
• using the thermal mass available in the building’s structure – for example in the concrete floor slab – to moderate temperatures and provide heating
• using correctly sized roof overhangs and other forms of shading to cut out unwanted light and heat
• carefully considered placement and sizes of windows and opening sashes
• good thermal insulation
• choice of materials.

Features that involve additional cost, which can be offset by a reduction in energy costs and improved comfort, include:

• a higher level of insulation
• using internal walls with high thermal mass
• double glazing where it is not a mandatory requirement
• solar water heating
• wet backs (to heat water for hot water supply and/or space heating)
• heat recovery devices
• photovoltaic panels and other on-site power generation.

Some passive design equipment, such as solar panels to heat domestic water supply or swimming pool water, may have a higher initial cost than conventional methods such as a gas-fired or electric water heater, but will cost much less to run. In many cases, savings will eventually exceed purchase costs. For some products, like solar water heaters, government assistance may be available.

The need for flexibility

New Zealand’s climate has large variations from region to region, between seasons, from year to year, and during each day. Climate change may alter some weather patterns and bring more extreme events. For this reason, flexibility is a key factor when designing New Zealand homes. Passive design features must be appropriate to the region, be able to cope with fluctuations in weather and other conditions, and be well integrated with conventional equipment such as heating and cooling appliances. In general, a home that is designed to be flexible enough to cope with existing fluctuations in weather is likely to cope with projected climate change for the foreseeable future.

For your clients

To help your clients understand passive design, refer them to www.smarterhomes.org.nz/design/passive-heating and www.smarterhomes.org.nz/design/passive-cooling.