Passive design for energy efficiency

The layout of the house in relation to the sun, and the use of features and materials that don’t maximise the use of solar energy, are important in keeping your house at the right temperature while saving on energy costs.

Passive design

Passive design is the control of ventilation and temperature without using any products that consume energy or money (such as heaters, dehumidifiers or fires).

Good passive design includes:

House orientation – positioning the house to allow maximum sun in the winter and coolness in the summer. This includes deciding which rooms you want to be the sunniest.
Solar energy – using solar panels for water heating.
Use of shading elements – for example, wide eaves protect from the sun in summer and provide increased weather protection in winter.
Placement and glazing of windows – the larger windows should face the sun to capture the warmth, use glazing to stop heat escaping, and have shading to limit summer overheating.
Ventilation – using window joinery that allows ventilation, such as security catches allowing windows to remain partially open, or vents in the joinery.
Insulation – to reduce heat loss.
Thermal Mass – using heavy building materials to store solar energy and limit overheating during the day but then release energy during the night to provide heating.

A note about solar power

On average there are about 2000 hours of sunshine a year in New Zealand, although this varies by location.

Solar energy is most commonly harnessed for heating water using solar panels mounted on the roof. Electricity can also be generated directly using photovoltaic cells mounted wherever they can best capture solar energy.

Many of the local councils also have information about using solar power.

Case study 1

Fiona and her partner recently built a rammed earth house in Alexandra. The walls are 450mm thick and have proved to have very good energy performance in Central Otago's winter temperatures, which drop to minus 8 degrees.

They chose double glazed joinery, with wooden trims on the inside to reduce heat loss through the aluminium. They also put a double layer of wool insulation in the roof cavity, and selected a log burner with eco-flue to reduce heat loss in the flue itself.

The house maintains its warmth well beyond their expectations. It is positioned to trap as much heat from the sun as possible, which helps heat the house during the day and means that on sunny days (even when the temperature only gets a few degrees above zero) they have no need for the fire.

Note: The basic principle of rammed earth construction is the ramming of moist earth into a movable formwork to form thick walls that can store heat energy. There are building firms who specialise in this type of construction, and you can find information about earth buildings on the Earth Building Association of New Zealand’s website: www.earthbuilding.org.nz.

Case study 2

Combining passive and mechanical systems

A house that won a New Zealand Institute of Architects Supreme Award in 2004 incorporates the following features and materials:

Parts of the house were benched into the land to help it blend into the coastal site and because the earth adds an extra layer of insulation, the inside temperature rarely drops below 15 degrees and summer overheating is limited.
Thermal blocks were used below ground and on all exterior walls for added insulation.
Pumice was used under the floor slab as insulation in lieu of polystyrene.
Polyester wall and ceiling insulation was used instead of fibreglass, but next time the owners would use wool insulation for its breathing qualities.
Solar water heating feeds to twin calorifiers (cylinders) designed to a European efficiency standard.
Pre-wired for future solar power installation (when it becomes more affordable).
A hydronic heating system fed from a condensing/modulating boiler and distributed over four levels through highly insulated multi-wall pipe to wall-hung radiators and underfloor in slabs.
Deep eaves over windows to shelter from summer sun but which allow low winter sun to enter and heat. Some passive thermal walls where overall design allowed.
A low particulate output (clean burning) wood-burning stove providing heat to the living, family, dining and kitchen area. It generates so little particulate after the initial start, that it is difficult to know if the fire is on from looking at the chimney.
Rain water is collected from the roof and fed through river gravel and geotextile cloth covering the roof, down polyethylene (food grade) pipes into a 30,000 litre subterranean (which keeps it cool) water cistern. This supplies four toilets, laundry, two ponds and irrigation. It can also supply all drinking water (through extra filters and UV treatment) at the throw of a switch.