Passive cooling
You don't need air conditioning to keep your home cool in summer.
Summer overheating is a common problem in New Zealand homes. Passive cooling means keeping your home comfortable throughout summer without using an air conditioner or other mechanical assistance. It works by:
- using shade and insulation to keep heat out of your home in summer
- using heat-storing materials such as concrete to absorb heat
- using breeze and air movement inside your home to keep you cool.
Whereas air conditioning is expensive to install and run, passive cooling is free.
When should you think about passive cooling?
Planning a home or renovation
If you are building or renovating, passive cooling should be considered early in the design process.
Good design should strike a balance between the need to keep your home warm in winter, the need to keep it cool in summer and the need to provide ventilation to bring fresh, healthy air into your home. So passive cooling should be considered alongside passive heating and ventilation options.
In your existing home
Many passive cooling options can be easily added to existing homes. Simple but effective options include using plants or blinds to provide shade.
Another way to improve passive cooling in an existing home is by installing extra insulation - the investment will be worthwhile.
Shading
Shading should be designed to take into account the sun's path in summer and winter on your site. The sun travels higher in the sky in summer, so shading can be designed to keep summer sun out but let winter sun in.
While your exact needs will vary according to your site and climate, most people will want to:
- shade high-angle summer sun from the north
- shade low-angle summer sun from the east and west
- let low-angle winter sun into your home from all directions.
In general, you'll need some form of shading above doors and windows on the east, north and west side of the house - but the size and type will depend on your circumstances.
Fixed shading options
Fixed shading options include:
- eaves
- pergolas
- fixed louvres
- covered balconies (note: a balcony on the north side of your home could block winter sun).
Eave design
Eaves should be designed to let as much sun as possible into your home in winter (when the sun is low in the sky) but keep the sun out in summer (when it's higher in the sky). The exact design of the eaves will depend on the amount of glazing in your home, which way it faces and the amount of direct sunlight you want to come into your home.
- Eaves can be designed to provide shade during summer and let sunlight in during winter
Planting for shade
Planting provides flexible shade options:
- Deciduous vines and trees let winter sun through, and provide summer shade.
- Ground-cover plants keep surface and ground temperatures lower in summer, as well as reducing glare - use plants instead of hard paving in outdoor living areas exposed to direct summer sun.
- A shaded courtyard next to the main living area can act as a cool air well.
- Deciduous trees provide shade in summer but let filtered light in through winter
Adjustable shading options
Adjustable shading provides flexibility. It can be especially useful where you need to deal with low-angle morning or evening sun.
Adjustable shading options include:
- manually-adjustable louvres
- shutters
- curtains
- sliding screens
- retractable awnings and sails
- removable shades that can be taken down at the end of summer.
Adjustable shading may be combined with fixed eaves or pergolas to provide deep shade in summer, but allow winter sun into these areas.
Insulation
A well-insulated home will retain heat in winter and keep heat out in summer.
Reflective insulation in ceilings and roof spaces will reduce the amount of heat absorbed through a roof (particularly metal roofs). It should be perforated to allow moisture transfer.
See insulation for more.
Thermal mass
Materials with high thermal mass (materials that are good at absorbing and storing heat) can be used to provide heating in winter and cooling in summer. These materials are usually heavy - such as concrete and brick - and are used in floors or walls.
Used properly - the right amount in the right place, with proper insulation - will ensure they help passive cooling by absorbing heat from the surrounding air. They will keep absorbing heat as long as the air temperature is higher than the temperature of the thermal mass.
Thermal mass designed to get direct sun in winter, but not in summer, will stay cool during summer days and keep air temperatures down.
See thermal mass for more.
Ventilation
Air movement keeps you cool by increasing the rate at which moisture evaporates from your skin.
There are two main ways to harness air movement:
- orient your home to catch the prevailing breeze
- use passive ventilation to get air circulating through your home.
Passive ventilation uses doors, windows, vents, louvres and other openings to let fresh air into and through your home. This helps to provide cooling, as well as removing moisture and airborne pollutants.
You’ll need more air movement as humidity increases.
See ventilation and orientation for more.
Cooling air with water
When water evaporates, it absorbs heat from surrounding air, so the air cools.
Evaporation works best when humidity is lower so the air can take up more water vapour.
Rates of evaporation are increased by air movement and the exposed surface area of water. Fountains and mist sprays are effective for cooling.
Other options include pools, ponds and water features immediately outside windows or in courtyards to cool air before it enters the house.
Challenges of windy sites
Slowing down or excluding strong summer winds can be a challenge.
In some cases, the prevailing hot summer wind, sun and views are all orientated north.
It is important to know your site's climate and microclimate when you are planning a new home or renovation, so that wind direction can be considered. See site selection and orientation for more.
On windy sites, consider:
- putting doorways and some windows into sheltered recesses so they can be opened even when it’s windy to let warm air out
- using windows and doors on the south and east side of your home to provide for air movement without gusts if the prevailing wind is from the north
- using sliding windows - they won’t slam shut in the wind
- using small openings - one at ground level and one in the roof or on an upper level - to provide for ventilation on windy days
- using wind breaks and mounded plantings around your home to slow the wind down and change the wind path.
Air conditioning
Air conditioning uses significant amounts of electricity and requires the home to be closed off from the outside environment to work best.
It dries the internal air and can harbour and spread bacteria if not maintained.
If passive design principles are followed and some supplementary cooling is still required, other options such as fans are more cost-effective than air-conditioning.
More information
From Smarter Homes
From other sites
The energywise website has a web page on room placement with information about passive cooling.
The website www.eartheasy.com has information on natural cooling systems.
You can buy copies of New Zealand Standards for indoor air quality from the Standards New Zealand website.
You can buy a BRANZ Ltd bulletin on passive ventilation, from the BRANZ website (click on the link to the BRANZ bookshop).
An Introduction to Designing Comfortable Homes (PDF, 377KB) is a booklet about energy efficient design, produced by the Energy Efficiency and Conservation Authority and the Cement and Concrete Association of New Zealand.
