Climate and location

The numbersCanberra's climate

Canberra sits 577 m above sea level, inland of the coastal ranges. Height and distance from the sea give it a climate of extremes. Long-term averages at Canberra Airport:

Four things in that table shape the design:

1. Winter is the main job. About 60 frosty nights a year and July nights averaging below freezing make keeping heat in the first priority.
2. Winter is sunny. Nearly six hours of sunshine a day in July means north-facing glass can collect a large share of the heating a house needs. Not every cold climate has that.
3. Days and nights are far apart. The gap between the daily maximum and minimum is about 15°C in summer and 11°C in winter. A swing that large is what makes thermal mass worth having. Heavy materials store the day's warmth into the evening, and in summer they can be cooled overnight by opening up the house.
4. Summers are dry. Afternoon humidity in January averages 37%. Cool nights and dry air make cross-ventilation and night cooling work well, so many Canberra homes can be kept comfortable with little or no air conditioning.

The averages are already shifting. At the current airport station, the January average maximum for 2008–2026 is 30.0°C, two degrees above the long-term figure.

Two mapsClimate zones and NatHERS zones

Two different climate maps apply to every house, and they do different jobs.

The NCC climate zones divide Australia into eight broad zones, from zone 1 (hot and humid) to zone 8 (alpine). They decide which deemed-to-satisfy rules apply, including minimum insulation and how roofs must manage condensation. Canberra and Goulburn are zone 7, cool temperate. Wollongong and eastern Sydney are zone 5, warm temperate. Western Sydney is zone 6, Dubbo is zone 4 and Thredbo is zone 8.

The NatHERS climate zones split the country into 69 smaller regions, each with its own year of hourly weather. They decide the weather a rating is run against and the star scale it is judged on. Canberra is NatHERS zone 24. A site's NatHERS zone comes from its postcode.

NSW uses both maps in BASIX. The NatHERS zone, by postcode, sets the maximum heating and cooling loads. The NCC zone, by local government area, sets the insulation under BASIX's simpler "DIY" method. The two don't always line up. BASIX puts Dorrigo, on the plateau inland from Coffs Harbour, in cool-temperate zone 7 even though the NCC map shows it as coastal.

The broad zones are coarse. Within one zone, local conditions often tell you more about a site than the zone does.

Cool temperateWhat a Canberra house needs

The aim in a cool-temperate climate is to cut winter heating without making summer uncomfortable. In practice that comes down to a handful of decisions:

• Plan. Put living areas on the north, and bedrooms, bathrooms and storage on the south. Keep east and west walls short, because low morning and afternoon sun is hard to shade and brings heat in summer.
• Glass. Use double glazing in thermally improved frames, with glass that admits winter sun on the north and limits heat gain on the east and west. Insulating blinds or heavy curtains with closed pelmets stop warmth escaping through the glass on winter nights.
• Shade. Size north eaves to block high summer sun and let low winter sun in. Use external shading on east and west glass. Adjustable shading on some west windows can let in welcome afternoon sun in winter and keep it out in summer.
• Mass. Where the daily temperature swing is more than about 6°C, extra thermal mass in sunlit rooms pays off. Canberra's swing is 11 to 15°C. Mass only works if the sun can reach it and it's insulated on the outside. A concrete slab under a dark, south-facing room just stays cold.
• Insulation and sealing. Insulate walls, ceilings, floors and slab edges, and seal the house against draughts. An entry airlock, or at least a door between the front door and the living areas, stops each arrival letting the warmth out.
• Ceilings. Very high ceilings collect warm air above where people sit. In a heated room, height costs energy.
• Cooling. Plan for cross-ventilation and for opening the house up on summer nights, and add ceiling fans. That handles most Canberra summer days. When it's not enough, an efficient reverse-cycle system or heat pump will do the rest.

Avoid wood heaters in town. They're a major source of Canberra's winter air pollution.

CondensationThe cold-climate risk

On a July morning in Canberra the outside air is close to saturated, with 9 am humidity averaging 85%, and every outer surface has been chilled by a frosty night. Moist indoor air that reaches a cold surface condenses. On a window you wipe it off. Inside a wall or roof it rots timber, wets insulation and grows mould. Tight, well-insulated houses are more exposed to this, and the code now requires Canberra walls and roofs to be built so moisture can drain and dry out. See Condensation and building breathability for what that means for walls, roofs and cladding.

MicroclimateThe site matters more than the zone

Two blocks in the same suburb can behave like different climates. The differences that matter most in Canberra:

• Winter sun and neighbours. At noon on the shortest day, the sun in Canberra is only about 31° above the horizon. A two-storey house 7 m tall casts a shadow about 11.5 m long at that hour, and longer before and after. A neighbour that close to the north can take most of a living room's winter sun. What the neighbour could build later under the planning rules matters as much as what's there now.
• Cold air drainage. On clear, still winter nights, cold air flows downhill and pools in low ground. Valley floors and the low end of a sloping block get the hardest frosts.
• Slope. A block falling to the north gets winter sun earlier and keeps it longer. A block falling to the south casts its own long shadows.
• Wind. Afternoon winds at the airport average about 17 km/h in every season. Exposed sites need sheltered entries and outdoor rooms that face away from the prevailing wind. Without them, a sunny terrace goes unused for half the year.
• Trees. Deciduous trees to the north give summer shade and let winter sun through. Evergreens in the same place block it all year.

Before I draw anything, I map winter shadows across the site, note where cold air will settle and where the wind comes from, and check what the neighbours could build. Those observations decide where the living areas go. Every other energy decision follows from that.