Passive Solar Design Basics For Australian Homes

Orientation is the foundation. Put the living areas, the outdoor living space and their main windows on the northern side so the low winter sun reaches in and warms them, and keep the driveway and garage to the south or west where they do not eat into the sunny north. Everything else in passive solar design builds on this, and it costs nothing to get right at the plan stage.

Get as many rooms onto the north as the block allows, and make the living areas the ones you never compromise. On a small block the living areas may be all you can point north, and that is fine, because that one move captures most of the benefit. On a larger or rural block, take advantage of the frontage and face nearly the whole house north, keeping only the garage and a study deliberately out of the sun. Even the laundry and bathrooms do better on the north, where the sun helps them dry and stay free of mould.

You do not need to hit true north exactly. The Australian government's Your Home guide notes that living areas facing solar north are ideal, but anything up to about 15 degrees west of north or 25 degrees east of north still gives good passive sun access. That tolerance is why so many real blocks can be made to work. Note that solar north is not the same as the magnetic north your phone compass shows, and the two can differ by a fair margin depending on where you live.

Windows are where heat comes in and goes out. Passive solar homes put generous glazing on the north, keep east and west windows modest because their low sun is hard to shade, and use south windows mainly for light and cross ventilation. The amount and placement of glass is one of the biggest levers in a BASIX or NatHERS assessment.

North glazing only works because the summer sun is high and the winter sun is low. A correctly sized eave or roof overhang shades the north windows in summer while still letting the winter sun in. East and west windows need different shading, because their sun is low and sideways, so external blinds, screens or planting do more than an eave. This is worth the effort because unshaded glass is usually the single biggest source of summer heat in a home, and good shading can block up to 90 per cent of it.

The same thinking applies to the alfresco. A roof that slopes the right way over the outdoor area, without a box gutter, lets the winter sun into the space while shading it through summer, so the outdoor room works alongside the living areas rather than against them.

Thermal mass is the heavy material inside the home, a concrete slab or a masonry wall, that soaks up warmth during the day and releases it slowly at night. Placed where the winter sun can reach it, thermal mass evens out the temperature swings and holds the free heat you collected. It only helps when it sits inside the insulated envelope and catches the sun.

Insulation and draught sealing keep the comfortable temperature you have created from leaking away. There is no point collecting winter sun if it escapes through an uninsulated ceiling or gaps around the doors. Insulation is the wrap that makes the other four ideas pay off.

The whole reason north works is the height of the midday sun through the year. In the temperate south of Australia the noon sun sits low, around 30 to 34 degrees above the horizon in midwinter, and climbs high, close to 80 degrees, in midsummer. On the Central Coast it is about 33 degrees in winter and 80 degrees in summer.

That gap is what a north eave exploits. Sized correctly it lets the low winter sun slide in under the overhang to warm the rooms, then blocks the high summer sun before it reaches the glass. Longer eaves suit the sunnier, warmer parts of the country, shorter ones the cool south where you want more winter sun. As a bonus, the same winter sun angle, about 33 degrees on the Central Coast, is close to the ideal tilt for rooftop solar panels.

None of these five ideas does much alone. Orientation without shading overheats in summer. Glazing without insulation leaks. Thermal mass in the shade does nothing. Passive solar design is the whole set working together, and it starts with a plan whose living areas already face north.

That is the one decision that is hard to change later and easy to get right now, which is why Dudils reviews every plan for it first.