How To Choose Windows and Glazing for an Australian Home

A good insulated wall resists heat flow many times better than even a very good window. That means glass is where you lose warmth in winter and gain unwanted heat in summer. The Australian government's Your Home guide puts numbers on it: up to 87 per cent of a home's summer heat can enter through the windows, and up to 40 per cent of its winter warmth can leak back out the same way. That does not mean fewer windows. It means the windows you do have should be as thermally efficient as you can afford, and placed where they earn their keep.

A useful rule from passive design is to keep a window within about 6 metres of anywhere in a room, because both daylight and natural ventilation reach roughly 6 metres into a space. Beyond that the room goes dark and stuffy and you start paying for lights and mechanical air.

Visible Light Transmittance, or VLT, is how much daylight the glass lets through. A very low VLT cuts heat but darkens the room, so you end up switching lights on, which defeats the point.

U value is how fast heat conducts through the whole window, glass and frame together. Lower is better. A low U value window holds warmth in during winter and keeps summer heat out. The heat lost through a window is roughly its U value times its area times the temperature difference across it, so a large window on a cold night moves a lot of heat.

Solar Heat Gain Coefficient, or SHGC, is how much of the sun's heat the window lets in, on a scale of 0 to 1. High SHGC is what you want on the north, where free winter sun is welcome. Low SHGC is what you want on the east and west, where the low sun is fierce and hard to shade. The real gain also depends on the angle the sun strikes the glass. A north window meets the high summer sun at a steep angle near 82 degrees, where most of the heat is reflected away, and the low winter sun at about 35 degrees, where it pours in. That is passive solar working through the glass itself.

This is the part builders rarely explain. The best window for one wall is the wrong window for another, because each direction gets a different sun.

On north glazing you want a high SHGC so the winter sun can warm the rooms, paired with an eave that shades it in summer. On east and west glazing, where the sun is low and blinding morning and afternoon, you want a lower SHGC and external shading, because no eave can block sun that comes in sideways. On the south you get almost no direct sun, so those windows are mostly for light, view and cross ventilation, and low e glass keeps them from bleeding heat.

Because the ideal glass differs by wall, a well specified home often uses more than one glazing type across the house. The thermal modelling behind a BASIX or NatHERS assessment is where those choices get locked in.

Single clear glass is the worst performer and the cheapest. Adding a low e coating, a microscopically thin metal layer, lifts a single pane a long way by letting daylight in while reflecting heat back to where it came from.

Double glazing, two panes sealed around a gas filled cavity, is the bigger step. The cavity does the insulating, so a 12 millimetre gap filled with argon performs better than a narrow air gap. Combine double glazing with a low e coating and you have a window that suits almost any Australian climate, not just cold ones. If replacing whole windows is out of budget, secondary glazing, a second sheet fixed inside the existing frame, captures much of the benefit and cuts noise as well.

There is a knock on saving too. Because efficient glazing lowers the peak load on a hot or cold day, it can shrink the size of air conditioner a home needs, by around 30 per cent on Your Home's figures, so you save on the equipment as well as the running cost.

People obsess over the glass and forget the frame, but heat pours through a bad frame just as fast. Plain aluminium is strong and cheap and conducts heat beautifully, which is exactly what you do not want. A thermally broken aluminium frame puts an insulating break between the inside and outside metal and performs far better.

Timber frames insulate naturally and suit many homes, provided they are sealed well against draughts. uPVC frames often outperform both, seal tightly and need little maintenance, which is why they are spreading fast in Australia. Composite frames pair a durable outer with an insulating inner. Whatever the material, look for the Window Energy Rating Scheme, or WERS, label, which rates the whole window so you can compare like for like.