Double glazing and windows for Perth homes

You can insulate the walls, get the orientation right and build in reverse brick veneer, and still lose much of that benefit through poor windows. Glass is the weakest point in any wall, so in a high-performance home the windows are one of the most important decisions you make, and one of the most common places people either overspend or underspend.

This article explains what double glazing actually does, the numbers worth understanding, why the frame matters as much as the glass, and whether double glazing is worth it in Perth's specific climate.

Why windows are the weak point

A good insulated wall might have an insulation value many times higher than a standard window. That means a plain single-glazed window can leak heat at several times the rate of the wall around it. In summer it lets radiant heat pour in; in winter it lets warmth escape. However well the rest of the house is built, the windows are usually where the comfort is won or lost.

That is why glazing is treated as a system in a passive solar home, not a finishing choice made at the end. How much glass, which way it faces, what type it is and how it is shaded all get decided together, early.

What double glazing actually does

Double glazing is two panes of glass with a sealed gap between them, usually filled with air or an inert gas such as argon. That trapped layer is a poor conductor of heat, so it slows the flow of heat through the window in both directions. The result is a window that keeps summer heat out and winter warmth in far better than a single pane, and as a side benefit cuts outside noise and stops the condensation you get on cold single glazing.

Double glazing on its own is only part of the story, though. The bigger gains in a hot climate come from what is done to the glass itself.

The numbers that matter: U-value and SHGC

Two figures describe how a window performs, and they are worth knowing because they let you compare products properly.

U-value measures how easily heat passes through the window. Lower is better. It matters most for winter warmth and for keeping conducted heat out on very hot days.

Solar heat gain coefficient, or SHGC, measures how much of the sun's radiant heat the window lets through. A high SHGC lets in lots of solar heat, which you want on north glass in winter and want to avoid on west glass in summer. This is the number that matters most in Perth.

The tool for controlling SHGC is a low-E coating, a microscopically thin, invisible layer on the glass that reflects radiant heat while still letting light through. Low-E is often more valuable than the second pane itself in our climate, because our problem is mostly keeping the summer sun's heat out. The table below shows the pattern.

Glazing type	Heat flow (U-value)	Solar heat (SHGC)	Best use
Single glazing	High, poor	High, uncontrolled	No longer suitable for a high-performance home
Standard double glazing	Much lower, good	Still fairly high	Good all round, especially where you want winter sun
Double glazing with low-E	Lowest, best	Tunable to suit each orientation	The strong choice for Perth, matched to which way the glass faces

The frame matters as much as the glass

It is easy to focus on the glass and forget the frame, but a great pane in a poor frame wastes much of the benefit. A standard aluminium frame conducts heat readily, creating a path for heat to bypass the glazing entirely. The better options are thermally broken aluminium, which puts an insulating break through the frame, or timber and uPVC frames, which conduct far less heat to begin with. In a home where the windows have been chosen carefully, the frame is specified with the same care as the glass.

So is double glazing worth it in Perth?

For a new high-performance home, yes, in almost every case, but the smarter framing is where to spend rather than whether to spend. Perth's mild winters mean the winter heat-loss argument for double glazing is weaker here than in Melbourne or Canberra. The strong argument here is summer: controlling solar heat gain with low-E glass, sensible glazing sizes and good shading.

So the priority in Perth is usually low-E double glazing on the glass that needs it most, west and east in particular, combined with correct orientation and shading so the windows work with the sun rather than against it. Glass is only ever as good as the design around it. A modest, well-oriented, well-shaded home with good glazing will outperform a poorly designed one with the most expensive windows on the market. It is also part of why a passive solar home typically costs around 10 to 20 per cent more than a custom-designed and built 7 star home, a premium repaid through comfort and lower bills, which we cover in our guide to what an energy-efficient home costs.

You can read our overview of passive solar design, understand how passive cooling keeps a home comfortable in summer, or get in touch to talk through your project.