The Australian federal government, through the Department of Industry and Science’s website, has released free architect‐designed plans and base specifications for homes to achieve a minimum 7 Star NatHERS rating in a range of climate zones.

A review of the plans and base specification, then the Star Ratings, however, leads to equal parts confusion and concern. A recommended choice for a major component of the building envelope “to create a comfortable home with low impact on the environment – economical to run, healthier to live in and adaptable to your changing needs” is, in all but one climate zone: single glazed, non‐thermally improved aluminium framed windows. The lone exception was for Darwin, where aluminium frames with low solar gain low e glazing is given.

This base specification recommended by appears not only to defy physics, but also to disregard the wide variety of climactic conditions present in Australia. It also appears to dismiss data from both the Window Energy Rating Scheme (WERS) of the Australian Window Association (AWA) and the National Fenestration Rating Council (NFRC) – a non‐profit US organization that was formed in response to the energy crises of the 1970s – by specifically allowing what is arguably the worst performing window configuration from an energy conservation standpoint.

The mere suggestion that using single glazed, basic aluminium framed windows will automatically achieve compliance to 7 stars is dubious at best and flies in the face of Australia’s push towards energy efficient products.

Energy Efficiency by Window Component

To understand energy transmission/loss through windows, it is important to remember that heat moves to cold, and does so via three avenues: conduction, convection and radiation. Window industry professionals keep these laws of physics in mind when designing window systems.

There are three areas of concentration when designing energy efficient windows: framing technology, spacer technology, and glazing technology. All three need to work in concert with each other to produce an energy efficient window.

First, let’s examine framing technology.

Since the late 1940s to the present day, framing materials have changed dramatically. Shortly after World War II, window framing transitioned from timber to lower cost aluminium. Due to high energy costs, Europe (primarily Germany) then led the way to more efficient framing through the introduction of uPVC in the early 1950s. The energy crisis in the 1970s drove this same transition in the US.

Window frames contribute to energy transfer primarily through conduction. With the addition of thermal breaks in aluminium frames, or a switch to alternative framing materials, window efficiency is improved dramatically.

A good way to compare the efficiency of window frames is by a comparison of their respective K-values, that is a measurement of their thermal conductivity (a lower value means less conductivity and better insulation):

  • Aluminium (non-thermally broken): 6.6
  • Aluminium (thermally broken): 2.2
  • uPVC: 1.18
  • Wood (pine): 0.96

While framing technology is certainly important, even more important is glazing technology, since glass represents the greatest area of a window (roughly 85 per cent). Additionally, glass can contribute to the transference of energy by conduction, radiation, and to a lesser extent, convection.

Glass efficiency is measured by its U‐value, which is also a measurement of its thermal conductivity. As is the case with K‐values, the lower the number, the more energy efficient the product. Dual panes have about half the U-value of a single glazed pane.

The final contributor to a window’s thermal efficiency is the spacer used between panes in dual and triple glazed units. Since this is the weakest part of the glazing unit from a conductivity standpoint, engineers look for ways to reduce conduction at this point.

Typical U values for windows:

  • Typical single glazed aluminium window: 7.0
  • Typical double glazed aluminium window: 4.5
  • Typical double glazed hardwood: 1.9
  • Typical uPVC double glazed window rates: <1.9
  • Typical uPVC triple glazed window rates: 0.7

Window metrics

What many people tend to focus on when considering energy efficient windows are heating costs as opposed to cooling costs. However, as we know, heat moves to cold, and present day building designs need to control unwanted heat transfer. Although it’s widely believed that it’s more important to focus on this in heating climates, the opposite is true.

Australia is geographically dominated by a cooling climate zone, with a small zone of mixed heating and cooling and a geographically small heating climate zone. However, when you combine the two cooling and heating/cooling regions, it includes nearly 60 per cent of the population of the country.

Consider that there are many fuel choices when it comes to heating buildings: coal, wood, natural gas, LPG and petroleum. However, when it comes to cooling a building, there is but one fuel source: electricity.

According to the Australian Bureau of Statistics, compared to a 15 per cent increase in the Consumer Price Index, Australia’s retail electricity prices rose by 72 per cent since 2012. The highest increases were seen in Melbourne (84 per cent) and Sydney (79 per cent), with the smallest increases being seen in Darwin and Canberra, at a still-hefty 42 per cent and 45 per cent respectively.

And what’s more, because of how NatHERS works, the rating system does not take into account thermal comfort. The single glazing allowed under these ‘7 Star’ home designs will almost certainly produce poor thermal comfort outcomes for these homeowners.

Whatever climate you live in, controlling energy transfer, and thereby energy costs are of critical importance. This leaves architects, builders and consumers with two choices: reduce the amount of glazing area in their homes or invest in energy efficient windows.

When you consider all the accumulated knowledge with regard to framing conductivity, glazing conductivity, conductive heat transfer, radiated heat transfer, and spacer configurations, it is dismaying that the Department of Industry and Science’s would allow the poorest performing option as the base specification when trying to encourage more energy efficient housing. Why haven’t they specified, for example, a minimum U-value for the windows?

Although the base plans may have been optimised on all other building elements to achieve 7 stars, it is hugely disappointing that they have overlooked the opportunities for higher performing windows to achieve the rating.

Come on Australia, we can do better than that.

Acknowledgement: Robert Maynes of Mathews Brothers Company provided some great research and assistance in developing this response.