Sustainable Window Design

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-By Jaime McKee

When speaking of a building’s environmental sustainability, it has become common practice to describe its green “envelope. The outer shell of a building, separating the interior from what lies beyond, the envelope includes such elements as the roof, walls, foundation, and floors. All of these features can be “greened in various ways – and in previous issues we’ve explored both green roofs and living walls – yet the installation of windows is akin to punching giant holes in that envelope. The wrong choice here can mean significant losses in energy efficiency, as the average home may lose as much as 40% of its heating or cooling through its windows – inexcusable losses in these carbon-crunched times.

Yet windows, too, are becoming available in increasingly eco-friendly varieties. New technologies can be found in every element from the framing, to the coating, to the glass itself, and are helping both residential and commercial builders round out their buildings’ green envelopes effectively and attractively.

Windows can tell us a lot about a building. They seem to be an ever-evolving facet of the construction process, and, as one that is by its nature impossible to hide, they can speak volumes about a building’s age, style, and degree of energy efficiency. It wasn’t until the late 17th century that glass windows became common in homes, and the first of these tended to be small, as glass was expensive and recognised to be a very poor insulator. Over the centuries, windows have evolved from a tiny hole in the wall designed to let in a little fresh air and light into an integral aspect of a building’s functionality and aesthetic. Yet for much of that time, they have remained largely inefficient, the weak link in most buildings’ envelopes, vulnerable to air leakage, condensation, and of course, physical damage from a stray tree branch or rogue baseball.

The Glass is Greener…

Recent developments in window technology have done much to change this. In terms of the glass itself, stronger and larger slabs are now much more accessible, allowing for the stunning walls of windows so often seen on many of Australia’s grand structures. There is a tipping point here – windows are still not as insulating as solid walls – but to an extent, allowing more natural sunlight in can help regulate a building’s temperature, and can lessen occupants’ reliance on artificial lighting, helping to keep energy costs down. Double or even triple glazing also drastically improves insulation and temperature regulation, and low-e glass coatings can be applied to further tweak the amount of sunlight which enters. These factors contribute to a window’s U-value, a number which indicates the amount of heat lost, the inverse of the R-value commonly applied to insulation. In the case of U-value, the lower the number, the better. A double pane window with a low-e coating might have a U-value of .35, while a triple pane window with a low-e coating will have a U-value of .25, but will cost substantially more. Inert gases such as argon can even be used as a filler between panes of glass to further prevent energy losses.

Of course, it is important to keep in mind not only energy efficiency but the entire life-cycle of the product when thinking green. To this end, the materials used in the window frame can play just as large a role as the glass itself. The most common type of window frame currently on the market is that made from uPVC – unplasticised polyvinyl chloride. While inexpensive and readily available, this material releases dioxins in both its manufacture and ultimate disposal, and is strongly discouraged by environmental groups and some countries, such as Germany and the Netherlands. Thermally unstable, uPVC windows may also warp and lose efficiency over time. Aluminium is another framing option, and is commonly seen in older window installations throughout Australia. While non-toxic, aluminium is a conductor and can allow approximately 87% of warm air and 49% of cool air to pass through. Wood is a renewable option with high aesthetic appeal, but must be maintained and treated to maximise its lifespan and prevent rot. Perhaps the most eco-friendly option is fibreglass: both physically and thermally stable, fibreglass is stronger than uPVC and will not rot like wood or conduct heat like aluminium. It is, however, significantly more expensive than other options and not as commonly available.

Finally there are the window mechanisms to consider. While any infiltration of air equals energy loss, practically speaking, people want windows that can be opened for fresh air. A hopper style window – hinged on the bottom and tilting open at the top – is the best performer here, followed closely by a casement window – hinged on either the left or right side and opening out with a crank mechanism. Among classical style windows, single hung perform better than double hung.

Window placement and coverings also play a role in their green performance; maximising daylighting while minimising heat gain is optimal. Views of the outdoors have also been shown to contribute positively to work and living environments, and this element is beginning to be woven into some of the very best Australian home and office designs.

WERS

With so many factors to consider, how does one avoid being overwhelmed by the process of choosing windows? In Australia, the WERS system acts as a comprehensive guide to windows’ energy impact, and can help guide the decisions of homeowners, contractors and architects when trying to think green.

A proprietary scheme, the Window Energy Rating Scheme is managed by the Australian Window Association (AWA). Members agree to a formal code of conduct and participate in an annual audit; membership enables windows to be rated and labelled for their annual energy impact on a whole house, in any climate of Australia. A New Zealand variant of WERS, the Window Efficiency Rating Scheme, is also available. Operating in both the residential and commercial sectors, the WERS system takes into account a window’s materials, framing specifications, glazing, and other components to arrive at a rating for each participating window on the market. WERS-rated residential windows feature star ratings, ranging from zero (poor) to ten (excellent), and percentage improvements to help provide easy comparison between windows; commercial ratings are more precise and are designed to provide industry members with custom ratings to distinguish their product on the commercial market.

To participate in WERS, window manufacturers must obtain energy ratings for their products from a rating organisation that is accredited by the Australian Fenestration Rating Council (AFRC). WERS is independent of any one manufacturer and acts as a fair, rigorous and credible system for testing performance claims. Described by the Australian Greenhouse Office as an “excellent example of government and industry working in partnership to address the greenhouse problem, the Window Energy Rating Scheme has done much of the heavy lifting for consumers in terms of compiling the relevant factors into a useful benchmark.

Back in the 17th century, it likely couldn’t be conceived that windows would one day feature so prominently in whole-house efficiency and sustainability. But with an important dual role to play, both in both improving quality of life and reducing greenhouse gas emissions, it is no wonder that windows are often the “make or break element in green building design. Much more than just “holes in the envelope, windows frame our view of the world, and impact upon it far more than we may realise.

October 17, 2018, 11:28 PM EDT