Assisted by design software, including AccuRate, FirstRate 5 and BERS pro, the understanding of high performance glass and window types is improving.
To encourage the evolution of the use of energy efficient glass, the Sustainable Windows Alliance (SWA) has produced three guides based around Australia’s three climate zones of Hot (WA, NT, QLD), Mixed (NSW, SA, areas of WA) and Cold (VIC, ACT, TAS and areas of NSW). The SWA’s findings demonstrate, as long thought by the industry, that SHGC (Solar Heat Gain Coefficient) and U-Value are critical considerations when specifying windows and glass.
A low U-Value is generally preferable regardless of climate, whereas SHGC should be based on the climate, as well as the orientation of the glazing.
In cooler climates, north facing windows should have a high SHGC to allow for passive solar energy in winter; fixed shading should be designed to shade the windows during summer. East and west facing windows should have a low SHGC to avoid overheating in summer. In hot climates, a low SHGC is always ideal.
The windows used in Australian homes have changed very little over the past few decades, largely because they haven’t needed to.
“Until now, increased insulation in the walls and ceilings has in many cases been enough to meet modest 4-star and 5-star energy ratings for most house designs and climates,” said Lachlan Austin from the Sustainable Windows Alliance (SWA) steering committee. “But with the implementation of 6-star ratings, builders and homeowners will have to start addressing the performance of their windows – in most cases ordinary windows will not be adequate for current home designs to meet increased building regulations.”
“The good news is there is no need for consumers to limit the size and scope of glazed areas – we just need to use better glass.”
What once was considered energy-efficient glass — double glazing with low-e coatings, argon fill and thermally broken aluminum framing — is now becoming more and more standard.
Evolving technologies, like dynamic glazing, triple glazing and vacuum glazing, are the future.
The University of Ulster in Ireland is leading the way in the development of these technologies and particularly with regards to vacuum glazing.
Nearly 60 per cent of dwellings in European countries are still single glazed, causing excessive heat losses and hence increased carbon dioxide emissions. By upgrading glass U-values from 5.7 to 1.6 Wm-2K-1, the corresponding European CO2 reduction was estimated at 82 million tonnes per year.
Vacuum glazing consists of two sheets of glass hermetically sealed around their periphery with a vacuum gap between the glass sheets. An array of tiny support pillars, typically 0.3 to 0.5 millimetres in diameter, are used to separate the glass panes and to prevent them from touching due to atmospheric pressure. Low emittance (low-e) coatings are used on one or both of the internal glass surfaces to reduce radiative heat transfer from the inside to the outside of the glazing.
The University of Ulster has developed and patented a novel sealing technique for the fabrication of vacuum glazing at low temperatures and has fabricated glazing with a mid-pane U-values down to 0.86 Wm-2K-1 for a 0.4m by 0.4m sample employing hard low-e coatings.
But there are still several obstacles which are preventing widespread use of these technologies.
Incentives for their use vary widely from country to country, although Europe is doing better in this regard, and there is a high cost differential. For triple glazing, for example, although they may be cost effective in the long-term, they remain expensive and can be used only with certain types of framing, strong enough to hold three layers.
Mathieu Meur, managing director of Meinhardt Façade Technology, is positive though that things are moving in the right direction.
“Environmentally-responsive glass is still immature and extremely expensive, to the point of being unusable, due to incredibly long ROI but tinted and low-e coated glass could almost be called old and established now. We have been pressing the glass manufacturers to produce better glass products that answer market expectations,” Meur said.
Michael Barclay, an engineer and manager of fenestration services at MMM Group Limited suggests lack of awareness is still an issue and specifiers would rather stick with what they know.
“It’s a matter of educating professionals from architects to fenestration designers,” he said.
New warm edge spacer bars, for example, reduce the amount of heat lost through a sealed unit. They are also more sustainable than aluminum spacers, but Barclay says people may mistrust the durability factor since warm edge spacer are made of foam or plastic and metal combination.
“The norm over the last decade is to use aluminum because it’s strong and cheap and people have always used that,” he said. “Unless we stay on top of energy performance the size of the window will shrink and opaque building materials, like brick or wood will take over the façade. What you’ll have is a trend towards a box.”