Window performance has increased dramatically on a number of measures over the past 25 years, driven by the fact that they are a major source of unwanted heat transfer, discomfort from glare and condensation problems in a building.

The best method for reducing the impact of your windows on a building’s thermal comfort and energy performance is to reduce their use wherever possible, but big expanses of glazing are widely considered attractive and unlikely to disappear from the facades of Australian commercial buildings anytime soon.

With that in mind, it is critical to take advantage of the best performing frame and glass technology, aimed at reducing heat transfer (U-Value) and Solar Hear Gain Coefficient (SHGC), and thereby cutting the energy lost through windows.

This includes thermally broken frames, double and triple glazed units, low-emissivity glass coatings, gas fills such as argon in the glazing space, and insulating spacers.

The latest glass and coatings

Double glazing has reached its upper limits in terms of insulating performance and no amount of low-emissivity coatings and gas cavity fills will get the centre of double glazing units (DGUs) to an R-value better than 5.

Manufacturers have since turned to adding one or more extra layers of glazing to improve this performance and when combined with gas fills and low-e coatings, quality quadruple glazing can achieve R-values of R10 and higher.

But adding layers means additional thickness and weight, and in response some suppliers now offer thinner krypton gas filled cavities, vacuum glazing units, closed cavity facades, and heat reflecting films if asked. These are tipped to be the next generation of window technology.

^Glassworks SolarAdapt is a Solar Responsive Thermochromic (SRT) adaptive glazing film which uses the sun’s energy to cause the tinting of the window, naturally controlling room temperatures as the sun changes orientation and radiation levels.

But the thermal qualities are only part of the story because glazing lets light in and results in solar radiation heating the inside of a room. A building’s air-conditioning system is almost always defined by the glazed area and while dark tinted windows and reflective coatings are used to reduce the Solar Heat Gain Coefficient (SHGC) of windows, these approaches reduce views and daylight. Manufacturers are now sometimes using spectrally selective coatings which reduce solar gain while having only a small effect on daylight.

Above Left: Recognised in Popular Science magazine’s list “The 100 Best Inventions of the Millennium”, Heat Mirror technology utilizes advanced coated films suspended in the middle of an IGU and reflect heat back to its source. They also provide multiple insulating cavities which can be filled with Krypton or Argon gas for improved performance. With three Heat Mirror films and krypton filled cavities, window R-Values of 20 are possible. Image: Paarhammer Windows and Doors frame and Heat Mirror Strips supplied by Viridian New World Glass.

Above Right: A renovation of the Empire State Building saw 6,500 windows upgraded to include Heat Mirror Strips and is tipped to save $50,000 on energy bills.

Frames:

Glass is only part of the heat transfer problem and a window will only perform as well as its spacers and framing. Warm-edge spacer bars separating the panes of glass within insulated glazing units (IGU) have become more affordable and widely available, but most of the heat transfer of even IGUs will still come through the spacer rather than the centre of the glass.

Window frames are a common source of thermal bridging, particularly in commercial applications where better thermal dampener variations such as timber and vinyl frames are not applicable to use and aluminium frames dominate.

Despite the technology, thermally broken aluminium frames rarely contribute to a window achieving an R-Value over R4, even if a R9 IGU is used for the glazing element.

Composite frames are beginning to surface in the market and these include foam-filled vinyl frames with aluminium exterior claddings, wood frames with polyurethane thermal breaks, and foam-filled fiberglass extrusions with wood interior and aluminium exterior finishes.

These innovations in window frames have improved their heat flow resistance however they are still behind the performance of glazing technology and will continue to be the source of most unwanted heat transfer in a building.

Nathan Johnson

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