topping heat at the window can be an effective way of saving on air- conditioning costs. One way of achieving this is with solar control film, a thin polyester-based material attached to windows or doors which can substantially reduce the amount of solar energy that is allowed through the glass.

"Generally, the most effective solar control films in terms of solar heat rejection are metalised films that have a low VLT (visible light transmission) in the 15 per cent to 20 per cent range, and a high heat rejection factor in the 70 per cent to 80 per cent region," says Rob Hamilton, president of the Window Film Association of Australia and New Zealand (WFAANZ).

Metalised film can reflect significant amounts of solar radiation. During the manufacturing process, extremely thin layers of metals are deposited directly onto the polyester film, onto which another layer of polyester film is laminated. "Polyester offers near-perfect clarity and can be treated with layers of metals or dyes to produce specific performance and colour characteristics," Hamilton says.

The two basic manufacturing processes for metalised films are vapour coating and sputtering. Sputtering allows several layers of different metals to be applied to a single piece of film, resulting in unique colours and higher levels of selective transmission. Achieving a large reduction in solar heat gain (SHG) comes with a trade-off: lower VLT. But technology is reducing this trade-off, with 'spectrally selective' films designed to reject glare and block heat while providing better VLT.

Other types of window film include clear film and dyed film, which are non-reflective and do not contain any metals to reflect solar radiation. Clear film is usually designed to offer safety protection and UV control to help reduce fading. Dyed film provides glare and fade control and is designed to reduce heat gain by solar absorption.

A greater variety of shades, colours and performance levels are now available, with scratch resistant coatings for protection, durable dyes and ceramic pigments, in addition to metals for colour stability. Many films offer glare and fade protection for building interiors and are popular for shopfronts. Some films also offer safety, security and anti-graffiti benefits.

One of the latest films manufactured by Film Pacific is OptiTune. "This product has a heat rejection of 70 per cent, with a VLT of 22 per cent - perfect for domestic or commercial use," says Kip Linden, national marketing and sales manager with Film Pacific. "The benefit of OptiTune is you can use it on laminated glass because of its low absorption rate and it has a neutral grey appearance."

While some old films can look ugly and visibility was low at night, technological advances have improved both the appearance and performance of window film, with a low reflective internal coating to enhance night-time visibility. Film Pacific has launched a new night vision film called Night Owl, which Linden says is one of the lowest- reflective films ever made.

Solargard Window Films has a similar product called Nightscene film: "The secret to this technology is minimising the visible light reflection by laminating film layers together and the use of special metal and non-metal materials," says Paul Mealing, managing director at Solargard. "Solargard Nightscene film combines a heat reflective middle layer with a low reflective layer either side."

Other Solargard products include LX70, "a nearly invisible film offering up to a 55 per cent reduction in heat gain. This highly spectrally selective metalised film can be used for high-end residential and commercial buildings," Mealing says.

Costs

Standard solar control films range in price from $50 per m2 to $80 per m2, including the supply, installation and warranty. Higher performance films may cost as much as $180 per m2. Safety film is around $75 to $120, depending on the thickness and specific requirements, while decorative films range from $65 per m2 to $130 per m2.

Modern technology allows films to last longer, which is reflected in the warranties. Most window film manufacturers provide a minimum 10-year warranty on their products and many come with a lifetime warranty.

Performance comparisons

When comparing solar control films and products such as Low E (low emissivity) glass, it is important to take into account the relevant performance measures, such as heat rejection and heat retention.

Linden says that installing Low E glass can be four or five times more expensive than applying window films, although "Low E glass does have fantastic heat retention that is unrivalled by window film at this stage. The benefits of window film are far greater in a warmer climate like Queensland, where Low E glass would be beneficial in places like Victoria or Tasmania," Linden says.

Another advantage of window films is that they can be quickly fitted to any existing glass. Occasionally, if window film is to be applied to glass before it is installed in the frame, there is a slight installation efficiency gained, but this is typically minimal and it may be better to apply films after the window is installed because installers need a clean and dust-free area.

However, the wrong film on the wrong glass can present a risk of glass breakage, but a good installer knows which films are safe for which glass types. For example, Solargard uses software tools to simulate a building's energy requirements and match the right film to a client's needs.

What next in window film technology?

One of the new frontiers is nanotechnology, "a fledgling process but an exciting advance expected to open up new capabilities for window films in the future," Hamilton says.

A nano is around one thousandth of the width of a strand of hair. Nanotechnology involves creating materials at the atomic level, one molecule at a time. "Current nanotechnology films offer good heat rejection abilities positioned above the abilities of dyed films and below what is available from metallised films. Metallised and dyed window films offer equivalent durability to nano films, which is reflected in the manufacturer-backed warranties," he says.

Nano-ceramic window films are said to be fade and corrosion free, outlasting other conventional material (dyed and metals) based films by more than 50 per cent. They are also 'spectrally selective', rejecting glare and blocking heat while providing better VLT.

Solargard offers UP75 film, "a similar product to LX70, offering spectral selectivity using advanced nano-particle technology," Mealing says.

According to Mealing, recent product developments with Solargard's range have involved the combination of exotic metals in full metal films. "New dyes have been developed to provide lifetime durability without colour change and the development of state-of-the-art nano-particle technology," he says. "In the near future we will bring to the market further advances in nano- particle products and low reflectivity films offering the highest visibility coupled with high heat and glare rejection."

John Daginis, state manager NSW/ACT with MEP, says scientists are now working on polarised films. "When the sun hits them they will go dark and when it's dark they will go lighter. We now have the ability to apply up to nine different metals onto a spectrally selective material. This technology allows for clear visibility and high heat reduction. This is ideal for heritage buildings and anyone with a view, like those living on the ocean. It's an eco film, letting light in while reducing the heat," Daginis says.

As many Australians look for ways to make their home more comfortable without increasing energy consumption, the window film industry continues to develop new products aimed at improving the energy efficiency of windows in providing a more comfortable place.

Jon Hoel

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