The power of solar heat should never be underestimated. According to the California Energy Commission (CEC), 30 per cent of a building’s cooling requirements are to remove heat that entered through existing glass. Solar heat gain through south and west-facing windows is a serious problem for buildings even in cold climate cities like Montreal, Toronto, Calgary, and Vancouver. Couple this with the knowledge that heat is also generated in office environments by computers, lights, and people, and it is easy to see how the addition of unchecked solar heat can add to air-conditioning costs. In the ongoing quest for greater energy efficiency, commercial buildings and residential homes are being designed to harness solar heat where it is advantageous- but repel it where it is not.

Along with increased energy costs, solar heat can lead to a number of problems, including an inability to use space adequately near windows; a potential for more heating, ventilating and air-conditioning (HVAC) system repairs and replacement due to increased use; and fading and heat damage to furnishing, carpets, window treatments and retail merchandise (large expanses of glass can raise store temperatures and result in uncomfortable and unproductive employees). The most effective way of dealing with this problem is stopping heat at the window. In new construction, this approach can allow for the specification of smaller and less expensive HVAC systems.

To that end, glass manufacturers have developed solar control glass, which provides optimum energy performance, but at a cost exceeding that of standard insulating glass. For existing buildings, the most expensive option in dealing with solar heat is replacing all existing glass and frames with a new window system designed to block heat. A less expensive solution is to keep the frames and only replace the glass. In either case, building managers may be reluctant to replace existing windows or glass that is performing well in all other areas of service.

Applied window films
The use of applied window films removes the need to retrofit the glass and frames of an entire building, because the film is simply placed on the existing windows. Performance expectations of applied window film include reflecting heat and ultraviolet (UV) radiation while transmitting visible light into the building. Its ability to do this depends on the type of applied film used and the type of glass on which it is applied. Performance numbers change when comparing films applied to single-pane versus insulating glass. Nevertheless, as an alternative, film can be used on existing windows and glass doors, on most insulating glass and over most types of low emissivity (Low-E) glass, although it may affect the performance of some Low-E coatings.

Low-E refers to the ability of a substance to reflect heat. Glass or window film will reflect more heat than it lets in if a transparent, heat-reflective Low-E coating is applied during the manufacturing process. Most Low-E coated glass is compatible with solar control applied film; however, the application of solar control film to some single-pane glass with a Low-E coating on the room (interior) side of the window may negate the ability of the glass to insulate against heat loss (from the interior to the exterior) while increasing its ability to block incoming solar heat.

Benefits of applied window film (depending on the type of film) are:

• It reflects anywhere from 40 to 80 per cent of the sun’s heat.
• It slows heat transfer through glass into a building from four to 44 per cent.
• It transmits up to 70 per cent of visible light through glass intio a building.
• It reduces glare.
• It blocks 95 to 99 per cent on UV radiation, which can contribute to fading.
• It reduces loss of room heat through the glass in winter by as much as 15 per cent.
• It can reduce energy consumption, thereby lowering cooling and heating costs.

Reflective films made of semi-transparent metallic coatings provide a high level of solar heat control, but like tinted films, they reduce visible light. Most reflective films are highly reflective in daylight, giving them a mirror-like appearance when viewed externally. While benefiting one building, they can also cause a glare problem and unintended heat gain in surrounding buildings. In artificial light and at night, they appear mirrored form the inside as well.

Where lighting is an important factors, as in retail stores, tinted and reflective films often lead to darkened interiors, which require increased lighting, which in turn leads to greater electricity consumption and higher temperatures, resulting in more use of air-conditioning. Most conventional window films transmit (let in) less than 58 per cent of visible light, 12 per cent less than the 70 per cent necessary to make film undetectable to the naked eye.

Clear spectrally selective applied window film is designed to offer the best ratio of visible light transmission to heat rejection. “Spectrally selective” refers to the film’s ability to “select” or let in desirable daylight while blocking out undesirable heat. This is a function of a transparent, heat-reflective Low-E coating that is wavelength-sensitive. The coating is designed to transmit most of one wavelength range (i.e. visible light_ and reject most of the another (i.e. solar heat). The coating is chemically bonded to the film by a process known as vacuum sputtering.

Blocking heat and UV radiation
The ideal window film would be totally clear yet capable of significantly blocking unwanted solar heat, while reducing glare. We do not live in an ideal world. In the battle between clarity and limiting solar heat gain, one must often be sacrificed to the benefit of the other. Most tinted (dark) films transmit over 65 per cent of solar energy, giving them an unacceptable shading coefficient of over 0.70 in dealing with heat gain. Tinted films also come up short in clarity as they transmit less than 58 per cent of visible light and appear unclear.

Reflective films perform much better, achieving a shading coefficient as low as 0.51, blocking more than tinted films; however, they may transmit as little as 15 per cent of the visible light. Spectrally selective films transmit approximately 45 per cent of solar energy and 70 per cent of the visible light, giving them a shading coefficient of 0.51.

Along with eliminating solar heat gain, some window films are designed to block UV radiation, which can cause fading damage to carpets, draperies, furnishings, and retail merchandise. Some clear applied spectrally selective films successfully block 99 per cent of UV radiation. The ability of a window to block UV is a function of both the coating on the glass and the film itself. Conventional single-pane glass lets in approximately 150 times more UV radiation than coated glass or glass with film. Fading damage, while caused by UV radiation, is also the result of exposure to visible light. When UV is blocked, the fading process is merely slowed down.

Retaining heat
In Canada, the seasons change. In summer, we want to block solar heat gain from entering in an effort to stay cooler. In winter, we want our windows to block heat on the inside form being lost to the outside.

Both conventional and spectrally selective window films are designed primarily to block near-infrared light (i.e. direct solar energy). Both, however, will enhance the ability of existing glass to insulate against heat loss to the outside during the cold weather by as much as 15 per cent, assisting interior far-infrared heat sources such as furnaces.

Other considerations
The look of a building’s windows is important to the overall design intent of the architect. They are there to enhance the aesthetic appeal of the building, complementing other exterior design choices. Window film, although used for practical purpose, should not harm the design intent. Conventional dark and reflective window films change the appearance of existing glass and therefore the external appearance of the building. Clear spectrally selective film does not, which allows it to be used either on the entire building or selectively on identified problem areas.

All types of film can be applied to single-pane and insulated fixed glass, windows and doors. Before applying either, however, it is important to identify the existing glass and follow the advice of a qualified film installer.

Improperly installed windows have their own set of problems. Seal failure is the inability of material sealing together two panes of glass in a single unit to prevent the intrusion of air and moisture into the air space between them. This results in the appearance of condensation between the panes and prevents Low-E coatings from performing properly. Due to concerns about the difference in film and glass heat absorption rates in relation to seal failure, the film industry commissioned the Association of Industrial Metallizers, Coaters and Laminators (AIMCAL) to research the matter. Tests commissioned by AIMCAL and conducted by independent laboratories determined that when properly installed on insulating glass, applied window film does not cause seal failure. Accordingly, most window film manufacturers offer an insulating glass warranty in the event of seal failure.

Applied films are guaranteed not to peel, discolour, blister, bubble, or demetallize for at least 10 years in a commercial installation. This guarantee depends on the film being applied by a professional film installer who is properly trained in installation procedures by a window film manufacturer. General contractors and glass contractors are generally not qualified to do so.

Improper uses of applied window film
Window films increase thermal stress on windows, because of uneven temperature distribution across the glass surface. Glass and window films absorb heat differently. In high-altitude cities, temperatures may drop at night, leaving the surface of window glass very cold. When the early morning sun strikes the cold window surface, the center of the glass will heat up more rapidly than the surrounding edges. Theoretically, glass can break from such thermal stress. It is more likely to break, however, if a window film has been applied whose heat absorption characteristics are radically different from those of the glass. In that case, the film will absorb more heat than the glass.

There are many factors that will increase thermal stress, such as partial shading of windows from overhangs, tight-fitting drapes, signs of decals on windows, and heating and cooling vents directed at the glass. Different types of glass, such as annealed versus tempered or clear versus tinted, have different solar absorption rates and will withstand different degrees of thermal stress. Annealed glass is the most common window glass available and the most fragile and susceptible to breakage. Tempered glass is produced through a process of heating and rapid cooling to induce a change in structure that makes it about four times stronger than annealed glass. If it breaks, it falls into small fragments as opposed to the large shards created by broken annealed glass. Due to its ability to resist thermal stress, tempered glass is particularly suitable for the application of window film.

According to AIMCAL, some types of solar control films may cause thermal stress problems when applied to the following types of glass, because of incompatible glass/film heat absorption rates:

• Single-pane glass larger than 9.2 sq.m. (100 sq.ft.).
• Double-pane glass larger than 3.7 sq.m. (40 sq.ft.).
• Clear glass thicker than 9.5 mm (0.375 in.).
• Tinted glass thicker than 6.35 mm (0.25 in.).
• Window framing systems of concrete, solid aluminum or solid steel.
• Glass where a sealant or glazing compound has visibly hardened.
• Chipped, cracked, or otherwise damaged glass.
• Reflective, wired, textured or patterned glass.
• Triple-pane glass.
• Laminated glass windows.

Professional film installers know which types of glass may be unsuitable for applied window films, but glass breakage due to the installation of window film is statistically insignificant.

Conclusion
The comfort of a building’s occupants has a direct impact on their productivity and their company’s bottom line. The purpose of a building, aside from providing an interior for offices and workspaces, is to deal with the complexities of the surrounding climate- rain, wind, snow or sunlight. While building codes and standards require that at least certain, effective minimum standards be met, enhancements can improve and complement existing performance levels,

Whether it is dealing with an increase in solar heat that tax a current HVAC system, or simply providing protection from UV radiation, the proper use of applied window films is a first step towards improving a building’s indoor environment.