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Window film is a glass act.

Thirty percent of a building's cooling requirements are a function of heat entering through existing glass. Stopping heat at the window is the most effective means of lowering temperatures and reducing HVAC operating cost. In new construction, reducing heat at the window can mean the need for smaller HVAC systems.

Applied window film is the least expensive solution to mitigate the impact of too much solar heat entering windows. The good news is conventional dark and reflective applied window films successfully block a significant amount of solar heat.

The bad news is that these same films reduce a significant percentage of visible light. Most of these films are highly reflective in daylight giving them a mirror like appearance when viewed externally. In retail establishments visible light is reduced inside the store and shoppers outside cannot clearly see inside.

Most conventional window films transmit less visible light than the 70% necessary to be undetected by the eye. The result is building interiors are correspondingly darkened often requiring the use of increased illumination. This leads to higher electricity consumption that may increase temperatures requiring more air-conditioning. Increased utility costs defeat the benefit of the film--cost savings.

The best solution to overheating--clear, spectrally-selective film Clear spectrally-selective applied window film offers the best ratio of visible light transmission to heat rejection. Spectrally-selective refers to the ability of the film to select or let in desirable daylight, while blocking out undesirable heat.

Most so-called spectrally selective films transmit no more than 58% of visible light. If a window film looks tinted and not clear it is not optimally selective in visible light transmission.

Building management should consider the following when evaluating spectrally selective vs. conventional window film:

How do they compare in clarity?

The ideal film would be totally clear yet able to significantly block unwanted solar heat and reduce glare.

How do they compare in blocking heat?

With a shading coefficient as low as 0.51, some reflective films block significant heat but many transmit as little as 15% of the visible light. When considering both heat rejection and light transmission, spectrally selective films out perform conventional competitors.

How do they compare aesthetically?

Clear spectrally selective film does not change the appearance of existing glass allowing its application on an entire building or on as few windows as necessary to deal with a localized over heating problem.

How do they compare in price?

The price of dark, tinted and reflective window film ranges from $4 to $6 dollars per installed square foot. The best spectrally selective applied window film ranges in price from approximately $9 to $12 a square foot installed. Installed prices are volume dependent. On larger projects such superior performing films may be installed for less.

How do they compare in payback?

Considering the cost of energy used for lighting and HVAC operation due to conventional films inability to transmit sufficient visible light, the payback for conventional film and spectrally selective film becomes comparable.

Installations of spectrally-selective window film

The following retailers use spectrally-selective window film in selected establishments: Hallmark Cards, Calico Corners, Public Storage, Esprit, McDonalds, and Exxon convenience stores.

V-Kool, Inc., Houston, TX, is a sales and marketing distribution company of spectrally selective applied films. For information contact V-Kool, Inc., at 800 2177046 and at
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Title Annotation:Technology; building materials
Author:Watts, Marty
Publication:Real Estate Weekly
Geographic Code:1USA
Date:Sep 15, 2004
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