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Spectrally selective vs. conventional window film: a comparison in energy conservation capabilities.

Forty percent of a building's cooling requirements is 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.

Applied window film is the least expensive solution to mitigate the impact of too much solar heat entering windows. The good news is conventional tinted and reflective applied window films successfully block a significant mount 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 than 34% of visible light, a good 36% less than the 70% necessary to be undetected by the naked eye. The result is building interiors are correspondingly darkened, often requiting the use of increased illumination. This may lead to higher electricity consumption that may increase inside temperatures requiring more air-conditioning. Increased utility costs defeat the major benefit of the film--cost savings.

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.

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 psf. The best spectrally selective applied window film ranges in price from approximately $9 to $12 psf 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.
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Comment:Spectrally selective vs. conventional window film: a comparison in energy conservation capabilities.(INSIDE CONSTRUCTION & DESIGN)
Author:Watts, Marty
Publication:Real Estate Weekly
Date:Sep 5, 2007
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