According to the California Energy Commission, as much as forty percent of a building’s cooling requirements are a function of heat entering through existing glass. Even a small amount of glass on the south or west side of a building can result in overheating and increased energy costs. Stopping heat at the window is the most effective way to save by lowering temperatures and reducing HVAC operating cost.

 

The best place to stop heat and reduce air conditioning cost is at the window. However, the most energy efficient window film is not necessarily the film that blocks the most heat.

 

Applied window film is the least expensive solution to mitigate the impact of rising energy cost from too much solar heat entering windows. The good news is conventional tinted 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 sun light. Most of these films are highly reflective in daylight giving them a mirror like appearance when viewed externally. In retail establishments visible sun light is reduced inside the store and shoppers outside cannot clearly see inside.

 

Most conventional window films transmit less than 34% of visible sun light, a good 36% less than the 70% necessary to be undetected by the naked eye. The result as building interiors are correspondingly darkened, often requiring the use of increased energy costs from the additional illumination. This may lead to higher energy consumption that may increase inside temperatures requiring more air-conditioning. Increased energy costs defeat the major benefit of the energy efficient films – cost savings.

 

The use of conventional tinted and reflective heat-blocking window films can negatively impact building occupants by denying them adequate levels of natural sun light. Studies conducted by the U.S. Department of Energy and others document a relationship between low levels of natural sun light and less than optimum well being and productivity.

 

 

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

 

While it is relatively easy to manufacture tinted and reflective film that blocks heat, it is another thing entirely to produce what is called spectrally selective energy efficient window film, capable of blocking undesirable heat while simultaneously transmitting desirable sun light. At least one spectrally selective energy efficient window film is available and for years has been successfully installed on windows and fixed glass in commercial, institutional and residential properties. Unfortunately, several manufacturers call their film spectrally selective when it is only marginally so.

 

The definitive test is how much visible light the film transmits. A true spectrally selective energy efficient window film transmits at least 70% of visible light. Most calling themselves spectrally selective actually transmit no more than 54%. Such films when installed on a window become visible to the naked eye. If a window film looks tinted and not clear it is not optimally selective in the all important category of visible light transmission.

 

Consider the following when evaluating spectrally selective energy efficient window film 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 window film sun block significant heat but many transmit as little as 15% of the visible light. When considering both heat rejection and sun 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 squares foot. The best spectrally selective applied window film ranges in price from approximately $9 to $12 dollars per 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 film’s inability to transmit sufficient visible light, the ability to save with conventional film and spectrally selective film becomes comparable.