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Comparing Standard and Low-E Window Film

Posted by Steve DeBusk on Oct 07, 2014

Your commercial building’s windows significantly impact energy efficiency. But you don’t have to take our word for it. The U.S. Department of Energy says that inefficient windows are to blame for 25% to 35% of wasted energy. And the California Energy Commission estimates that around 40% of a building’s cooling requirements are due to solar heat gain through windows.

Window film often wins out over window replacement when it comes to improving the performance of existing windows (unless windows are broken or leaking). To make windows as energy-efficient as possible, there are different window film options to compare.

Standard Window Film
Emissivity (which determines a film’s insulating performance) varies from 0.70 to 0.81 with standard window films. These numbers mean that 19% to 30% of a room’s near-infrared heat is reflected back into the space in the winter to help reduce heating costs. Standard window film offers energy-efficiency savings during cooling season, but it reduces solar gain all year long – even during winter (when heat gain could help keep the building warm).

In some cases, standard window film may also affect outdoor views. Window films with less than 20% visible light transmission, or reflective films with visible reflectance of 30% or more, may make it difficult to enjoy views on cloudy or rainy days (or after the sun goes down). Outdoor scenery may also appear darker with a low visible light transmission film (less than 20%). Reflective window film may need shades or blinds to diminish reflection and provide nighttime privacy.

  • Typical ROI for reflective standard window film: 2 to 5 years
  • Typical ROI for lower reflectance window film: 3 to 6 years
  • Total installed cost of standard window film: $4 to $6 per square foot

Low-E Window Film
Emissivity values of low-e window film range, depending on the film. Conventional low-e window film has an emissivity rating of 0.33, which means that 67% of a room’s near-infrared heat is reflected back into the building in the winter. It also enhances insulating performance of existing windows by 44%. However, traditional low-e window film can sometimes turn iridescent when installed near energy-efficient lighting (compact fluorescents, for example).

High-performance low-e window film offers emissivity ratings as low as 0.07, which means that 93% of the building’s near-infrared heat is reflected back into the room. With high-performance low-e window film, building owners experience improved insulating performance levels of up to 92%.

Because low-e window films improve window insulating performance year-round, they typically provide both cooling and heating energy savings. Low-e film prevents heat loss in the winter, and prevents solar heat gain in the summer. New low-e window films also reflect infrared heat while still allowing visible light in through the window. This technology allows facilities managers to make use of natural light while minimizing the amount of heat allowed through the windows.

  • Typical ROI for low-e window film: 2 to 5 years
  • Total installed cost of low-e window film: $5 to $15 per square foot

Have you installed either traditional or low-e window film? Which did you select, and why?

Steve DeBusk is global energy solutions manager for Eastman Performance Films, LLC. Steve has 30 years of experience in energy efficiency. He is a Certified Energy Manager, a Certified Measurement and Verification Professional, and a Certified Sustainable Development Professional. Follow Steve on Twitter @greenbldgs.

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