Control fading damage with window film: block heat and UV while letting in natural light--can film do all three?
Window film applied to existing glass can cost-effectively mitigate fading and heat problems. It does so by effectively blocking ultraviolet (UV) radiation, which accounts for 40 to 45 percent of fading damage; heat, which accounts for 25 percent; and visible light, which accounts for 25 percent. The remaining 5 to 10 percent of fading damage is due to the quality of the fabric, indoor lighting, and humidity level. Most applied window films successfully block more than 95 to 99 percent of the UV (which is the primary cause of fading damage). Many also block a good deal of infrared energy that accounts for overheating. That's good news. The bad news is that most window films block UV and heat while simultaneously blocking significant levels of natural light.
So what's a building manager to do? First, understand that because fading is caused by exposure to UV, heat, and visible light, unless you eliminate all three, fading will occur. From a practical point of view, fading can be appreciably slowed--but not entirely eliminated by--blocking significant amounts of UV and heat while letting in visible light.
The key to selecting the right kind of applied window film to deal with both UV and overheating is to understand how fading and overheating happen. Carpets, fabrics, artwork--even paints and wood--may fade upon exposure In the sun's energy. UV radiation--while only 3 percent of that energy--is the primary source of fading damage. Heat is a function of infrared energy. Both UV and infrared radiate as different wavelengths of energy. Visible light radiates on yet another wavelength.
Property managers and retailers need a non-tinted, non-reflective applied window film that can selectively block UV and heat while transmitting visible light. Such clear, colorless films are called spectrally selective because they "select" which wavelengths of energy they will block and which they will let through the glass. (Consult Talking Points, below, to compare spectrally selective and conventional window film.) Most spectrally selective films transmit no more than 58 percent of visible light--12 percent less than the 70 percent necessary to be undetected by the naked eye. If a window film looks tinted or reflective and not clear, it is not optimally selective in the all-important category of visible light transmission.
Differences between spectrally selective and conventional films in:
... blocking UV. Blocking UV is a function of both the film's components and of the physical properties of the film itself. Both films block a significant amount of UV.
... blocking heat. The lower the shading coefficient, the lower the solar heat gain. Conventional, tinted films transmit more than 65 percent of solar energy and have a shading coefficient of 0.70. Some reflective films with a shading coefficient as low as 0.28 transmit as little as 15 percent of visible light. When considering heat rejection and light transmission, spectrally selective films outperform conventional.
... clarity. The best spectrally selective film transmits up to 70 percent of visible light, and in doing so possesses a clear appearance.
... cost. Tinted and reflective window film costs range from $4 to $6 per installed square foot. Spectrally selective window film ranges in price from approximately $9 to $12 per square foot installed.
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Marty Watts is president and CEO at Houston-based V-Kool Inc. (www.v-kool-usa. com).
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|Title Annotation:||SmarterBUILDINGS: The Building Envelope|
|Date:||Apr 1, 2005|
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