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3 Key Terms Used to Describe Window Film

House Window
Your home's windows play a huge rule in your overall energy efficiency, thanks to the often unwanted phenomenon known as solar heat gain. While beneficial for those living in cold climates, for those living in hot climates solar heat gain can drastically increase the amount of energy needed for summertime cooling.
Fortunately, window films offer a reliable and cost-effective way to minimize solar heat gain. Yet many homeowners shy away from investing in window films because they don't understand all of the terminology used. If you would like to improve your knowledge of window film ratings, keep reading. This article outlines three key terms used to describe window film.
1. Visible Light Transmission
Visible light transmission, or VLT, provides one of the most basic pieces of information about a window film. VLT tells you how much actual light a window film will allow into your home. Manufacturers generally designate VLT as a percentage. Generally speaking, the higher the VLT of a window film, the darker it is.
For instance, a film with a VLT rating of 20 percent blocks 20 percent of the light passing through your window. A film with a VLT rating of 50, by contrast, will prevent half of the incoming light from entering your home. Homeowners must heavily weigh VLT when deciding how much ambient light they want to allow into their home.
2. Solar Heat Gain Coefficient
VLT bears some relation to energy efficiency, since darker tints in general will also block more solar radiation from entering your home. Yet VLT simply can't paint an accurate picture about how much energy actually makes it through a window film. To better understand how well a film can protect against solar heat gain, you'll need to know its solar heat gain coefficient, or SHGC.
The solar heat gain coefficient doesn't just describe window films, it also gets used as a tool for measuring the efficiency of a window's glass. SHGC denotes the fraction of solar radiation that manages to get into your home, as measured on a scale from 0 to 1. The lower the SHGC, the less heat enters your home. The best window films, used with low emissivity glazing, can achieve an SHGC as low as 0.2.
3. Light-to-Solar Gain Ratio
Not all window films have the same relationship between the visible light transmission rating and the solar heat gain coefficient. In other words, selecting a darker film doesn't always mean that you're selecting the one that will best keep heat out of your home. Likewise, choosing a film with a high SHGC doesn't necessarily mean that you'll be living in a darkened cave.
To give a clearer picture of a particular film's performance, pay attention to the light-to-solar gain, or LSG, ratio. If the manufacturer doesn't provide this value, you can calculate it for yourself simply by dividing the visible light transmission rating by the solar heat gain coefficient. The LSG ratio gives a more accurate idea of how spectrally selective a film is.
The higher the LSG, the more daylight and less solar heat is allowed in. Higher LSG ratings thus make a better choice for those who live in heat-prone regions. Higher LSG ratings also help to keep your home as bright as possible during the daylight hours.
Many people express frustration when attempting to decode the technical ratings used to describe window films. Yet once you have grasped the basics, these rating systems provide a highly useful way to understand the properties of a given film. For more information about selecting the best possible window film for your home, please contact the experts at Affiliated Auto Glass.