Facility managers have good reason to make thoughtful decisions about glazing, and fortunately, the toolbox of glazing options has grown quite large. A third or so of an average building’s cooling load is caused by solar heat gain through windows, and energy-efficient glazing can bring about significant reductions in heating and cooling costs. The glazing product chosen, coupled with the design concept, can keep out a lot of sunlight, heat, and cold — or let them in. It can make efficient, money-saving use of daylight — or squander that opportunity. It can limit — or add significantly to — the load on the HVAC system. It can do a good job of holding in a building’s heat, or insulate poorly. And it can stop — or allow — annoying glare.
For facility managers faced with glazing choices, a logical starting point is the sunlight coming in the windows. The light coming at the glass actually consists of three types of radiation. Nearly all of it is visible light (44 percent) and infrared light (53 percent), and a little bit (3 percent) is ultraviolet light.
The light entering a building not only illuminates an interior space but warms it up, acting like a “mini-heater,” in the words of Helen Sanders, vice president for technical business development at SAGE Electrochromics. For commercial office buildings most of the year, “unmanaged solar energy creates a thermal heating load that must be removed by air-conditioning,” notes the National Fenestration Rating Council (NFRC). The best way to manage the amount of solar heat that makes it into a building is to block it before it gets there, and the simplest way to do that is to install windows with low solar heat gain, the council says.
Natural light has another side, too. If used skillfully, it can reduce the need for electric lighting (about 38 percent of total electricity use in a building) and the heat produced by electric lighting, cutting energy costs. In fact, the best thing you can do for energy efficiency, according to Sanders, is “tying dimmable lighting controls into a good daylighting and envelope design that allows you to harvest that free daylight and turn off the electric lights.”
The toolbox of glazing choices available for today’s buildings in both retrofits and new projects has grown large, thanks to a revolution in technology and quality. Consider: Once upon a time there was the lowly single-pane window. Now, to mention just a few of the options, facility managers can add one more pane of glass, or two, or even three more, and can put argon gas in between each pane to increase thermal efficiency.
Another option is toned or tinted glass to hold down glare and heat transfer, or perhaps a layer of window film to the inside of the glass. Window films are increasingly versatile — one new film redirects incoming daylight up to the ceiling and drives it as much as 40 feet into a building.
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