Ultimate Daylighting
Along with new digital tools that allow the architect to evaluate heat loss and heat gain from all sides of a building, the architect has many choices for specifying the appropriate glass for a successful green project. According to Chris Dolan, director, Commercial Glass Marketing, Guardian Industries: “Architects may be interested in the many high-performance glazing products now available. There are new coating technologies that allow architects to help meet new energy codes, move toward net-zero requirements, and support LEED® and other stringent rating system requirements. It's important to consider climate zone, building orientation, and design intent for appearance when selecting the right glazing product.” A review of some of these new initiatives will assist designers as they design for transparency and energy efficiency. From technology to research, there are many innovations in window design.
Dynamic Glass: Sunglasses for Your Building
Electrochromic glass is another energy-efficient technology that incorporates low-voltage current into window glazing. This product allows a window to “track the sun,” changing the values of visible light transmittance (VLT) and solar heat gain coefficient (SHGC) throughout the day. The conductors can be switched on or off through sensors automatically or manually by building occupants. These windows provide built-in sunshades for buildings and allow occupants greater control of glare or too much daylighting during their workday.
This custom home in a remote town along the Pacific Coast in Mendocino County, California, designed by Mork-Ulnes Design, uses large, energy-efficient windows to frame views. Photography by Bruce Damonte |
Exploring Low-E Window Coatings and Vision Clarity
To meet the demand for a window that maximized transparency without causing increases in energy consumption and excessive heat gain, glass manufacturers introduced low-emissivity or low-E glass. Emissivity is the measure of the ability of a material to radiate energy. The lower the emissivity, the less heat is transferred through the glass. Low-E glass has a very thin coating applied to one or more surfaces of an insulated glass unit.
Not all low-E products are equal in performance and the manufacturing process—the type of coatings and the placement of coatings on the surfaces of a double or triple glass pane window can affect window performance. Two common low-E options are sputter coat, or soft coat glass, and pyrolytic, or hard coat glass. In comparisons of building performance by manufacturers between sputter- and pyrolytic-coated products, the results have shown that sputter-coated glass provides a combination of higher visible light transmission and lower solar heat gain. In addition, low-E glass prevents heat loss in cooler climates. Higher-performance windows result in better occupant comfort, a decrease in energy consumption, and a reduction of heating and cooling system loads.
Performance data for sputter- and pyrolytic-coated low-E windows. Chart courtsey of Guardian Industries |
Demanding Performance in Tall Buildings | ||
Skidmore Owings Merrill LLP (SOM) designed the Burj Khalifa in Dubai, which at 2,650 feet is the world’s tallest building. More than 1.8 million square feet of high-performance low-E glazing reduces the amount of glare from the strong desert sun, and provides substantial solar protection to keep the interior from overheating. Gregory Smith, associate director, SOM (Chicago office), and technical coordinator on this project, worked with the cladding contractor and glass manufacturer to choose high-performance windows. Because of the change in wind pressure at different levels of the building, the windows are engineered to have many different glass thicknesses in the structure. The glazing has to withstand extreme desert temperature swings as well as strong winds. To achieve a mirrored appearance, to protect against ultraviolet radiation, and to maximize the views from inside, the architects chose a silver reflective coating. To achieve high-performance energy efficiency the architect also specified a low-E coating. To maximize performance, the silver coating was placed on the number two surface of the insulating glass unit and the low-E coating on the third surface. When specifying coatings, it is important to understand the correct application of each type of coating as well as the surfaces of the inboard and outboard lites of an insulating glass unit. The number one surface is the glass on the outboard lite facing the outside of the building, number two is the glass surface on the outboard lite facing the air gap. Number three is the glass surface facing the air gap on the inboard lite and number four is the glass surface on the inboard lite facing the interior of a room. (See accompanying image.) For improved performance and better appearance, low-E coatings are typically applied to the number two surface. Certain low-E coatings can be placed on the number three surface to improve performance of insulated units with coated or tinted outboard lites. It is important to work with the glass manufacturer at an early stage in the process to assure correct specifications. Reflecting on this project, Smith comments: “There are definitely ongoing improvements in the coating technology.” As the project progressed, even higher performance glass values were achieved. Architects can achieve a window with a neutral appearance on the inside looking out because of these improvements. The visible light transmission for this project was 19 percent, shading coefficient 0.26, and solar heat gain coefficient 0.23—high-performance values for this tall building in the desert.
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Chart courtesy of Guardian Industries |
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