![]() We wish to block solar heat (SHGC) as much as possible, which usually contradicts the high-visible transmittance requirement of the item (2). In hot climates, the third item is the opposite. In cold climates, there are three functions desired for windows: (1) prevent heat loss (which requires blocking IR spectra), (2) inspire good human feelings, which requires transparency to visible light with friendly colors, and (3) let as much solar heat as possible into the room, which requires transparency to whole solar spectrum up to 2500 nm. Second, let us explain how low-E coating could save energy, which is discussed in two climate conditions: It is an overall coefficient of heat transfer, the lower the U-factor, the better the insulating properties of the windows. U-factor is a measure of air-to-air heat transmission (loss or gain) in indoor and outdoor temperatures of a 1-m high glazing due to the thermal conductance and the difference. Solar heat gain coefficient (SHGC) is the percentage of the solar energy passing through the window over the incident solar energy (including direct solar transmittance and indirect reradiation). Visible light transmittance (Tvis) is the transmitted percentage of visible light (380–780 nm) through glass, (or insulating glass unit, IGU). Thus, energy saving windows are very popular in developed countries. ![]() were double-glazed windows, and 50% have a low-E coating. Modern coating technologies provide architectural window coatings with adequate illumination levels in building interiors, while controlling energy transfer to save either cooling or heating energy. ![]() A National Academy of Sciences study concluded that, “by an order of magnitude, the largest apparent benefits were realized as avoided energy costs in the buildings sector in energy efficiency”. Efficiency upgrades that improve the energy efficiency of windows are among the most promising and cost-effective energy technology options available now. Windows are often considered the least energy-efficient component in a building. “Building heating, ventilation, and air conditioning (HVAC) accounted for 14% of primary energy consumption in the United States in 2013”. The growing awareness of global warming has intensified efforts to make buildings and vehicles more energy efficient.
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