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Home Improvement

2003-05-01 12:22:00

How do they work?

ANSWER:  This answer may be a bit more technical than you asked for… but it should help tell you how everything works.The sun radiates enormous amounts of energy, a tiny portion of which reaches the earth in the form of electromagnetic radiation. The electromagnetic spectrum of solar energy is split into three bands by wavelength: the ultraviolet band, the visible band and the near infrared band. The ultraviolet band is the element most responsible for fading. The visible band is the only part of the solar spectrum actually seen with our eyes and is responsible for glare. The near infrared band is the heat element. We do not see it, but we sense it as heat.     Glass is increasingly used in American homes and offices. Clearly, there is a conflict between our desire for a spacious, airy interior incorporating large expanses of glass and the ravages wrought by the sun. Furthermore, there is an energy price to pay. Heat passes readily through glass - approximately 50 percent of a home's utility bills are wasted by loss of heat and heat gain through untreated glass.While you are researching your replacement windows you need to be aware of the U-factor, or U-value, which is the measure of a window's thermal efficiency, based on its resistance to the flow of energy. The lower the U-factor, the more thermally efficient the window. Most new window technology has been aimed at lowering U-factors through the use of three important innovations: warm-edge seals, low emissivity glass coatings and argon gas.     First, modern window systems are generally made with dual-pane glass that provides an air space between the two panes which greatly increases the window's insulation qualities.      Secondly, there is low emissivity glass, commonly referred to as Low-E, which is glass that has been coated with a micro-thin metallic film to improve thermal performance. Low-E is a measurement of a surface's ability to absorb heat. The lower the emissivity, the less heat is absorbed by the glass and the more room heat is reflected back into the home. The lower the emissivity rating, the better. For example, a window with an emissivity rating of 0.45 would reflect back 55 percent of the room heat that would be lost through untreated glass. An emissivity rating of 0.36 means 64 percent of the room heat would be retained.      Low-E glass reduces the transfer of heat (infrared) rays, virtually eliminates ultraviolet radiation (the main cause of fading), through the glass while still allowing light to pass through. In essence, the coating reflects heat back toward its source. Thus, heat from the furnace stays inside the home during cold weather, and heat from the sun stays outside in hot weather, resulting in greater comfort and lower energy costs. Low-E coatings also reduce harmful ultraviolet rays that cause drapes, furniture and art works to fade. This allows home owners to make optimum use of outdoor views afforded by modern spacious windows.     Lastly, some windows utilize argon gas which is a safe, odorless, colorless, non-toxic, non-flammable inert gas that is commonly used in place of air between the glass panes of an insulated Low-E glass unit. Argon's heat conductivity is lower than that of air, and thus it is a better insulator and contributes to a lower U-factor. Argon is not an effective solar performer by itself; that is why it is only used in conjunction with Low-E insulated glass.