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The Science of Window Insulation
While values that apply to replacement windows can give you an idea of their performance under certain conditions and temperatures, there is more to insulation in windows than simply heat coming in and going out. The movement of heat through solid materials like vinyl as well as glass can be classified in three ways: convection, conduction, and radiation.
Conduction is the movement of heat through solid materials. When you touch a metal object to another metal object that is hot, the first object also becomes heated. The molecules within an object become excited and move around more. When solid objects touch, the molecules of one excite the molecules of another. Some substances are more or less likely to be good conductors of heat, which is why vinyl can control heat entering your home better than substances such as aluminum can. However, glass can also play a role in preventing heat conduction. Argon or krypton filled panes resist the excitement of molecules and slow down the movement of hot particles into your house. Were those inert gases not in place, warm particles would have moved freely through.
Convection is the transmission of warmth through the movement of individual particles from a warmer area to a colder one. Unlike conduction, which transfers energy to other particles and warms them, convection is the actual movement of warm matter to another area. When cold air touches a pane of glass that is not filled with a noble gas such as argon, it cools the glass which in turn cools warm air directly opposite it inside your home. As any person can tell you, cold air sinks and warm air rises. The newly cooled air sinks to the floor of your house, forcing warm air up into its former place at the window. This convective cycle leads to extreme cooling of a room without properly insulate replacement windows.
Light that we can see as colors and brightness is simply one portion of the Electro-Magnetic scale, and is also the only one that human beings can see. X-Rays, Ultra-Violet rays and radio waves also exist on this scale. One other type of electro-magnetic energy is Infrared. Emitted from objects that are hot, Infrared rays radiate from the sun constantly. It is this emission that causes a room or car to be warmed after being in direct sunlight while being insulated to outside temperatures. Infrared energy has a longer wavelength or wave and contains less energy per wave than does visible light. Thus, it is possible for glass coatings such as Low-E to reflect the long wave of heat radiation but to allow the shorter wave of light to enter a window.