Windows might not show it, but they have become advanced piece of equipment. Low-end windows still use single-pane glass but provide little insulation. Double-pane windows were invented in 1935, and they will greatly improve insulation of your house. However, "double-pane window" is a generic term that masks the complexity of the offering.
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The main advantage of the double-pane windows is better insulation. That insulation is not a result of the thickness of the glass but a result of the thickness of the resulting sandwich of materials. Double-pane windows usually use an airtight spacing between the sheets of glass of one-half inch to 1 inch. Depending on the double-pane window you choose, it will be between two and 10 times more insulated than a single-pane window.
Simple double-pane windows use air between the two sheets of glass; higher-end windows use noble gas such as argon or krypton in place of air. The main disadvantage of air as a filler for that space is that when temperature differences are high, condensation can occur. Noble gases do not contain water in gaseous form, and, therefore, condensation is not a problem. Air also conducts energy better than both argon and xenon, reducing the efficiency of the window. Of the noble gas used in window making, argon is the most common and the cheapest, but is also the gas providing the least improvement over air. Krypton and xenon follow in that order for efficiency and price.
The glass used in double-pane windows is also specialized. Low-emissivity glass is a type of glass specifically designed to reflect heat in one direction. If you are in a hot climate, you will want to have the low-e glass reflect the heat out of your home. If you are in cold climate, you will want the glass to reflect heat inside your home.
Frame and Spacer
The frame and the spacer materials also will make a difference in window efficiency. The spacer is the element of the windows that holds the two panes of glass and that provides the airtight environment between those two panes. The low-end offering usually uses aluminum, which is both cheap and, unfortunately, a very good heat conductor. Because of it, condensation can form on the inside edge of the window. Higher-end windows use different forms of plastic. That spacer is less prone to heat and cold conduction and prevents condensation.
Beyond Double Pane
Double pane is an old technology, and in recent years several manufacturers have tried to improve the technology. Triple-pane windows, with three-pane and two noble gas pockets, provide even more energy efficiency. The relatively new heat-mirror film can also be used to provide better efficiency to windows. Regular double- or triple-pane windows usually have an R value (amount of energy that can pass through a material--the higher the value the better insulation is) of 3 to 5, which is good for a window. Windows with the heat-mirror can provide R value of up to 12, equal to the R value of a regular wall with bat insulation.