Insulating glass (IG) consists of two or more glass window panes separated by a space to reduce heat transfer across a part of the building envelope. A window with insulating glass is commonly known as double glazing or a double-paned window, triple glazing or a triple-paned window, or quadruple glazing or a quadruple-paned window, depending upon how many panes of glass are used in its construction. Insulating glass units (IGUs) are typically manufactured with glass in thicknesses from 3 to 10 mm (1⁄8 to 3⁄8 in). Thicker glass is used in special applications. Laminated or tempered glass may also be used as part of the construction. Most units are produced with the same thickness of glass on both panes but special applications such as acoustic attenuation or security may require different thicknesses of glass to be incorporated in a unit. The space in between the panes provides the bulk of the insulation effect. It can be filled with air, but argon is often used as it gives far superior insulation, and sometimes other gases or a vacuum are employed.
History
Possibly the earliest use of double glazing was in Siberia, where it was observed by Henry Seebohm in 1877 as an established necessity in the Yeniseysk area where the bitterly cold winter temperatures regularly fall below −50 °C, indicating how the concept may have started:
One of the peculiarities of this part of the country is that it is a land of dear glass. You rarely see a window with square panes. In the houses of some of the poorer peasants it is not an uncommon thing to find one entirely composed of broken pieces of glass of all sizes and shapes, fitted together like a puzzle, and carefully sewn into a framework of birch bark which has been elaborately cut to fit each piece. Sometimes glass is dispensed with altogether, and pieces of semi-transparent fish-skin are stitched together and stretched across the window-frame. In winter double windows are absolutely necessary to prevent the inmates of the houses from being frozen to death. The outside windows project about six inches in front of the inside ones. If the inside window reveals the poverty of the inhabitants, the outside window seemingly displays his extravagance. To all appearances it is composed of one solid pane of plate-glass nearly three inches thick. On closer examination this extravagant sheet of plate-glass turns out to be a slab of ice carefully frozen into the framework with a mixture of snow and water in place of putty. Fitting a second pane of glass to improve insulation began in Scotland, Germany, and Switzerland in the 1870s. Insulating glass is an evolution from older technologies known as double-hung windows and storm windows. Traditional double-hung windows used a single pane of glass to separate the interior and exterior spaces.
In the summer, a window screen would be installed on the exterior over the double-hung window to keep out animals. In the winter, the screen was removed and replaced with a storm window, which created a two-layer separation between the interior and exterior spaces, increasing window insulation in cold winter months. To permit ventilation, the storm window may be hung from removable hinge loops and swung open using folding metal arms. No screening was usually possible with open storm windows, though in the winter, insects typically are not active. Traditional storm windows and screens are relatively time-consuming and labor-intensive, requiring removal and storage of the storm windows in the spring and reinstallation in the fall and storage of the screens. The weight of the large storm window frame and glass makes replacement on upper-stories of tall buildings a difficult task requiring repeatedly climbing a ladder with each window and trying to hold the window in place while securing retaining clips around the edges. However, current reproductions of these old-style storm windows can be made with detachable glass in the bottom pane that can be replaced with a detachable screen when desired. This eliminates the need for changing the entire storm window according to the seasons. Insulated glazing (IG) forms a very compact multi-layer sandwich of air and glass, which eliminates the need for storm windows. Screens may also be left installed year-round with insulated glazing, and they can be installed in a manner that permits installation and removal from inside the building, eliminating the requirement to climb up the exterior of the house to service the windows. It is possible to retrofit insulated glazing into traditional double-hung frames, though this would require significant modification to the wood frame due to the increased thickness of the IG assembly. Modern window units with IG typically completely replace the older double-hung unit and include other improvements such as better sealing between the upper and lower windows and spring-operated weight balancing that removes the need for large hanging weights inside the wall next to the windows, allowing for more insulation around the window and reducing air leakage. IG provides robust protection against the sun and keeps the house cool in the hot summer and warm in winter. The spring-operated balancing mechanisms also typically permit the top of the windows to swing inward, permitting cleaning of the exterior of the IG window from inside the building. The insulating glazing unit, consisting of two glass panes bound together into a single unit with a seal between the edges of the panes, was patented in the United States by Thomas Stetson in 1865. It was developed into a commercial product in the 1930s, when several patents were filed, and a product was announced by the Libbey-Owens-Ford Glass Company in 1944. Their product was sold under the Thermopane brand name, which had been registered as a trademark in 1941. The Thermopane technology differs significantly from contemporary IGUs. The two panes of glass were welded together by a glass seal, and the two panes were separated by less than the 0.5 inches (1.3 cm) typical of modern units. The brand name Thermopane has entered the vocabulary of the glazing industry as the genericized trademark for any IGU.
Construction
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