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 . 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:
Fitting a second pane of glass to improve insulation began in Scotland, Germany, and Switzerland in the 1870s.History of Double Glazing, Retrieved 11 February 2022
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. See note 25. 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 typical of modern units. The brand name Thermopane has entered the vocabulary of the glazing industry as the genericized trademark for any IGU.
Construction
Glass Single pane glass is a very poor insulator (R-value of around 1, RSI below 0.2), so single panes provide very little insulation. Glass coatings are frequently employed such as partially reflective or colored coatings to reduce insolation, and coatings to reflect infrared.
Low emissivity glass (low E glass) is a commercially available option for IGU construction. Low E glass is made by applying a Low E coating to a pane of glass. These are generally metallic coatings, usually applied onto the second or third glass surfaces of the unit, that have the effect of reflecting infrared light, and blocking or attenuating portions of the ultraviolet and visible light spectra. This can significantly reduce the solar gain of the IGU, which impacts both the thermal performance (R-value) and the Solar Heat Gain Coefficient (SHGC). Two types of low E coatings are available: hard coatings and soft coatings. Hard coatings are produced using tin oxide that is applied when the glass is still hot, and is absorbed into the glass, and are hard wearing and usually cheaper. Soft coatings are vacuum-sputtered onto the glass surface and have higher performance but are easily oxidized and damaged, and thus have to be protected by an inert gas fill.Popular Science Aug 1990 Vol. 237, No. 2 ISSN 0161-7370 page 42
Spacer
The glass panes are separated by a "spacer". A spacer, which may be of the warm edge type, is the piece that separates the two panes of glass in an insulating glass system, and seals the gas space between them. The first spacers were made primarily of steel and aluminum, which manufacturers thought provided more durability, and their lower price means that they remain common.
However, metal spacers conduct heat (unless the metal is thermally improved), undermining the ability of the insulated glass unit (IGU) to reduce heat flow. It may also result in water or ice forming at the bottom of the sealed unit because of the sharp temperature difference between the window and surrounding air. To reduce heat transfer through the spacer and increase overall thermal performance, manufacturers may make the spacer out of a less-conductive material such as structural foam. A spacer made of aluminum that also contains a highly structural thermal barrier reduces condensation on the glass surface and improves insulation, as measured by the overall U-value. A spacer that reduces heat flow in glazing configurations may also have characteristics for sound dampening where external noise is an issue. Typically, spacers are filled with or contain desiccant to remove moisture trapped in the gas space during manufacturing, thereby lowering the dew point of the gas in that space, and preventing condensation from forming on surface #2 when the outside glass pane temperature falls. New technology has emerged to combat the heat loss from traditional spacer bars, including improvements to the structural performance and long-term-durability of improved metal (aluminum with a thermal barrier) and foam spacers.
Fill gas An established way to improve insulation performance is to replace air in the space with a lower thermal conductivity gas. Gas convective heat transfer is a function of viscosity and specific heat. Monatomic gases such as argon, krypton, and xenon are often used since (at normal temperatures) they do not carry heat in rotational modes, resulting in a lower heat capacity than poly-atomic gases. Argon has a thermal conductivity 67% that of air; krypton has about half the conductivity of argon. Argon comprises nearly 1% of the atmosphere and is industrially isolated at moderate cost, whereas krypton and xenon are only trace elements which are expensive to extract. All particular noble gases are non-toxic, clear, odorless, chemically inert, and readily available because of their widespread application in industry. Some manufacturers also offer sulfur hexafluoride as an insulating gas, particularly as sound proofing. It has only 2/3 the conductivity of argon, but is stable, inexpensive, and dense. However, sulfur hexafluoride is an extremely potent greenhouse gas. In Europe, falls under the F-Gas directive which controls and even bans its usage for various applications. Since 1 January 2006, is banned as a tracer gas and in all applications except high-voltage switchgear.
Practically speaking, the more effective a fill gas is at its optimum thickness, the thinner the optimum thickness is. For example, the optimum thickness for krypton is lower than for argon, and lower for argon than for air.ASHRAE Handbook, Volume 1, Fundamentals, 1993 However, since it is difficult to determine whether the gas in an IGU has become mixed with air at time of manufacture (or becomes mixed with air once installed), many designers prefer to use thicker gaps than would be optimum for the fill gas if it were pure. Argon is commonly used in insulated glazing as it is the most affordable. Krypton, which is considerably more expensive, is not generally used except to produce very thin double glazing units or extremely high performance triple-glazed units. Xenon has found very little application in IGUs because of cost.
Vacuum technology is also used in some non-transparent insulation products called vacuum insulated panels as well as vacuum glazed glass.
Manufacture IGUs are often manufactured on a made to order basis on factory production lines, but standard units are also available. The width and height dimensions, the thickness of the glass panes and the type of glass for each pane as well as the overall thickness of the unit must be supplied to the manufacturer. On the assembly line, spacers of specific thicknesses are cut and assembled into the required overall width and height dimensions and filled with desiccant. On a parallel line, glass panes are cut to size and washed to be optically clear.