Insulated glazing
In 1877, Henry Seebohm observed a practical necessity in the Yeniseysk area of Siberia. Winter temperatures there regularly fell below negative fifty degrees Celsius. Local builders had fitted a second pane of glass to improve insulation against the bitter cold. This observation suggests how the concept of double glazing may have started in human history. Fitting a second pane began in Scotland, Germany, and Switzerland during the 1870s. The technology evolved from older methods known as double-hung windows and storm windows. Traditional double-hung windows used a single pane of glass to separate interior and exterior spaces. In summer, a window screen was installed on the exterior over the double-hung window to keep out animals. In winter, the screen was removed and replaced with a storm window. This created a two-layer separation between the interior and exterior spaces. The process increased window insulation in cold winter months but required significant labor. Workers had to remove and store storm windows in spring and reinstall them in fall. Climbing ladders repeatedly to secure retaining clips around edges made upper-story replacement difficult. Thomas Stetson patented an insulating glazing unit consisting of two glass panes bound together into a single unit with a seal between the edges in 1865. It was developed into a commercial product in the 1930s when several patents were filed. 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 glass panes are separated by a component called a spacer. A spacer may be of the warm edge type and seals the gas space between them. The first spacers were made primarily of steel and aluminum. Manufacturers thought these materials provided more durability. Their lower price means that they remain common today. However, metal spacers conduct heat unless the metal is thermally improved. This undermines the ability of the insulated glass unit to reduce heat flow. It may also result in water or ice forming at the bottom of the sealed unit due to sharp temperature differences. To reduce heat transfer through the spacer, manufacturers make it out of less-conductive material such as structural foam. A spacer made of aluminum containing a highly structural thermal barrier reduces condensation on the glass surface. It improves insulation as measured by the overall U-value. Typically, spacers are filled with or contain desiccant to remove moisture trapped in the gas space during manufacturing. This lowers the dew point of the gas in that space. It prevents condensation from forming on surface number two when the outside glass pane temperature falls. New technology has emerged to combat heat loss from traditional spacer bars. Improvements include enhanced structural performance and long-term durability for both improved metal and foam spacers.
Insulating glass units are often manufactured on factory production lines on a made-to-order basis. Standard units are also available but require specific dimensions. The width and height dimensions must be supplied to the manufacturer along with glass thickness. On the assembly line, spacers of specific thicknesses are cut and assembled into required overall width and height dimensions. They are then filled with desiccant. On a parallel line, glass panes are cut to size and washed to be optically clear. An adhesive primary sealant known as polyisobutylene is applied to the face of the spacer on each side. Panes are pressed against the spacer. If the unit is gas-filled, two holes are drilled into the spacer of the assembled unit. Lines are attached to draw out air from the space. Air is replaced with desired gas or left as vacuum. The lines are removed and holes sealed to contain the gas. The more modern technique uses an online gas filler which eliminates the need to drill holes in the spacer. The purpose of primary sealant is to keep insulating gas from escaping and water vapor from entering. Units are enveloped on the edge side using polysulfide or silicone sealant as secondary sealant. This restraints movements of the rubbery-plastic primary sealant. The desiccant removes traces of humidity from the air space so no condensation appears on inside faces during cold weather.
The maximum insulating efficiency of a standard insulated glazing unit is determined by thickness of the space. Greater space increases insulation value up to a point. Eventually convection currents begin to flow carrying heat between panes within the unit. Typically most sealed units achieve maximum insulating values using a space measured at center of the unit. A standard IGU consisting of clear uncoated panes of glass with air in cavity typically has RSI-value of 0.35 K·m2/W. Using US customary units, each change in component results in increase of one R-value to efficiency. Adding argon gas increases efficiency to about R-3. Using low emissivity glass on surface number two adds another R-value. Properly designed triple-glazed IGUs with low emissivity coatings on surfaces two and four filled with argon gas result in higher ratings. Certain multi-chambered IG units reach R-values as high as R-24. Vacuum Insulating Glass units result in R-values as high as R-15 at center of glass. Combining VIG unit with another glass pane and warm edge spacer reaches R-18 or more depending upon coating. Double VIG units with warm edge spacer reach R-25 or more depending upon factors. Quadruple glazing is produced for cold environments such as Alaska or Scandinavia. Even quintuple and six-pane glazing is available with mid-pane insulation factors equivalent to walls.
In some situations insulation refers to noise mitigation rather than heat transfer. In these circumstances a large air space improves noise insulation quality or sound transmission class. Asymmetric double glazing uses different thicknesses of glass rather than conventional symmetrical systems. This improves acoustic attenuation properties of the insulated glazing unit. If standard air spaces are used, sulfur hexafluoride can replace or augment inert gas. It improves acoustical attenuation performance. The most widely used glazing configurations for sound dampening include laminated glass with varied thickness of interlayer. Including structural thermally improved aluminum thermal barrier air spacer reduces transmission of exterior noise sources. Reviewing glazing system components ensures overall sound transmission improvement. Some types of light include radio waves. Many low-e glass and semi-reflective metalized coatings greatly attenuate Wi-Fi and cell phone signals. Sulfur hexafluoride has only two-thirds conductivity of argon but is stable and dense. It is used particularly as sound proofing in Europe under F-Gas directive which controls usage.
The life of an IGU varies depending on quality of materials used and size of gap between panes. Temperature differences and workmanship affect longevity along with geographic location. IG units typically last from 10 to 25 years. Windows facing equator often last less than 12 years. IGUs typically carry warranty for 10 to 20 years depending upon manufacturer. If IGUs are altered such as installation of window insulation film the warranty may be voided by manufacturer. Condensation collects between layers when perimeter seal fails and desiccant becomes saturated. This can generally only be eliminated by replacing IGU. Seal failure results in significant factor in overall cost of owning IGUs. Large temperature differences stress spacer adhesives which eventually fail. Units with small gap between panes are more prone to failure due to increased stress. Atmospheric pressure changes combined with wet weather lead to gap filling with water in rare cases. Flexible sealing surfaces preventing infiltration around unit degrade or tear over time. Replacement of these seals can be difficult or impossible due to extruded channel frames without retention screws. In Canada since beginning of 1990 some companies offer servicing of failed IG units. They provide open ventilation to atmosphere by drilling holes in glass or spacer. This solution reverses visible condensation but cannot clean interior surface staining.
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Common questions
When did Henry Seebohm observe the practical necessity for double glazing in Siberia?
Henry Seebohm observed the practical necessity for double glazing in 1877. He noted that winter temperatures in the Yeniseysk area of Siberia regularly fell below negative fifty degrees Celsius. Local builders had fitted a second pane of glass to improve insulation against the bitter cold.
Who patented an insulating glazing unit consisting of two glass panes bound together into a single unit with a seal between the edges in 1865?
Thomas Stetson patented an insulating glazing unit consisting of two glass panes bound together into a single unit with a seal between the edges in 1865. The technology evolved from older methods known as double-hung windows and storm windows. It was developed into a commercial product in the 1930s when several patents were filed.
What year did the Libbey-Owens-Ford Glass Company announce their Thermopane brand name insulated glazing product?
The Libbey-Owens-Ford Glass Company announced their Thermopane brand name insulated glazing product in 1944. Their product was sold under the Thermopane brand name which had been registered as a trademark in 1941. This followed the development of the technology into a commercial product in the 1930s when several patents were filed.
How long do insulated glazing units typically last before requiring replacement due to seal failure?
Insulated glazing units typically last from 10 to 25 years depending on quality of materials used and size of gap between panes. Windows facing equator often last less than 12 years due to temperature differences and geographic location. IGUs typically carry warranty for 10 to 20 years depending upon manufacturer.
Which gas is used particularly as sound proofing in Europe under F-Gas directive which controls usage?
Sulfur hexafluoride is used particularly as sound proofing in Europe under F-Gas directive which controls usage. It has only two-thirds conductivity of argon but is stable and dense. Sulfur hexafluoride can replace or augment inert gas to improve acoustical attenuation performance.