Glazing, which derives from the Middle English for 'glass', is a part of a wall or window, made of glass. Glazing also describes the work done by a professional "glazier". Glazing is also less commonly used to describe the insertion of ophthalmic lenses into an eyeglass frame. Common types of glazing that are used in architectural applications include clear and tinted float glass, tempered glass, and laminated glass as well as a variety of coated glasses, all of which can be glazed singly or as double, or even triple, glazing units. Ordinary clear glass has a slight green tinge, but special colorless glasses are offered by several manufacturers. Glazing can be mounted on the surface of a window sash or door stile, usually made of wood, aluminium or PVC. The glass is fixed into a rabbet (rebate) in the frame in a number of ways including triangular glazing points, putty, etc. Toughened and laminated glass can be glazed by bolting panes directly to a metal framework by bolts passing through drilled holes. Glazing is commonly used in low temperature solar thermal collectors because it helps retain the collected heat.
History
The first recorded use of glazing in windows was by the Romans in the first century AD. This glass was rudimentary, essentially a blown cylinder that had been flattened out, and was not very transparent. In the eleventh century, techniques were developed where the glass was spun into a disc, creating a thinner circular window, or a cylinder was again formed, but this time it was cut from edge to edge and unrolled to make a rectangle-shaped window. The newer cylinder method remained the dominant method until the 19th century, and individual panes of glass were therefore limited in size to the dimensions of those cylinders. Continuous plate production was invented in 1848 by Henry Bessemer, who drew a ribbon of glass through rollers. This standardized the thickness of the glass, but its use in mass-production was limited by the need to polish both sides of the glass after manufacture, which was time-consuming and expensive. The process was slowly refined throughout the next century, with automated grinders and polishers being added to bring the cost down.
The breakthrough in large, mass-produced, continuous glass production happened in the 1950s with the development of the float glass manufacturing process. Molten glass is poured over a surface of molten tin, where it flattens out and can be drawn off in a ribbon. The advantage of this process is that it is scalable to any size and produces high quality panes without any further polishing or grinding. Float glass has continued to be the most used type of glazing to the present day.
Composition The most common glass used for glazing is Soda–lime glass, which has many advantages over other glass types. Silica (SiO2) makes up the bulk of the composition of this material at 70–75% by weight. Pure silica has a melting point that would be prohibitively expensive to reach with large-scale manufacturing, so sodium oxide (soda, Na2O) is added, which reduces the melting point. However, the sodium ions are water-soluble, which is not a desired property, so calcium oxide (lime, CaO) is added to reduce the solubility. The end result is a product which is high quality, clear, relatively cheap to produce, and recycles easily.
Role in energy conservation Approximately 25% to 30% of HVAC energy costs stem from heat gain and loss through the glazing in windows. Multiple methods have therefore been developed to minimize heat transfer through the glass. The glazing itself is a barrier to transfer via convection, so the two strategies for reducing heat transfer focus on minimizing conduction and radiation.
Double-paned windows
The strategy to reduce conduction is the use of Insulated glazing, where two or more panes of glass are used in series, each separated from each other by a space. Double-paned windows are the norm in new residential installations, as they offer substantial energy savings in comparison to single-paned glass. Each individual glass pane has poor insulation properties, with an R-value (insulation), or measure of an object's resistance to heat conduction, of 0.9. However, when two panes are placed in series with a gap between them, held in place and sealed by a spacer, the still gas in the gap acts as an insulator. The ideal gap size varies by location, but on average it ranges from 15–18 mm thick, giving a final assembly size of 23–26 mm assuming a typical glazing thickness of 4 mm. A double-paned window with air in the gap has an R-value of 2.1, which is much better than the 0.9 that a single pane of glass yields. A triple-paned window, which is not as popular but is used occasionally in environments with extreme temperatures, has an R-value of 3.2. While these values are much lower than those of walls, which have R-values starting at 12-15, the reduction in heat transfer is nevertheless substantial. Higher R-values still can be obtained by filling the gap with a less conductive gas such as argon (or less commonly, krypton or xenon). One final alternate method to reducing conduction is by creating and maintaining a vacuum in between the panes of glass, achieving a very high R-value of 10 while also greatly minimizing the required gap between the panes to 2 mm, yielding an assembly size as small as 10 mm. This technology was first launched commercially in 1996, and while several million units have been produced in the ensuing decades, it remains prohibitively expensive for most use cases and has yet to see widespread adoption.
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