Almost all glass produced today in developed countries is done so by the “float process.” The glass composition materials, mainly silica sand, soda ash and limestone, are melted in a furnace and then flowed on to a bath of molten tin. The glass is formed and gradually cools as it moves from the tin bath to an annealing lehr, which is a controlled cooling chamber. As it moves through this process, the glass is in the form of a continuous ribbon, which is cut to size and packaged at the final stage.
All float glass, as it is initially made in the above description, is called annealed and is the most common. Float glass is made in a variety of colors or tints, in addition to basic clear glass. With annealed glass, the starting point can be further fabricated in many ways. Coatings of various types can be applied to achieve many visual effects and affect the optical properties. In addition, the glass can be heat-treated to increase its strength and give it safety glazing properties. Glass can be put into an insulating glass unit, meaning two or more pieces of glass are separated by a dry air space to improve the insulating properties.
Proper window design in the south must account for solar heat gain in order to help reduce air-conditioning. Therefore, glass products should have a low solar heat gain coefficient or low shading coefficient; the u-value, or insulating performance is of lesser importance.
These terms are mathematically related and both describe the solar energy blocked from passing through a glass material. The shading coefficient is the ratio of solar energy that passes through a piece of glass relative to piece of 1/8” clear glass (which has a shading coefficient of 1.0). Solar heat gain coefficient represents the solar gain through the glass relative to the incident solar radiation; it is equal to 86% of the shading coefficient. In either case, a lower number indicates improved solar control over the 1/8” clear glass baseline.
In colder, heating-dominated climates such as Canada or the northern United States, windows with higher shading coefficients generally are preferred and conserve energy due to the fact that in the longer heating season, more solar radiation, which becomes “free” heat, is allowed to pass into a home.
In the south, with a long air-conditioning season, it is most important to reduce solar gain and therefore reduce air conditioning loads.
Any heat source, such as furnaces or engines, which consume and combust fossil fuels, release long wave energy. Objects such as a sidewalk, road or windowsill which have been exposed to short wave solar radiation will also emit long wave infrared energy. Therefore, the sun is not responsible for long-wave infrared energy, instead it provides short-wave solar radiation.