Various agencies monitor the quality of the glass in windows and how well it performs. One such agency is the non-profit Insulating Glass Certification Council, which tests the durability of insulating glass seals that window manufacturers use. The IGCC rates durability of glass by assigning it one of four categories: CBA, CB, C, or unrated. The CBA rating is the highest. If a window dealer has unrated windows, you are not getting the best grade of insulated glass.
Here is a primer on glass, which may be of particular interest if you desire to preserve a house’s historic qualities.
Sheet glass may still be found in houses built before 1940. It is recognizable by its wavy distortion and obvious imperfections. It was made by allowing a ribbon of molten glass to spread into a wide sheet before it cooled. This air-cooling technique gave the glass a bright sheen that is not seen on standard glass today.
Plate glass supplanted sheet glass and was made by rolling molten glass flat and then polishing it. This technique removed distortions and allowed these types of window to be made much thicker and stronger, which made them of particular value in large store windows.
Float glass is the most common type of glass, made by pouring molten glass over a bed of tin. The glass floats on top of the tin and cools with no distortions.
Tempered glass is highly resistant to breaking and when it does, it crumbles into thousands of tiny pieces with no sharp edges. It is made by reheating float glass and then cooling it quickly. Tempered glass is normally required within 3 feet of doors. It cannot be cut but instead must be made specifically for your needs.
Laminated glass is a safety glass, made by laminating clear plastic between two sheets of glass.
Electronic privacy glass is created from liquid crystals suspended between two panes of glass and connected to an electrical circuit. When the crystals are not electrified, they merge into random patterns that give the glass a frosted appearance. When a current passes through the crystals, they align themselves and the pane is clear.