Glass is used throughout buildings for its optical clarity, longevity and structural properties; it can offer unobstructed views out of a building, or if specified so, can provide natural light into rooms where privacy is required.
Glasses are amorphous, non-crystalline, solid materials and are typically brittle. The most common glass used in windows for many years is soda-lime glass, composed mainly of silica. There are also many modern thermoplastic materials used as glasses, such as polycarbonate and acrylics, which are lighter and possess different properties to common silica glasses.
In the majority of external envelope applications, glass is fitted as a Double Glazed Unit (DGU), or increasingly, as a Triple Glazed Unit (TGU), in order to provide improved resistance to the passage of heat, thus improving energy efficiency and comfort to building users. These sealed units of differing pane numbers are collectively known as Insulated Glass Units (IGU).
A typical hermetically sealed DGU mounted within a rooflight frame would comprise a toughened outer pane, argon filled cavity, and preferably a laminated inner pane; although under certain circumstances a toughened pane may be considered acceptable. The DGU assembly may look similar to the image shown right:-