For example, the substructure of the outer layer of glass is normally composed of aluminum or steel cantilevers. The glazing is then fastened with a point-fixed system or top and bottom rails. An important criteria for the classification of glass facades is the way in which they divide the gap between outer and inner layers. The gap may be undivided, or it may be partitioned by walls, glazing, wings, or other design techniques. This determines the functional characteristics of the facade, such as its heat and sound insulation, as well as the means of fire protection used within the structure.
Undivided facades are simple systems without structural elements between the inner and outer skins.
Fire and smoke can also spread freely through the gap. In addition, air that accumulates at the top of the air space between the two layers is likely to get hot on sunny days. Openings in the outer skin at the top of the structure siphon out the warm air, while cooler replacement air is drawn from near the base of the building. Atria are used for various purposes, such as meeting areas or cafeterias. Offices may open onto the atria, creating a more inviting and temperate work space.