Glass is a transparent or translucent material made by melting sand into a homogeneous, moldable form. Almost all buildings use glass for windows, so a facade is only labeled as glass if the opaque parts of the surface (i.e. areas that are NOT windows) are also clad in glass.

The facade designation concerns glass over the non-window portions of the exterior. Ordinary window glazing alone is expressly insufficient, even when windows occupy much of the facade.

Glass cladding can cover an opaque backing while retaining glass as the exposed material. The transparency normally associated with glass therefore does not require those clad wall areas to function as windows.