As the outer envelope of a building, the facade fulfills several essential tasks: it protects the interior from weather conditions, regulates heat and moisture exchange, and gives the structure its visual identity. Architecturally, the facade articulates the building volume through openings, materials, reliefs, and colors. A distinction is made between load-bearing and non-load-bearing facades, as well as ventilated and non-ventilated systems. Modern systems such as curtain walls of glass or metal allow great transparency and flexible design. The energy quality of the facade is governed by legal requirements. In historic preservation, facade conservation is of particular importance as it documents the historical character of a building.