A curtain wall is an exterior covering of a building in which the outer walls are non-structural, instead serving to protect the interior of the building from the elements). Curtain walls are thin and lightweight, usually made of aluminum and glass, and are designed to carry their own weight while transferring the load of wind and gravity to the structure of the building. They are air and water-resistant, making them energy-efficient and cost-effective for heating, cooling, and lighting the building. Curtain walls may be designed as "systems" integrating frame, wall panel, and weatherproofing materials). They can be installed in small or large units, depending on the needs and preferences of the project, and may span the distance of the floor to ceiling or be installed across multiple floors. Curtain walls can be installed with several glazing options, including glass, metal panels, stone veneer, louvres, and operable windows or vents). Unlike storefront systems, curtain wall systems are designed to span multiple floors, taking into consideration building sway and movement and design requirements such as thermal expansion and contraction, seismic requirements, water diversion, and thermal efficiency).