Curtain walls are lightweight exterior cladding systems that attach to a building or structure, usually from floor to floor, to provide a façade. They modify the exterior appearance, but do not carry the dead load of the building or structure. Rather, curtain walls carry their own dead loads as well as the horizontal loads or wind loads that are incident upon them. Dead loads are constant loads upon a building or structure because of the weight of the supported structure itself. Wind loads or horizontal loads are the pressure on a building or structure from the wind blowing in any direction. By design then, curtain walls must resist shear, sliding, overturning, and uplifting. These wind loads are transferred to the building or structure through connections at the floors or columns. Most curtain walls are made with extruded aluminum members. By pushing or drawing a billet of aluminum through a die, curtain wall suppliers can create members with a fixed cross-sectional profile. A lightweight metal, aluminum offers good corrosion resistance and strength. Typically, the aluminum frame is filled with glass for architectural purposes and to admit daylight. Solar gain control parameters such as thermal comfort and visual comfort are more difficult to control when highly-glazed curtain walls are used. Other common fill materials for curtain walls include stone veneer, metal panels, louvers, and operable vents or windows. Older curtain walls may use steel structural members. Curtain walls differ from storefront systems in two important ways. First, curtain walls are designed to span multiple floors in office buildings, hospitals, hotels, and academic buildings. Second, curtain walls incorporate critical design requirements such as thermal expansion and contraction; building sway and movement; water diversion; and thermal efficiency parameters for cost-effective heating, cooling, and lighting. Thermal expansion is a change in volume because of temperature increase. Conversely, thermal contraction represents a change in volume because of a decrease in temperature. Building sway and building movement are caused by high winds and harmonic vibration. Curtain walls that divert water are also available. To control or reduce energy costs, thermal efficiency is an important consideration when selecting curtain walls.