A louver (American English) or louvre (British English) is a window blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain and direct sunshine. The angle of the slats may be adjustable, usually in blinds and windows, or fixed.
Louvers originated in the Middle Ages as lantern-like constructions in wood that were fitted on top of roof holes in large kitchens to allow ventilation while keeping out rain and snow. They were originally rather crude constructions consisting merely of a barrel. Later they evolved into more elaborate designs made of pottery, taking the shape of faces where the smoke and steam from cooking would pour out through the eyes and mouth, or into constructions that were more like modern louvers, with slats that could be opened or closed by pulling on a string.
Modern louvers are often made of aluminium, metal, wood, or glass. They may be opened and closed with a metal lever, pulleys, or through motorized operators.
Often used interchangeably by mistake, the key difference between louvers and jalousies is that louvers are fixed position. Jalousies are installed within a movable adjustable mechanism which positions all jalousies into any parallel position with respect to each other.
Louvers are rarely seen as primary design elements in the language of modern architecture, but rather simply a technical device. Louvers are part of the design of Demerara windows to help keep 18th and 19th century buildings cool in hot climates and block direct sunlight. Some modern louver systems serve to improve indoor daylighting. Fixed mirrored louver systems can limit glare and of redirect diffuse light. Such louvers may be integrated in between two panes of double glazing. In industrial facilities such as steel foundries and power plants, louvers are very common. They are utilized for natural ventilation and temperature control.
Louvers may be used as a type of flood opening, usually covered by one or more moving flaps. They are designed to allow floodwaters to enter and leave the building, equalizing hydrostatic pressure on the walls and mitigating structural damage due to flooding.
There are examples of architects who use louvres as part of the overall aesthetic effect of their buildings. The most well-known example is Finnish modernist architect Alvar Aalto who would create aesthetic effects in the facades of his buildings through the combination of different types and sizes of louvers, some fixed some moveable, and made mostly from wood (e.g., the various buildings of the Helsinki University of Technology). A second example, taking influence from Aalto, is the second-generation modernist architect Juha Leiviskä.