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Window Design and Construction
A window is an opening in the wall of a building designed to admit light and air. The admission of light is accomplished through the use of transparent or translucent panes; ventilation, by opening the sash to the outside air. The frame in which a window is set is a casing. It has a sill at the bottom, jambs at the sides, and a lintel as a straight, horizontal member at the top, or the head may be arched or pointed. The sash is the movable part of the window and is called a casement if it opens on hinges at the side. It is glazed with panes, or lights, usually of transparent glass, but the panes in church and cathedral windows are commonly of stained glass, and in buildings where a diffused light is preferred, or where visibility from the outside is undesirable, they may be frosted or tinted. The pane may completely fill the sash in which it is set, but several smaller ones are frequently used instead of a single sheet. They are separated by slender wooden, metal, or lead bars, the vertical divisions being called mullions, the horizontal ones transoms. These bars are sometimes less correctly referred to as muntins or muntings, a corruption of montant mullion. When the bars radiate from the center, the window is known as a wheel, or as a rose window if of stained glass, as in a church.
Rise of Metal in Window Construction
The development of new construction methods and materials has greatly increased the importance of windows in architecture. Where before 1945 modern window sashes and casings were made almost exclusively of wood, a growing proportion have since been constructed of aluminum, steel, and stainless steel. Use of comparatively narrow strips of these metals, in place of wider wooden members, has permitted an increase in the size of glass panes, thus increasing the size of the opening to light and air. Metal window sashes and casings were first tried in schools, hospitals, and other public buildings, but low maintenance costs, especially of aluminum, which requires no repainting because of resistance to rot and rust, soon made them popular for commercial and industrial structures and also for residential construction. The entire walls of some office and factory buildings are now made of glass, much of it double glazing to create a dead-air space between the twin panes and prevent loss of heat during cold weather.
Types of Windows
Windows in common use are of several types, depending upon the style of architecture and the taste or preference of the builder. Perhaps still the most popular is the double-hung window. It has two sashes that slide up and down with the aid of weights on cords or chains running over pulleys at the top and concealed in pockets inside the casing. More modern hardware includes spring clips that hold the sash in place by compression against the jamb. Casement windows are hinged vertically at the sides and usually open outward, manually or by means of a crank or handle operating a simple chain of gears, although they may be hinged to open inward instead. French windows reach to the floor, for which reason they are also called French doors. They usually open inward. Awning windows consist of three or more horizontal sashes hinged at the top so that they open outward, acting as awnings to keep out rain when open. They are usually controlled from the inside by means of a crank, some being arranged so that all sections open or close simultaneously; in others a delaying action allows the bottom unit to open part way before the upper sections move.
Hopper windows are hinged at the bottom and open inward. Bay windows are three or more windows, frequently double hung, with the outer pair at an angle to the wall from which they project outward. If they are curved to form an arc of a circle, they are called bow windows. Properly speaking, bay and bow windows appear only on the ground-floor level; on upper floors they are known as oriels. A dormer window is one placed at a right angle to a pitched roof and covered by a small pitched roof of its own. Fixed windows are those with a sash that does not open. Especially when large and opening on a view, they are known as picture windows and have become very popular in the design of modern homes. They are frequently flanked by narrow casement windows or others that will open to provide ventilation, but if the room is air conditioned, this is not important. A window wall, as the name implies, is an entire wall, sometimes of basic windows, sometimes of glass panels, some of which may be arranged to slide horizontally for entry to a terrace or patio; or the wall may be built of glass brick.
Sliding windows with wall space between sill and floor were originally designed for ranch-style and other modern houses where the accent is on the horizontal, but now they are considered compatible with many types of architecture. Jalousie, or louvered, windows consist of multiple horizontal glass slats, or louvers, fixed on pivots at the ends so they can be opened to any angle like Venetian blinds. They overlap slightly when closed. Somewhat wider slats are pivoted vertically and are classed as pivoted windows or solar shades. Some are controlled by sun-powered motors. Another style still found in modern buildings is the clerestory, a shallow window or row of windows installed at the top of a wall under the roof line.