A window is an opening in an otherwise solid and opaque surface that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material. Windows are held in place by frames, which prevent them from collapsing in.
Windows of a brick building in Washington, DC
A stained glass panel depicting Biblical scenes at a historic church in Scotland
Definition and Types of Windows
A window is an opening in a wall that lets light and possibly air into the room and allows occupants to see out. Primitive windows were just holes. Later, they were covered with animal hide, cloth, or wood. Shutters that could be opened and closed came next. Over time, windows were built that both protected the inhabitants from the elements and transmitted light: mullioned glass windows, which joined multiple small pieces of glass with leading; paper windows; flattened pieces of translucent animal horn; and plates of thinly sliced marble. Mullioned glass windows were the windows of choice among the European well-to-do, whereas paper windows were economical and widely used in ancient China, Korea, and Japan. In England, glass became common in the windows of ordinary homes only in the early 17th century, whereas windows made up of panes of flattened animal horn were used as early as the 14th century in northern Britain. Modern-style floor-to-ceiling windows became possible only after the industrial glass-making process was perfected. Evidence of glass window panes in Italy dates back nearly 3000 years.
View from window in Fort Sam Houston, Texas
Woven bamboo window of the Joan tea house in Inuyama
Basic Window Types
The double-hung sash window is the traditional style of window in the US and in many other places that were formerly colonized by the UK. It has two parts (sashes) that overlap slightly and slide up and down inside the frame. The two parts are not necessarily the same size. Nowadays, most new double-hung sash windows use spring balances to support the sashes, but traditionally, counterweights held in boxes on either side of the window were used. These were and are attached to the sashes using pulleys of either braided cord or, later, purpose-made chain. Double-hung sash windows were traditionally often fitted with shutters. Sash windows may be fitted with simplex hinges, which allow the window to be locked into hinges on one side, while the rope on the other side is detached, allowing the window to be opened for escape or cleaning. In the single-hung sash window, one sash is movable (usually the bottom one), and the other is fixed. This form of sliding sash window is less expensive than the double-hung version.
The horizontal sliding sash window has two or more sashes that overlap slightly but slide horizontally within the frame. In the UK, these are sometimes called Yorkshire sash windows, presumably because of their traditional use in that region.
A casemet window has a hinged sash that swings in or out like a door. This type of window has either a side-hung, a top-hung, or (occasionally) a bottom-hung sash, or a it may have a combination of these types, sometimes with fixed panels on one or more sides of the sash. In the US these are usually opened using a crank, but in Europe they tend to use projection friction stays and espagnolette locking. Formerly, plain hinges were used with a casement stay.
A tilt-and-slide window is usually a door-sized window. The sash tilts inwards at the top and then slides horizontally behind the fixed pane. A tilt-and-turn window can either tilt inwards at the top or can open inwards from hinges on the side.
A transom window is a window situated above a door. For an exterior door, the transom window is often fixed. When used with an interior door, it might have hinges at the top or bottom, or on the sides, that allow the window to open. Before the advent of forced-air heating and cooling, transom windows were important for providing ventilation.
Windows for Daylighting
Windows positioned high in the walls or near the roof area of the structure are often fixed and are used mostly for daylighting—providing natural light for effective internal illumination during the day. A clerestory window, for example, is a vertical window usually set high in a wall and used for daylighting. Skylights, which serve the same function, are flat or sloped windows built into a roof structure that is out of reach. A roof window is a sloped window built into a roof structure that is within reach. A roof lantern is a multi-paned glass structure, resembling a small building, built on a roof to allow day or moon light into the interior. Roof lanterns sometimes include additional clerestories and may also be called cupolas.
Decorative Windows
There are a variety of windows that serve decorative purposes in addition to their functional ones. One common decorative window is the bay window, a multi-panel window in which the panels are set at different angles to create a protrusion from the wall line. Bay windows are commonly used in cold regions that get a lot of snowfall. The panels are set in three different directions to provide optimum viewing from the interior of the building. Another type of multi-panel window, the oriel, is most often seen in the typical Tudor-style house or monastery. An oriel window projects from the wall and does not extend to the ground. Oriel windows originated as a form of porch. They are often supported by brackets or corbels. A picture window is very large fixed window in a wall, typically without glazing bars, or glazed with only perfunctory glazing bars near the edge of the window. Picture windows are intended to provide an unimpeded view, as if framing a picture. A multi-lit, or divided-lite window is glazed with small panes of glass separated by wooden or lead "glazing bars," or "muntins," arranged in a decorative "glazing pattern" often dictated by the architectural style. Due to the historic unavailability of large panes of glass, multi-lit windows were the prevailing style of window until the beginning of the 20th century. They are still used today.
Insulated Window Frames
Windows can be a significant source of heat transfer. Different kinds of glazing and window frames can reduce thermal losses and gains.
Frames and sashes are traditionally made of wood, but metal, vinyl or PVC, and composites are also common. Their cost and availability may vary from country to country. Solid metal frames and sashes are poor insulators because metals conduct heat quickly. Vinyl frames are popular in Europe because they conduct heat poorly. However, vinyl frames are not as strong as metal, wood, or composite window frames.To compensate for this lack of strength, some vinyl frames are reinforced with metal. However, the metal reinforcement will reduce the thermal efficiency of the vinyl window frame. Wood is also a good insulator. Composite frames may combine materials to obtain the aesthetics of one material while achieving the functional benefits of another. Modern metal window parts typically consist of two surfaces separated by insulating spacer material.
Air infiltration and hence convective heat losses can be reduced by good window seals and attention to construction. Evacuated or argon-filled insulated glazing units are also dependent on meticulous frame construction to prevent entry of air and loss of efficiency.
Window Construction
Modern windows are usually glazed with one large sheet of glass per sash, while windows in the past were glazed with multiple panes separated by "glazing bars," or "muntins," due to the unavailability of large sheets of glass. Today, glazing bars tend to be decorative, separating windows into small panes of glass even though larger panes of glass are available. Glazing bars are typically wooden, but occasionally lead glazing bars soldered in place are used for more intricate glazing patterns. A beam over the top of a window is known as the lintel or transom.
Windows and the Sun
Sun incidence angle
Historically, windows are designed with surfaces parallel to vertical building walls. Such a design allows considerable solar light and heat penetration due to the most commonly occurring incidence of sun angles. In passive solar building design, extended eaves are typically used to control the amount of solar light and heat entering the windows.
An alternate method would be to calculate a more optimum angle for mounting windows; this calculation would account for summer sun load minimization given the actual latitude of the particular building. An example of a building in which this process has been implemented is the Dakin Building in Brisbane, California. Much of the fenestration in that building has been designed to reflect summer heat load and to assist in preventing summer interior over-illumination and glare.
Solar Window
Solar windows not only provide a clear view and illuminate rooms, but also use sunlight to efficiently help power the building.
External Links
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Window."
