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A sash window or hung sash window is made of one or more movable panels, or "sashes". The individual sashes are traditionally paned windows, but can now contain an individual sheet (or sheets, in the case of double glazing) of glass.
The sash window is often found in Georgian and Victorian houses, and the classic arrangement has three panes across by two up on each of two sash, giving a six over six panel window, although this is by no means a fixed rule. Innumerable late Victorian and Edwardian period houses were built in England using standard sash window units approximately 4 feet (1.2 m) in width, but older, hand-made units could be of any size.
The term "sash windows" is used interchangeably with the term "box sash windows" in the United Kingdom, and frequently used to describe the same thing. Historically box sash windows are heavier and more stately in nature than modern sash windows, but both terms are used within the industry when referring to the same type of window.
The name "hung sash window", which is more usual in the United States than in the United Kingdom, typically refers to a double-hung window with two sashes that can move up and down in the window frame. These windows are commonly found in older buildings in warmer climates, as they promote airflow and are easy to clean. A significant advantage of double-hung windows is that they provide efficient cooling of interiors during warm weather. Opening both the top and bottom of a sash window by equal amounts allows warm air at the top of the room to escape, thus drawing relatively cool air from outside into the room through the bottom opening. A double-hung window where the upper sash is smaller (shorter) than the lower is termed a cottage window.
A single-hung window has two sashes, but normally the top sash is fixed and only the bottom sash slides. Triple- and quadruple-hung windows are used for tall openings, common in New England churches. Construction is usually of softwood, and these sashes were traditionally only single-glazed.
Modern double-glazed sash windows are available. These may have true muntins or "glazing bars", or may imitate them by applying them to the surface of the glazing, giving the appearance of multiple small panes, whereas each sash consists of only one large double-glazed unit.
uPVC or vinyl sash windows have become available since the first PVC window frames were being introduced in Germany in the 1960s. They have come to dominate the window industry with more than 50% of all window sashes being made of vinyl in the US as of 2017. The waterproof nature of uPVC and ease of installation are also benefits. Problems with uPVC include durability and the environmental issues. Over time the plasticizer chemicals used to make vinyl sashes flexible evaporate leaving the material brittle and prone to breaking. Vinyl is also weaker than wood and aluminum sashes and requires extra support in large window openings. Vinyl is also used on wood windows as an outside cladding to protect the wood from the weather but allow the inside of the window to take advantage of the appearance of fine wood. Vinyl is also available in a large range of colours and textures although white is by far the most common. Vinyl is limited mainly to residential usage and low rise commercial due to its flammability and the toxic fumes it produces when burned. In the future fire rated versions of UPVC windows may be used in high rise construction, but currently flammability and toxic fumes are a significant obstacle.
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