A window is a transparent opening in a wall (or other 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.
Contents [hide]
1 Etymology
2 History
3 Types of windows
3.1 Double-hung sash window
3.2 Single-hung sash window
3.3 Horizontal sliding sash window
3.4 Casement window
3.4.1 Awning window
3.4.2 Hopper window
3.5 Tilt and slide window
3.5.1 Tilt and turn window
3.6 Transom window
3.7 Jalousie window
3.8 Clerestory window
3.9 Skylight
3.10 Roof window
3.11 Roof lantern
3.12 Bay window
3.13 Oriel window
3.14 Thermal window
3.15 Fixed window
3.16 Picture window
3.17 Multi-lit window
3.18 Emergency exit/egress window
3.19 Stained glass window
3.20 French window
4 Technical terms
5 Window construction
5.1 Grids or Muntins
5.2 Frame and sash construction
5.3 Glazing and filling
5.4 Other construction details
6 Windows and the sun
6.1 Sun incidence angle
6.2 Solar window
7 See also
8 References
9 External links
[edit] Etymology
Arab-style windows in JerusalemThe word Window originates from the Old Norse �vindauga�, from �vindr � wind� and �auga � eye�, i.e. "wind eye". In Norwegian Nynorsk and Icelandic the Old Norse form has survived to this day (in Icelandic only as a less used synonym to gluggi), in Swedish the word vind�ga remains as a term for a hole through the roof of a hut, and in the Danish language �vindue� and Norwegian Bokm�l �vindu�, the direct link to �eye� is lost, just like for 'window'. The Danish (but not the Bokm�l) word is pronounced fairly similar to window.
Window is first recorded in the early 13th century, and originally referred to an unglazed hole in a roof. Window replaced the Old English �eag�yrl�, which literally means �eye-hole,� and �eagduru� �eye-door�. Many Germanic languages however adopted the Latin word �fenestra� to describe a window with glass, such as standard Swedish �f�nster�, or German �Fenster�. The use of window in English is probably due to the Scandinavian influence on the English language by means of loanwords during the Viking Age. In English the word fenester was used as a parallel until the mid-1700s and fenestration is still used to describe the arrangement of windows within a fa�ade.
From Webster's 1828 Dictionary, Window, n. [ G. The vulgar pronunciation is windor, as if from the Welsh gwyntdor, wind-door.]
[edit] History
A half-glazed window of the 17th century from ScotlandPrimitive windows were just holes in a wall. Later, windows 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. The Romans were the first to use glass for windows. In Alexandria ca. 100 AD, cast glass windows, albeit with poor optical properties, began to appear. Mullioned glass windows were the windows of choice among 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[citation needed] 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.
Woven bamboo window in JapanIn the Sanghabhedavastu legend of the Buddhists, it is said that the Buddha Siddartha Gautama, who dates to 500 B.C., was viewed by his father through a grand window because he was unable to enter the synagoge (Sanskrit samsthagare).[citation needed]
[edit] Types of windows
[edit] Double-hung sash window
This sash window is the traditional style of window in the USA, and many other places that were formerly colonized by the UK, with 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 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.
[edit] Single-hung sash window
One sash is movable (usually the bottom one) and the other fixed. This is the earlier form of sliding sash window, and is also cheaper.
[edit] 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 county.
[edit] Casement window
Main article: Casement window
Casement windowA window with a hinged sash that swings in or out like a door comprising either a side-hung, top-hung (also called "awning window"; see below), or occasionally bottom-hung sash or a combination of these types, sometimes with fixed panels on one or more sides of the sash. In the USA 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. Handing applies to casement windows to determine direction of swing. The casement window is the dominant type now found in the UK and much of Europe.
[edit] Awning window
An awning window is a casement window that is hung horizontally, hinged on top, so that it swings outward like an awning.
[edit] Hopper window
A hopper window is a bottom hung casement window that opens similar to a draw bridge typically opening to the outside.
[edit] Tilt and slide window
A window (more usually a door-sized window) where the sash tilts inwards at the top and then slides horizontally behind the fixed pane.
[edit] Tilt and turn window
A window which can either tilt inwards at the top, or can open inwards hinged at the side.
[edit] Transom window
Jalousie or louvered window
Clerestory windowA window above a door; in an exterior door the transom window is often fixed, in an interior door it can open either by hinges at top or bottom, or rotate on hinges. It provided ventilation before forced air heating and cooling. A fan-shaped transom is known as a fanlight, especially in the British Isles.
[edit] Jalousie window
Also known as a louvered window, the jalousie window consists of parallel slats of glass or acrylic that open and close like a Venetian blind, usually using a crank or a lever. They are used extensively in tropical architecture. A jalousie door is a door with a jalousie window.
[edit] Clerestory window
Main article: Clerestory
A window set in a roof structure or high in a wall, used for daylighting.
[edit] Skylight
Main article: Daylighting
A flat or slope window used for daylighting, built into a roof structure that is out of reach.
[edit] Roof window
A sloped window used for daylighting, built into a roof structure that is within reach.
[edit] Roof lantern
Main article: Cupola
Bay windowA roof lantern is a multi-paned glass structure, resembling a small building, built on a roof for day or moon light. Sometimes includes an additional clerestory. May also be called a cupola.
[edit] Bay window
Main article: Bay window
A multi-panel window, with at least three panels set at different angles to create a protrusion from the wall line.
[edit] Oriel window
Main article: Oriel window
A window with many panels. It is most often seen in Tudor-style houses and monasteries. 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. Buildings in the Gothic Revival style often have oriel windows.
[edit] Thermal window
Main article: Thermal window
Thermal, or Diocletian, windows are large semicircular windows (or niches) which are usually divided into three lights (window compartments) by two vertical mullions. The central compartment is often wider than the two side lights on either side of it.
[edit] Fixed window
Church windowA window that cannot be opened, whose function is limited to allowing light to enter (Unlike an unfixed window, which can open and close). Clerestory windows are often fixed. Transom windows may be fixed or operable.
[edit] Picture window
A 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.
[edit] Multi-lit window
A window 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 at use. Due to the historic unavailability of large panes of glass, this was the prevailing style of window until the beginning of the twentieth century, and is traditionally still used today.
[edit] Emergency exit/egress window
A window big enough and low enough so that occupants can escape through the opening in an emergency, such as a fire. In the United States, exact specifications for emergency windows in bedrooms are given in many building codes. Vehicles, such as buses and aircraft, frequently have emergency exit windows as well.[1]
[edit] Stained glass window
Main article: stained glass
A window composed of pieces of colored glass, transparent or opaque, frequently portraying persons or scenes. Typically the glass in these windows is separated by lead glazing bars. Stained glass windows were popular in Victorian houses and some Wrightian houses, and are especially common in churches.
[edit] French window
A French window, also known as a French door is really a type of door, but one which has one or more panes of glass set into the whole length of the door, meaning it also functions as a window.
[edit] Technical terms
5-chamber plastic window profileIn insulated glass production, the term "lite", or "light", refers to a glass pane, several of which may be used to construct the final window product. For example, a sash unit, consisting of at least one sliding glass component, is typically composed of two lites, while a fixed window is composed of one lite. The terms "single-light", "double-light" etc refer to the number of these glass panes in a window.
The lites in a window sash are divided horizontally and vertically by narrow strips of wood or metal called muntins. More substantial load bearing or structural vertical dividers are called mullions, with the corresponding horizontal dividers referred to as transoms.
In the USA, the term replacement window means a framed window designed to slip inside the original window frame from the inside after the old sashes are removed. In Europe, however, it usually means a complete window including a replacement outer frame.
The USA term new construction window means a window with a nailing fin designed to be inserted into a rough opening from the outside before applying siding and inside trim. A nailing fin is a projection on the outer frame of the window in the same plane as the glazing, which overlaps the prepared opening, and can thus be 'nailed' into place.
In the UK and mainland Europe, windows in new-build houses are usually fixed with long screws into expanding plastic plugs in the brickwork. A gap of up to 13mm is left around all four sides, and filled with expanding polyurethane foam. This makes the window fixing weatherproof but allows for expansion due to heat.
A beam over the top of a window is known as the lintel or transom.
In the USA, the NRFC Window Label lists the following terms:
Thermal transmittance (U-factor). Best values are around U-0.15 (equal to 0.8 W/m2/K).
Solar heat gain coefficient (SHGC). (ratio of solar heat (infrared) passing through the glass to incident solar heat)
Visible transmittance (VT) (ratio of transmitted visible light divided by incident visible light)
Air leakage (in cubic foot per minute per linear foot of crack between sash and frame)
[edit] Window construction
Windows can be a significant source of heat transfer[2]. Insulated glazing units therefore consist of two or more panes to reduce the heat transfer.
[edit] Grids or Muntins
These are the decorative, usually wood pieces that separate a larger pane into smaller segments. The mutins are generally held to the window frame using a pin of some sort.
[edit] Frame and sash construction
Factory site in Berlin-Spindlersfeld: window with letter S for SpindlerFrames and sashes can be made of the following materials:
Material thermal resistance Durability Maintenance Cost Recycled content Comment
Wood very good variable high high low shrinks and swells with humidity changes
vinyl or PVC very good good* low low very low emits dioxins if burned[3]
Aluminum bad** superior very low low typically >95% used in most large structures
Fiberglass good very good* very low high medium
* Vinyl and fiberglass frames perform well in accelerated weathering tests. Because vinyl is not as strong as other materials, some vinyl frames are reinforced with metal or composite materials to improve their structural strength.
** Modern metal window frames are typically separated by a thermal break made of a non-conducting material. This greatly increases thermal resistance, while retaining virtually all of the structural strength.
Composites may combine materials to obtain aesthetics of one material with the functional benefits of another.
[edit] Glazing and filling
Examples of modern plastic and wooden window profiles with insulated glazingLow-emissivity coated panes reduce heat transfer by radiation, which, depending on which surface is coated, helps prevent heat loss (in cold climates) or heat gains (in warm climates).
High thermal resistance can be obtained by evacuating or filling the insulated glazing units with gases such as argon or krypton, which reduces conductive heat transfer due to their low thermal conductivity. Performance of such units depends on good window seals and meticulous frame construction to prevent entry of air and loss of efficiency.
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, generally in a pattern dictated by the architectural style at use. Glazing bars are typically wooden, but occasionally lead glazing bars soldered in place are used for more intricate glazing patterns.
[edit] Other construction details
Many windows have movable window coverings such as blinds or curtains to keep out light, provide additional insulation, or ensure privacy.
[edit] Windows and the sun
Main article: Daylighting
[edit] 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, an extended eave is typically used to control the amount of solar light and heat entering the window(s).
An alternate method would be to calculate a more optimum angle for mounting windows which accounts for summer sun load minimization, with consideration of the actual latitude of the particular building. An example where this process has been implemented is the Dakin Building, Brisbane, California; much of the fenestration has been designed to reflect summer heat load and assist in preventing summer interior over-illumination and glare, by designing window canting to achieve a near 45 degree angle.
[edit] Solar window
Main article: Photovoltaics
Solar windows not only provide a clear view and illuminate rooms, but also use sunlight to efficiently help generate electricity for the building.[4]
[edit] See also
Irving Wightman Colburn
Defenestration
Equip'baie
G-value
Insulated glazing
Porthole
Replacement Windows
Shop window
Trickle vent
Window blind
Window tax
Wood preservation
the concentric arcs that distort some of these panes indicate they are crown glass, possibly of the 16th century.Main article: Crown glass (window)
One of the earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and trimmed to form a rectangular window to fit into a frame.
At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain, hence the name "bullseye." Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diaper latticed windows was in part because three regular diamond-shaped panes could be conveniently cut from a piece of Crown glass, with minimum waste and with minimum distortion.
This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.
[edit] Cylinder glass
Main articles: Cylinder blown sheet and Machine drawn cylinder sheet
In this manufacturing process glass is blown into a cylindrical iron mould. The ends are cut off and a cut is made down the side of the cylinder. The cut cylinder is then placed in an oven where the cylinder unrolls into a flat glass sheet.
[edit] Drawn Sheet glass (Fourcault process)
The uneven surface of old glass is visible in the reflection on this window pane.Drawn Sheet glass (or Fourcault process) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continuously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.
[edit] Cast plate glass
Developed by James Hartley, 1848. The glass is taken from the furnace in large iron ladles, which are carried upon slings running on overhead rails; from the ladle the glass is thrown upon the cast-iron bed of a rolling-table; and is rolled into sheet by an iron roller, the process being similar to that employed in making plate-glass, but on a smaller scale. The sheet thus rolled is roughly trimmed while hot and soft, so as to remove those portions of glass which have been spoiled by immediate contact with the ladle, and the sheet, still soft, is pushed into the open mouth of an annealing tunnel or temperature-controlled oven called a lehr, down which it is carried by a system of rollers.
[edit] Polished plate glass
Main article: Polished plate
The polished plate glass process starts with sheet or rolled plate glass. This glass is dimensionally inaccurate and often created visual distortions. These rough panes were ground flat and then polished clear. This was a fairly expensive process.
Before the float process, mirrors were plate glass as sheet glass had visual distortions that were akin to those seen in amusement park or fun-fair mirrors.
[edit] Rolled plate (Figured) glass
Figure rolled glassThe elaborate patterns found on figured (or 'Cathedral') rolled-plate glass are produced in a similar fashion to the rolled plate glass process except that the plate is cast between two rollers, one of which carries a pattern. On occasions both rollers can carry a pattern. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.
The glass used for this purpose is typically whiter in colour than the clear glasses used for other applications.
This glass can be laminated or toughened depending on the depth of the pattern to produce a safety glass.
[edit] Float glass
Main article: Float glass
90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces.
A very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass. The tin side of glass is also softer and easier to scratch.
Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.
[edit] Prism Glass
Prism glass is architectural glass used around the turn of the century to provide lighting to underground spaces and areas that would otherwise be too difficult to light. Prism glass uses a unique convex lens design to help illuminate more than ordinary glass. Prism glass can sometimes be found on sidewalks and in this form is known as vault lighting.[2]
[edit] Glass block
Main article: Glass brick, also known as Glass block.
Glass brick, also known as glass block, is an architectural element made from glass used in areas where privacy or visual obscuration is desired while admitting light, such as underground parking garages, washrooms, and municipal swimming baths. Glass block was originally developed in the early 1900s to provide natural light in industrial factories.
[edit] Annealed glass
Annealed glass is glass without internal stresses caused by heat treatment, i.e., rapid cooling, or by toughening or heat strengthening. Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched. Float glass is annealed during the process of manufacture. However, most toughened glass is made from float glass that has been specially heat-treated.
Annealed glass breaks into large, jagged shards that can cause serious injury, and thus, the reason it is considered a hazard in architectural applications. Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools or domestic houses.
[edit] Laminated glass
Main article: Laminated glass
Broken tempered laminated glass "wet blanket effect"Laminated glass is manufactured by bonding two or more layers of glass together with layers of PVB, under heat and pressure, to create a single sheet of glass. When broken, the PVB interlayer keeps the layers of glass bonded and prevents it from breaking apart. The interlayer can also give the glass a higher sound insulation rating.
There are several types of laminated glasses manufactured using different types of glass and interlayers which produce different results when broken.
Laminated glass that is made up of annealed glass is normally used when safety is a concern, but tempering is not an option. Windshields are typically laminated glasses. When broken, the PVB layer prevents the glass from breaking apart creating a "spider web" cracking pattern.
Tempered laminated glass is designed to shatter into small pieces, preventing possible injury. When both pieces of glass are broken it produces a "wet blanket" effect and it will fall out of its opening.
Heat strengthened laminated glass is stronger than annealed, but not as strong as tempered. It is often used where security is a concern. It has a larger break pattern than tempered, but because it holds its shape (unlike the "wet blanket" effect of tempered laminated glass) it remains in the opening and can withstand more force for a longer period of time, making it much more difficult to get through.
[edit] Toughened glass (tempered glass)Main article: Toughened glass
Toughened glass in a vandalized phone booth in BritainToughened glass (also known as tempered glass) is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations. Using toughened glass on automobile windshields would be a problem when a small stone hits the windshield at speed, as it would shatter into small squares endangering the driver and passengers. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and door lites and vision lites adjacent to doors. Toughened glass is typically four to six times the strength of annealed glass.
[edit] Heat-strengthened glass
Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses.
[edit] Chemically strengthened glass
Main article: Chemically strengthened glass
Chemically strengthened glass is a type of glass that has increased strength. When broken it still shatters in long pointed splinters similar to float (annealed) glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. Chemically strengthened glass is typically six to eight times the strength of annealed glass.
The glass is chemically strengthened by submerging the glass in a bath containing a potassium salt (typically potassium nitrate) at 450 �C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution.
Unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength.
Chemically strengthened glass was used on some fighter aircraft canopies.
[edit] Low-emissivity glass
See Low-emissivity glass
[edit] Self-cleaning glass
A recent innovation is so-called self-cleaning glass, aimed at building, automotive and other technical applications. A nanometre-scale coating of titanium dioxide on the outer surface of glass introduces two mechanisms which lead to the self-cleaning property. The first is a photo-catalytic effect, in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface; the second is a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds.
[edit] Insulated glazing
Main article: insulated glazing
Insulated glazing, or double glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers. This type of glazing has functions of thermal insulation and noise reduction.
[edit] Evacuated glazing
Another recent innovation for insulated glazing is evacuated glass, which as yet is produced commercially only in Japan and China. The extreme thinness of evacuated glazing offers many new architectural possibilities, particularly in building conservation and historicist architecture, where evacuated glazing can replace traditional (much less energy-efficient) single glazing.
An evacuated glazing unit is made by sealing the edges of two glass sheets, typically by using a solder glass, and evacuating the space inside with a vacuum pump. The evacuated space between the two sheets can be very shallow and yet be a good insulator, yielding insulative window glass with nominal thicknesses as low as 6 mm overall. The reasons for this low thickness are deceptively complex, but the potential insulation is good essentially because there can be no convection or gaseous conduction in a vacuum.
Unfortunately, evacuated glazing does have some disadvantages; its manufacture is complicated and difficult. For example, a necessary stage in the manufacture of evacuated glazing is outgassing; that is, heating it to liberate any gases adsorbed on the inner surfaces, which could otherwise later escape and destroy the vacuum. This heating process currently means that evacuated glazing cannot be toughened or heat-strengthened. If an evacuated safety glass is required, the glass must be laminated. The high temperatures necessary for outgassing also tend to destroy the highly effective "soft" low-emissivity coatings that are often applied to one or both of the internal surfaces (i.e. the ones facing the air gap) of other forms of modern insulative glazing, in order to prevent loss of heat through infrared radiation. Slightly less effective "hard" coatings are still suitable for evacuated glazing, however.
Furthermore, because of the atmospheric pressure present on the outside of an evacuated glazing unit, its two glass sheets must somehow be held apart in order to prevent them flexing together and touching each other, which would defeat the object of evacuating the unit. The task of holding the panes apart is performed by a grid of spacers, which typically consist of small stainless steel discs that are placed around 20 mm apart. The spacers are small enough that they are visible only at very close distances, typically up to 1 m. However, the fact that the spacers will conduct some heat often leads in cold weather to the formation of temporary, grid-shaped patterns on the surface of an evacuated window, consisting either of small circles of interior condensation centred around the spacers, where the glass is slightly colder than average, or, when there is dew outside, small circles on the exterior face of the glass, in which the dew is absent because the spacers make the glass near them slightly warmer.
The conduction of heat between the panes, caused by the spacers, tends to limit evacuated glazing�s overall insulative effectiveness. Nevertheless, evacuated glazing is still as insulative as much thicker conventional double glazing and tends to be stronger, since the two constituent glass sheets are pressed together by the atmosphere, and hence react practically as one thick sheet to bending forces. Evacuated glazing also offers very good sound insulation in comparison with other popular types of window glazing.
[edit] Building code seismic requirements
The most current building code enforced in most jurisdictions in the United States is the 2006 International Building Code (IBC, 2006). The 2006 IBC references for the 2005 edition of the standard Minimum Design Loads for buildings and other Structures prepared by the American Society of Civil Engineers (ASCE, 2005) for its seismic provisions. ASCE 7-05 contains specific requirements for nonstructural components including requirements for architectural glass.
Contents [hide]
1 Etymology
2 History
3 Types of windows
3.1 Double-hung sash window
3.2 Single-hung sash window
3.3 Horizontal sliding sash window
3.4 Casement window
3.4.1 Awning window
3.4.2 Hopper window
3.5 Tilt and slide window
3.5.1 Tilt and turn window
3.6 Transom window
3.7 Jalousie window
3.8 Clerestory window
3.9 Skylight
3.10 Roof window
3.11 Roof lantern
3.12 Bay window
3.13 Oriel window
3.14 Thermal window
3.15 Fixed window
3.16 Picture window
3.17 Multi-lit window
3.18 Emergency exit/egress window
3.19 Stained glass window
3.20 French window
4 Technical terms
5 Window construction
5.1 Grids or Muntins
5.2 Frame and sash construction
5.3 Glazing and filling
5.4 Other construction details
6 Windows and the sun
6.1 Sun incidence angle
6.2 Solar window
7 See also
8 References
9 External links
[edit] Etymology
Arab-style windows in JerusalemThe word Window originates from the Old Norse �vindauga�, from �vindr � wind� and �auga � eye�, i.e. "wind eye". In Norwegian Nynorsk and Icelandic the Old Norse form has survived to this day (in Icelandic only as a less used synonym to gluggi), in Swedish the word vind�ga remains as a term for a hole through the roof of a hut, and in the Danish language �vindue� and Norwegian Bokm�l �vindu�, the direct link to �eye� is lost, just like for 'window'. The Danish (but not the Bokm�l) word is pronounced fairly similar to window.
Window is first recorded in the early 13th century, and originally referred to an unglazed hole in a roof. Window replaced the Old English �eag�yrl�, which literally means �eye-hole,� and �eagduru� �eye-door�. Many Germanic languages however adopted the Latin word �fenestra� to describe a window with glass, such as standard Swedish �f�nster�, or German �Fenster�. The use of window in English is probably due to the Scandinavian influence on the English language by means of loanwords during the Viking Age. In English the word fenester was used as a parallel until the mid-1700s and fenestration is still used to describe the arrangement of windows within a fa�ade.
From Webster's 1828 Dictionary, Window, n. [ G. The vulgar pronunciation is windor, as if from the Welsh gwyntdor, wind-door.]
[edit] History
A half-glazed window of the 17th century from ScotlandPrimitive windows were just holes in a wall. Later, windows 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. The Romans were the first to use glass for windows. In Alexandria ca. 100 AD, cast glass windows, albeit with poor optical properties, began to appear. Mullioned glass windows were the windows of choice among 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[citation needed] 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.
Woven bamboo window in JapanIn the Sanghabhedavastu legend of the Buddhists, it is said that the Buddha Siddartha Gautama, who dates to 500 B.C., was viewed by his father through a grand window because he was unable to enter the synagoge (Sanskrit samsthagare).[citation needed]
[edit] Types of windows
[edit] Double-hung sash window
This sash window is the traditional style of window in the USA, and many other places that were formerly colonized by the UK, with 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 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.
[edit] Single-hung sash window
One sash is movable (usually the bottom one) and the other fixed. This is the earlier form of sliding sash window, and is also cheaper.
[edit] 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 county.
[edit] Casement window
Main article: Casement window
Casement windowA window with a hinged sash that swings in or out like a door comprising either a side-hung, top-hung (also called "awning window"; see below), or occasionally bottom-hung sash or a combination of these types, sometimes with fixed panels on one or more sides of the sash. In the USA 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. Handing applies to casement windows to determine direction of swing. The casement window is the dominant type now found in the UK and much of Europe.
[edit] Awning window
An awning window is a casement window that is hung horizontally, hinged on top, so that it swings outward like an awning.
[edit] Hopper window
A hopper window is a bottom hung casement window that opens similar to a draw bridge typically opening to the outside.
[edit] Tilt and slide window
A window (more usually a door-sized window) where the sash tilts inwards at the top and then slides horizontally behind the fixed pane.
[edit] Tilt and turn window
A window which can either tilt inwards at the top, or can open inwards hinged at the side.
[edit] Transom window
Jalousie or louvered window
Clerestory windowA window above a door; in an exterior door the transom window is often fixed, in an interior door it can open either by hinges at top or bottom, or rotate on hinges. It provided ventilation before forced air heating and cooling. A fan-shaped transom is known as a fanlight, especially in the British Isles.
[edit] Jalousie window
Also known as a louvered window, the jalousie window consists of parallel slats of glass or acrylic that open and close like a Venetian blind, usually using a crank or a lever. They are used extensively in tropical architecture. A jalousie door is a door with a jalousie window.
[edit] Clerestory window
Main article: Clerestory
A window set in a roof structure or high in a wall, used for daylighting.
[edit] Skylight
Main article: Daylighting
A flat or slope window used for daylighting, built into a roof structure that is out of reach.
[edit] Roof window
A sloped window used for daylighting, built into a roof structure that is within reach.
[edit] Roof lantern
Main article: Cupola
Bay windowA roof lantern is a multi-paned glass structure, resembling a small building, built on a roof for day or moon light. Sometimes includes an additional clerestory. May also be called a cupola.
[edit] Bay window
Main article: Bay window
A multi-panel window, with at least three panels set at different angles to create a protrusion from the wall line.
[edit] Oriel window
Main article: Oriel window
A window with many panels. It is most often seen in Tudor-style houses and monasteries. 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. Buildings in the Gothic Revival style often have oriel windows.
[edit] Thermal window
Main article: Thermal window
Thermal, or Diocletian, windows are large semicircular windows (or niches) which are usually divided into three lights (window compartments) by two vertical mullions. The central compartment is often wider than the two side lights on either side of it.
[edit] Fixed window
Church windowA window that cannot be opened, whose function is limited to allowing light to enter (Unlike an unfixed window, which can open and close). Clerestory windows are often fixed. Transom windows may be fixed or operable.
[edit] Picture window
A 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.
[edit] Multi-lit window
A window 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 at use. Due to the historic unavailability of large panes of glass, this was the prevailing style of window until the beginning of the twentieth century, and is traditionally still used today.
[edit] Emergency exit/egress window
A window big enough and low enough so that occupants can escape through the opening in an emergency, such as a fire. In the United States, exact specifications for emergency windows in bedrooms are given in many building codes. Vehicles, such as buses and aircraft, frequently have emergency exit windows as well.[1]
[edit] Stained glass window
Main article: stained glass
A window composed of pieces of colored glass, transparent or opaque, frequently portraying persons or scenes. Typically the glass in these windows is separated by lead glazing bars. Stained glass windows were popular in Victorian houses and some Wrightian houses, and are especially common in churches.
[edit] French window
A French window, also known as a French door is really a type of door, but one which has one or more panes of glass set into the whole length of the door, meaning it also functions as a window.
[edit] Technical terms
5-chamber plastic window profileIn insulated glass production, the term "lite", or "light", refers to a glass pane, several of which may be used to construct the final window product. For example, a sash unit, consisting of at least one sliding glass component, is typically composed of two lites, while a fixed window is composed of one lite. The terms "single-light", "double-light" etc refer to the number of these glass panes in a window.
The lites in a window sash are divided horizontally and vertically by narrow strips of wood or metal called muntins. More substantial load bearing or structural vertical dividers are called mullions, with the corresponding horizontal dividers referred to as transoms.
In the USA, the term replacement window means a framed window designed to slip inside the original window frame from the inside after the old sashes are removed. In Europe, however, it usually means a complete window including a replacement outer frame.
The USA term new construction window means a window with a nailing fin designed to be inserted into a rough opening from the outside before applying siding and inside trim. A nailing fin is a projection on the outer frame of the window in the same plane as the glazing, which overlaps the prepared opening, and can thus be 'nailed' into place.
In the UK and mainland Europe, windows in new-build houses are usually fixed with long screws into expanding plastic plugs in the brickwork. A gap of up to 13mm is left around all four sides, and filled with expanding polyurethane foam. This makes the window fixing weatherproof but allows for expansion due to heat.
A beam over the top of a window is known as the lintel or transom.
In the USA, the NRFC Window Label lists the following terms:
Thermal transmittance (U-factor). Best values are around U-0.15 (equal to 0.8 W/m2/K).
Solar heat gain coefficient (SHGC). (ratio of solar heat (infrared) passing through the glass to incident solar heat)
Visible transmittance (VT) (ratio of transmitted visible light divided by incident visible light)
Air leakage (in cubic foot per minute per linear foot of crack between sash and frame)
[edit] Window construction
Windows can be a significant source of heat transfer[2]. Insulated glazing units therefore consist of two or more panes to reduce the heat transfer.
[edit] Grids or Muntins
These are the decorative, usually wood pieces that separate a larger pane into smaller segments. The mutins are generally held to the window frame using a pin of some sort.
[edit] Frame and sash construction
Factory site in Berlin-Spindlersfeld: window with letter S for SpindlerFrames and sashes can be made of the following materials:
Material thermal resistance Durability Maintenance Cost Recycled content Comment
Wood very good variable high high low shrinks and swells with humidity changes
vinyl or PVC very good good* low low very low emits dioxins if burned[3]
Aluminum bad** superior very low low typically >95% used in most large structures
Fiberglass good very good* very low high medium
* Vinyl and fiberglass frames perform well in accelerated weathering tests. Because vinyl is not as strong as other materials, some vinyl frames are reinforced with metal or composite materials to improve their structural strength.
** Modern metal window frames are typically separated by a thermal break made of a non-conducting material. This greatly increases thermal resistance, while retaining virtually all of the structural strength.
Composites may combine materials to obtain aesthetics of one material with the functional benefits of another.
[edit] Glazing and filling
Examples of modern plastic and wooden window profiles with insulated glazingLow-emissivity coated panes reduce heat transfer by radiation, which, depending on which surface is coated, helps prevent heat loss (in cold climates) or heat gains (in warm climates).
High thermal resistance can be obtained by evacuating or filling the insulated glazing units with gases such as argon or krypton, which reduces conductive heat transfer due to their low thermal conductivity. Performance of such units depends on good window seals and meticulous frame construction to prevent entry of air and loss of efficiency.
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, generally in a pattern dictated by the architectural style at use. Glazing bars are typically wooden, but occasionally lead glazing bars soldered in place are used for more intricate glazing patterns.
[edit] Other construction details
Many windows have movable window coverings such as blinds or curtains to keep out light, provide additional insulation, or ensure privacy.
[edit] Windows and the sun
Main article: Daylighting
[edit] 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, an extended eave is typically used to control the amount of solar light and heat entering the window(s).
An alternate method would be to calculate a more optimum angle for mounting windows which accounts for summer sun load minimization, with consideration of the actual latitude of the particular building. An example where this process has been implemented is the Dakin Building, Brisbane, California; much of the fenestration has been designed to reflect summer heat load and assist in preventing summer interior over-illumination and glare, by designing window canting to achieve a near 45 degree angle.
[edit] Solar window
Main article: Photovoltaics
Solar windows not only provide a clear view and illuminate rooms, but also use sunlight to efficiently help generate electricity for the building.[4]
[edit] See also
Irving Wightman Colburn
Defenestration
Equip'baie
G-value
Insulated glazing
Porthole
Replacement Windows
Shop window
Trickle vent
Window blind
Window tax
Wood preservation
the concentric arcs that distort some of these panes indicate they are crown glass, possibly of the 16th century.Main article: Crown glass (window)
One of the earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and trimmed to form a rectangular window to fit into a frame.
At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain, hence the name "bullseye." Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diaper latticed windows was in part because three regular diamond-shaped panes could be conveniently cut from a piece of Crown glass, with minimum waste and with minimum distortion.
This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.
[edit] Cylinder glass
Main articles: Cylinder blown sheet and Machine drawn cylinder sheet
In this manufacturing process glass is blown into a cylindrical iron mould. The ends are cut off and a cut is made down the side of the cylinder. The cut cylinder is then placed in an oven where the cylinder unrolls into a flat glass sheet.
[edit] Drawn Sheet glass (Fourcault process)
The uneven surface of old glass is visible in the reflection on this window pane.Drawn Sheet glass (or Fourcault process) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continuously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.
[edit] Cast plate glass
Developed by James Hartley, 1848. The glass is taken from the furnace in large iron ladles, which are carried upon slings running on overhead rails; from the ladle the glass is thrown upon the cast-iron bed of a rolling-table; and is rolled into sheet by an iron roller, the process being similar to that employed in making plate-glass, but on a smaller scale. The sheet thus rolled is roughly trimmed while hot and soft, so as to remove those portions of glass which have been spoiled by immediate contact with the ladle, and the sheet, still soft, is pushed into the open mouth of an annealing tunnel or temperature-controlled oven called a lehr, down which it is carried by a system of rollers.
[edit] Polished plate glass
Main article: Polished plate
The polished plate glass process starts with sheet or rolled plate glass. This glass is dimensionally inaccurate and often created visual distortions. These rough panes were ground flat and then polished clear. This was a fairly expensive process.
Before the float process, mirrors were plate glass as sheet glass had visual distortions that were akin to those seen in amusement park or fun-fair mirrors.
[edit] Rolled plate (Figured) glass
Figure rolled glassThe elaborate patterns found on figured (or 'Cathedral') rolled-plate glass are produced in a similar fashion to the rolled plate glass process except that the plate is cast between two rollers, one of which carries a pattern. On occasions both rollers can carry a pattern. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.
The glass used for this purpose is typically whiter in colour than the clear glasses used for other applications.
This glass can be laminated or toughened depending on the depth of the pattern to produce a safety glass.
[edit] Float glass
Main article: Float glass
90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces.
A very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass. The tin side of glass is also softer and easier to scratch.
Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.
[edit] Prism Glass
Prism glass is architectural glass used around the turn of the century to provide lighting to underground spaces and areas that would otherwise be too difficult to light. Prism glass uses a unique convex lens design to help illuminate more than ordinary glass. Prism glass can sometimes be found on sidewalks and in this form is known as vault lighting.[2]
[edit] Glass block
Main article: Glass brick, also known as Glass block.
Glass brick, also known as glass block, is an architectural element made from glass used in areas where privacy or visual obscuration is desired while admitting light, such as underground parking garages, washrooms, and municipal swimming baths. Glass block was originally developed in the early 1900s to provide natural light in industrial factories.
[edit] Annealed glass
Annealed glass is glass without internal stresses caused by heat treatment, i.e., rapid cooling, or by toughening or heat strengthening. Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched. Float glass is annealed during the process of manufacture. However, most toughened glass is made from float glass that has been specially heat-treated.
Annealed glass breaks into large, jagged shards that can cause serious injury, and thus, the reason it is considered a hazard in architectural applications. Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools or domestic houses.
[edit] Laminated glass
Main article: Laminated glass
Broken tempered laminated glass "wet blanket effect"Laminated glass is manufactured by bonding two or more layers of glass together with layers of PVB, under heat and pressure, to create a single sheet of glass. When broken, the PVB interlayer keeps the layers of glass bonded and prevents it from breaking apart. The interlayer can also give the glass a higher sound insulation rating.
There are several types of laminated glasses manufactured using different types of glass and interlayers which produce different results when broken.
Laminated glass that is made up of annealed glass is normally used when safety is a concern, but tempering is not an option. Windshields are typically laminated glasses. When broken, the PVB layer prevents the glass from breaking apart creating a "spider web" cracking pattern.
Tempered laminated glass is designed to shatter into small pieces, preventing possible injury. When both pieces of glass are broken it produces a "wet blanket" effect and it will fall out of its opening.
Heat strengthened laminated glass is stronger than annealed, but not as strong as tempered. It is often used where security is a concern. It has a larger break pattern than tempered, but because it holds its shape (unlike the "wet blanket" effect of tempered laminated glass) it remains in the opening and can withstand more force for a longer period of time, making it much more difficult to get through.
[edit] Toughened glass (tempered glass)Main article: Toughened glass
Toughened glass in a vandalized phone booth in BritainToughened glass (also known as tempered glass) is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations. Using toughened glass on automobile windshields would be a problem when a small stone hits the windshield at speed, as it would shatter into small squares endangering the driver and passengers. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and door lites and vision lites adjacent to doors. Toughened glass is typically four to six times the strength of annealed glass.
[edit] Heat-strengthened glass
Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses.
[edit] Chemically strengthened glass
Main article: Chemically strengthened glass
Chemically strengthened glass is a type of glass that has increased strength. When broken it still shatters in long pointed splinters similar to float (annealed) glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. Chemically strengthened glass is typically six to eight times the strength of annealed glass.
The glass is chemically strengthened by submerging the glass in a bath containing a potassium salt (typically potassium nitrate) at 450 �C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution.
Unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength.
Chemically strengthened glass was used on some fighter aircraft canopies.
[edit] Low-emissivity glass
See Low-emissivity glass
[edit] Self-cleaning glass
A recent innovation is so-called self-cleaning glass, aimed at building, automotive and other technical applications. A nanometre-scale coating of titanium dioxide on the outer surface of glass introduces two mechanisms which lead to the self-cleaning property. The first is a photo-catalytic effect, in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface; the second is a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds.
[edit] Insulated glazing
Main article: insulated glazing
Insulated glazing, or double glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers. This type of glazing has functions of thermal insulation and noise reduction.
[edit] Evacuated glazing
Another recent innovation for insulated glazing is evacuated glass, which as yet is produced commercially only in Japan and China. The extreme thinness of evacuated glazing offers many new architectural possibilities, particularly in building conservation and historicist architecture, where evacuated glazing can replace traditional (much less energy-efficient) single glazing.
An evacuated glazing unit is made by sealing the edges of two glass sheets, typically by using a solder glass, and evacuating the space inside with a vacuum pump. The evacuated space between the two sheets can be very shallow and yet be a good insulator, yielding insulative window glass with nominal thicknesses as low as 6 mm overall. The reasons for this low thickness are deceptively complex, but the potential insulation is good essentially because there can be no convection or gaseous conduction in a vacuum.
Unfortunately, evacuated glazing does have some disadvantages; its manufacture is complicated and difficult. For example, a necessary stage in the manufacture of evacuated glazing is outgassing; that is, heating it to liberate any gases adsorbed on the inner surfaces, which could otherwise later escape and destroy the vacuum. This heating process currently means that evacuated glazing cannot be toughened or heat-strengthened. If an evacuated safety glass is required, the glass must be laminated. The high temperatures necessary for outgassing also tend to destroy the highly effective "soft" low-emissivity coatings that are often applied to one or both of the internal surfaces (i.e. the ones facing the air gap) of other forms of modern insulative glazing, in order to prevent loss of heat through infrared radiation. Slightly less effective "hard" coatings are still suitable for evacuated glazing, however.
Furthermore, because of the atmospheric pressure present on the outside of an evacuated glazing unit, its two glass sheets must somehow be held apart in order to prevent them flexing together and touching each other, which would defeat the object of evacuating the unit. The task of holding the panes apart is performed by a grid of spacers, which typically consist of small stainless steel discs that are placed around 20 mm apart. The spacers are small enough that they are visible only at very close distances, typically up to 1 m. However, the fact that the spacers will conduct some heat often leads in cold weather to the formation of temporary, grid-shaped patterns on the surface of an evacuated window, consisting either of small circles of interior condensation centred around the spacers, where the glass is slightly colder than average, or, when there is dew outside, small circles on the exterior face of the glass, in which the dew is absent because the spacers make the glass near them slightly warmer.
The conduction of heat between the panes, caused by the spacers, tends to limit evacuated glazing�s overall insulative effectiveness. Nevertheless, evacuated glazing is still as insulative as much thicker conventional double glazing and tends to be stronger, since the two constituent glass sheets are pressed together by the atmosphere, and hence react practically as one thick sheet to bending forces. Evacuated glazing also offers very good sound insulation in comparison with other popular types of window glazing.
[edit] Building code seismic requirements
The most current building code enforced in most jurisdictions in the United States is the 2006 International Building Code (IBC, 2006). The 2006 IBC references for the 2005 edition of the standard Minimum Design Loads for buildings and other Structures prepared by the American Society of Civil Engineers (ASCE, 2005) for its seismic provisions. ASCE 7-05 contains specific requirements for nonstructural components including requirements for architectural glass.
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