Glazing

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Introduction

The term 'glazing' refers to the glass component of building's façade or internal surfaces. Historically, the installation of glazing was generally undertaken by a specialist glazier, but today it is possible to purchase an entire window which can be fitted by a general contractor. In older buildings, re-glazing can be done by specialist firms that restore rather than replace windows.

Whereas in the past windows were generally single glazed consisting of just a single layer of glass, today there are many different options for glazing. A substantial amount of heat is lost through the window, and so double and triple glazed units have been developed to provide more insulation thereby improving the energy efficiency of a building. Glass can also be tinted to reject sunlight, coated in a translucent film to increase energy efficiency, or be self-cleaning. It is most common for glazing to be clear glass; however, there are also many varied forms of etched, textured, frosted, stained or tinted glass for privacy or aesthetic purposes.

Insulated glass unit (IGU)

An insulated glass unit (IGU) combines multiple glass panes into a single unit system, for example a window. Double glazed units are made of two sheets of glass with a cavity between them that is generally 4-20mm which creates an insulating barrier. Triple glazed units have three sheets of glass. IGUs help combat heat loss, making a building warmer, quieter and cheaper to run. The sheets of glass are generally tempered (toughened) or laminated for safety. To keep the panes of glass apart, pane spacers are set around the inside edges. The more efficient windows use spacers that contain little-or-no metal, often known as 'warm edge' spacers.

The cavity between the panes can be a vacuum (the efficiency is dependent upon the quality of the sealing), or a heavy inert gas. This improves insulation and prevents condensation within the unit. Argon is the most commonly used gas, but Krypton or Xenon are more efficient and more expensive.

R-values are a measure of the thermal resistance of a material of a specific thickness, that is, its resistance to the transfer of heat across it. The higher the R-value of a material, the more effective it is as an insulator. Double-glazed units filled with air have an R-value of around R-2. Filling with Argon gas raises the value to R-3. Changing the glass to Low-E (see below), can raise the value to R-4. Triple glazed IGUs can have a value of R-5 or higher.

Types of glazing

There are many different types of glazing with different manufacturing processes, strength, energy efficiency, appearances and so on. The most common include:

Float glass

This is named after the modern process used to create large, thin, flat panels from molten glass which is floated onto a pool of molten metal such as tin. This process produces a very smooth sheet of glass with a highly consistent thickness.

Annealed glass

This is a piece of float glass that has been cooled in a slow and controlled manner. The internal stresses within the sheet of glass are reduced by this process making the resulting glass stronger and less likely to break than it would otherwise be. There can be safety concerns using annealed glass as it can break into large jagged shards.

Heat strengthened glass

This is made from a sheet of annealed glass reheated beyond its annealing point of around 1200 degrees Fahrenheit and then cooled slowly. Heat strengthened glass may be twice as strong as annealed glass, but may still need to be laminated for use in buildings.

Fully tempered glass

Tempering is the process by which annealed glass is heated in the same way as heat strengthened glass. The glass is cooled more rapidly which allows the internal portion of the glass to remain fluid for longer than the outer surfaces. This means that an equal amount of tensile and compressive stresses are formed across the glass which allows it to become in the order of four times as strong as annealed glass. This is used as a safety glass, as it shatters into small granular pieces rather than sharp shards, reducing the risk of injury.

Heat soaked tempered glass

This is used as a means of testing glass panes that are to be used in safety critical situations, such as a glass railing. Tempered glass panes are heated to a temperature of around 550 degrees Fahrenheit for a few hours. This causes any unstable nickel sulfide inclusions (imperfections that may cause spontaneous breakage of the pane) to expand disproportionately to the glass, making the glass break.

Laminated glass

Laminated glass is used where glazing must remain intact if it is broken, either for safety or security reasons. It is made by fusing two or more layers of glass with inter-layers of polyvinyl butyral (PVB) through the use of heat and pressure. If it is made using heat strengthened glass, the pane will break into large pieces but will be held in the frame by the PVB inter-layer. If it is made from tempered glass, the sheet may fall out of the frame but will mostly stay together due to the inter-layer.

Wired glass

This is most often used as a fire resistant glass because the wire holds the glass in place if high temperature causes it to break. The wire mesh is better at holding glass in place than the PVB films used in laminated glass.

Low-emissivity (Low-E) glass

The term 'low-e glass' is used to describe glass that has a coating added to one or more of its surfaces to reduce its emissivity so that it reflects, rather than absorbs, long-wave infra-red radiation.

In cooler climates this means that long-wave infra-red radiation that builds up inside a building is reflected by the glass back into the space, rather than being absorbed by the glass and then partially re-radiated to the outside. This reduces heat loss and so the need for artificial heating.

In hotter climates, a low-e coating means that long-wave infra-red radiation outside the building is reflected back out of the building, rather than being absorbed by the glass and then partially re-radiated to the inside. This reduces the heat build-up inside the building and so the need for cooling. In hotter climates, a low-e coating might be used in conjunction with solar-control glass to reduce the amount of short-wave solar radiation entering the building.

The two main types of low-e coating are tin and silver. Tin oxide is applied to the glass at high temperatures to create a very hard and durable low-e coating. Silver coating must be enclosed within the glazing unit so that oxidation doesn't cause the degradation of the silver over time.

Self-cleaning glass

A transparent coating can be applied to glass during the manufacturing which reacts with the sun's UV rays to break down dirt and grime which forms on the outside of the windows, and when it rains, the decomposed dirt natural rinses away. The coating has hydrophilic properties which mean it attracts water over its entire surface, avoiding unsightly uneven water marks.

Window Energy Ratings

The Window Energy Ratings (WER) is a scale developed by the British Fenestration Ratings Council (BFRC) to measure the thermal performance of windows. The BFRC label indicates the rating of the window on a scale running from A+ (the most energy efficient) to G (the least efficient). The overall performance is dependent on the combined efficiency of the frame, the glazing and the air tightness of the finished windows. Low-e windows generally get the highest performing energy ratings. Professional installers should be registered with a Competent Person Scheme or register the installation through Local Authority Building Control. English and Welsh competent person schemes are:

• Fenestration Self-Assessment Scheme (FENSA): They guarantee that its installers and frames comply with building regulations.
• British Standards Institution (BSI).
• Certass Glazing Scheme: They also register and approve installers.

Building regulations

Under the building regulations in England and Wales, new and replacement windows must meet certain energy efficiency requirements, as well as other requirements relating to safety, means of escape and ventilation.

Approved Document L classifies an external window as a 'controlled fitting', which refers to the whole unit including the frame. New and replacement windows in existing homes in England, Wales and Scotland must be at least WER band C or have a U-value of 1.6. However, the regulations state that:

'Replacing the glazing whilst retaining an existing frame is not providing a controlled fitting, and so such work is not notifiable and does not have to meet the Part L standards, although where practical it would be sensible to do so.'

Part K of the Building Regulations require that where 'building work' is carried out in a critical location involving glass, that safety glazing is used. Safety glazing is required:

• In any glazed area within a window below 800mm from floor level.
• In any glazed area within a window that is 300mm or less from a door and up to 1500mm from floor level.
• Within any glazed door up to 1500mm from floor level.

Find out more

Related articles on Designing Buildings Wiki

• BFRC window rating scheme.
• Emissivity.
• Glass.
• Low-E glass.
• Overheating.
• Preventing overheating.
• R-value.
• RB12, Rio de Janeiro.
• Rights to light.
• U-value.

External references

• The Green Age - What is double glazing?
• Planning Portal - Doors/windows
• Double Glazing Info - Choosing your windows
• The Glass and Glazing Federation