Double Glazed Units

Property

Classification

Single Glazing/Monolithic

This term refers to the basic flat glass which is either annealed (un-tempered), tempered, cleared, opaque, tinted, coated, etc. and can be one or two component of laminated and double glazing unit.

Multi-glazing

Consisting of two or three parallel sheets of glass separated by a thin dehydrated air space maintain by an airtight glass seal between the sheets and located around their edges. Because of the seal and the dehydrated air, there is no condensation or accumulation of dirt within the air space. The outer sheet is usually a heat absorbing glass. Various other combination, which may include tempered and/ or laminated glass are used. Multi-glazing unit also reduces sound transmission, and known as insulating glass.

The u value for this type of glass with the air space ranges from 6.35 to 12.70mm is between 2.28 and 6.47 W/m² ºC, whilst for single glazing is around 2.8 W/m² ºC.

Heat absorbing glass (tinted/reflective)

This type of glass is a kind of glass which because of its special composition, excludes a high percentage of UV rays to which it is exposed, transmit less solar heat into building, and has less bleaching effort on coloured fabric. The advantage of using this glass in one can reduce the air conditioning loads and cost.

Various processes have been developed for the application of thin coatings to large areas of flat glass. Coating are applied to alter the performance of the basic float glass, i.e. light and radiant heat transmission, light and radiant heat reflection, and surface emissivity. Coating which have been developed to have high transparency but high reflectivity of long wave radiation is used to improve the thermal insulation properties of the treated glass. Some coatings provide distinctive colours, whilst others are neutral in appearance.

Two groups of coatings:

• Metallic coating/ Metallic alloy coating: applied by means of a vacuum deposition.
• Metallic oxide coating: can be applied in normal atmosphere.

Laminated glass

It is produced by bonding two glasses together with a plastic material or a resin. The interlayer, which is usually polyvinyl butyral (PVB) or fibreglass, can be either clear or tinted. The number of interlayer may varied from 1 layer to 6 layers. The bonding is achieved by heating the glass/ interlayer sandwich and applying pressure. Laminated glass can incorporate multiple layers of interlayer and glass plus other material such as polycarbonate to achieve specific performance characteristics. Toughened glasses can also be used as components of a laminated product.

Other than façade, laminated glass is also used as overhead glazing, as it can minimize the risk of dangerous broken glass fragments falling since it holds to the frame when it breaks. It is suitable for protection against vandalism, hampers, or break-in.

Toughened / Tempered glass

Toughened glass is usually produced by reheating a piece of annealed glass to a temperature of approximately 700 ºC at which point it begins to soften. The surfaces of the heated glass are then cooled rapidly.

As the strength of this glass is almost 4 to 5 times than annealed glass, it has a good impact performance and safety based on standard EN 12150 ” Glass in Buildings- thermally toughened safety glass” and BS 6206: 1982. When broken, it disintegrates into small fragments with dulled edges that are unlikely to cause serious injury.

One of disadvantage of this types of glass is that the component contains nickel sulphide which prone to expand or shrink on drastic temperature difference, resulting on sudden breakage.

Heat Strengthened glass

It is thermally treated, first to over 600ºC, then cooled down at a carefully controlled speed. This process imbues the surface of the glass with permanent compression stress and gives it its special characteristic.

It is thermally treated, first to over 600ºC, then cooled down at a carefully controlled speed. This process imbues the surface of the glass with permanent compression stress and gives it its special characteristic.

Figured glass

Mainly for decorative purposes or diffusing light. Includes surface modified such as acid etched glass and sand blasted glass.

Safety glass

Safety glass is defined as glass which must have passed an impact test (BS 6202: 1981and graded as per SS 341:2001). The critical location in internal and external walls, where safety glass required to be used is specified in BS 6262: Part 4.

• Bullet proof glass
• Wired glass (one kind of glass reinforcement which incorporate steel mesh into the glass body to ensure the strength and to have better fire resistance property)

• Laminated and tempered glasses are also fall under this category.

Component

A typical composition for architectural glass/ “flat glass” is as follows:

• Sand (silica) 72%
• Soda ash (sodium carbonate) 13%
• Limestone (calcium carbonate) 10%
• Dolomite (calcium magnesium carbonate) 4%
• Various other oxides such as alumina and magnesia, to improve the physical properties of the glass, including its resistance to atmospheric pollutant.
• For body tinted glass, metal oxide can be also be incorporated [2,3]

Density

Density of glass is 2.5 g/cm³ [7]

Sizes

Typical maximum sizes for toughened glass

Source: Doran, DK, Newnes Construction Material Pocket Book; Gutterworth- Heinemann Ltd, Oxford, 1994.

Sound Transmission

The sound transmission is normally determined by Weight Reduction Index (R) in dB.

As glass has a very low sound absorption coefficient, glass used in a single sheet or ordinary thickness is a poor sound insulator, and the differences in insulation value between the ordinary thicknesses are very small. The sound reduction of single glazing is from about 29 to 34 dB.

Double glazing system is able to improve the sound reduction to 52 dB, assumed there is no problem with the ventilation through the window. The types of glass used as sound insulator would be varied, depend on the function of the room and the individual property of the glass i.e. thickness (total*: 22-41 mm), treatment (coated, tinted, etc).

* Thickness includes the thickness of the 2 glasses + air gap.

However, the sound insulation performance cannot be determined from that of the glass element alone. The sound reduction index for a window can only be verified after testing the complete assembly.

Heat Transmission

Recent glass technology manages to improve the u-value.

Low emissivity is coating on glass which has the ability to reflect the long wave radiation. In air-filled cavities with uncoated surfaces, the long wave radiation exchange between glass surfaces is high, amounting to about 60% of the total heat exchange across the cavity. With one of the glass surfaces having a coating with emissivity less than 0.2 (compared with 0.88 for uncoated glass surface), the radiation exchange is reduced by approximately 75% and consequently the U-value is reduced.

Argon and Krypton are known as the ideal gas since it is an inert gas that does not react chemically and no smell. Although Krypton is more expensive, it is still preferred since it gives better u-value.

Light Transmission (Solar Radiation)

This may varies depending on the glass thickness, whether it is a tinted glass, coated or multiglazing unit with and without coating.

Source: Doran, DK, Newnes Construction Material Pocket Book; Gutterworth- Heinemann Ltd, Oxford, 1994.

Compressive Strength

620.55 to 1241.10 MPa [3]

Tensile Strength

Annealed glass: 40 MPa

Toughened glass: 120-200 MPa (depending on thickness, edgework, holes, notches, etc)

Elasticity

Glass is a perfectly elastic material. It does not exhibit permanent deformation until breakage, but it is fragile, and will break without warning if subjected to excessive stress.

E= 70 GPa [7]

Coefficient of Linear Expansion

The coefficient of linear expansion of glass is: 9 x 10^-6 m/mk [7]

UV Radiation

UV radiation can be virtually eliminated by the use of PVB laminated glass. This type of glass can transmit only about 0.4% UV, compared to 44% for the normal type of glass.