Glass options available for aluminium windows and doors
To help you in choosing the right glass for your home or office, we provide you with the following which is an extract from the Glass & Glazing Handbook (Noel C. Stokes) and will assist customers in the terminology and features of glass.
Glass is a product as old as civilisation itself and, yet, it is so versatile that it can meet our needs in the buildings of today and, because it is being developed continuously, using new techniques, to meet our needs in the buildings of the future.
These days we can choose from glass that will minimise the risk of injury when broken, glass that resists penetration and provides security, and glass that can reduce the transmission of unwanted noise and significantly reduce the fading of furnishings caused by ultraviolet rays and heat from the sun. There is also a growing demand for glass that provides increased comfort and energy efficiency by controlling the entry of heat in summer and the loss of warmth in winter.
Some types of glass incorporate one of these benefits while others combine several, so the designer or consumer is faced with a range of technically complex choices before reaching a decision on the product to be specified.
More than 90% of the world’s flat glass is made by the float process. Molten glass is poured continuously from a furnace onto a large bed of molten tin. It floats on the tin, spreading out and seeking a controlled level in the same manner as water poured onto a smooth, flat surface. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off the bath.
Clear glass is produced by the float process, combining silica sand, soda, lime, potash and alkaline salts.
Tinted glass is also produced by the float process with small quantities of metal oxides added to the normal clear composition to create the desired colour. The three colours produced in Australia are green, grey and bronze.
Rolled glass is produced by pouring molten glass between rollers to produce the desired thickness. There are two types of rolled glass produced in Australia: patterned and wired.
Patterned glass is produced by a rolling process whereby one of the rollers carries a surface pattern that is impressed into the glass. The non-patterned surface remains smooth.
Wired glass is also a rolled product, produced by sandwiching a steel wire mesh between two separate ribbons of glass and passed between a pair of consolidating rollers which can also impress a pattern.
Primary glass products are further processed or modified to achieve different levels of performance, and these secondary processes make changes to basic glass to develop products which meet specific requirements.
Toughened (or tempered) glass is produced from ordinary annealed glass by thermal toughening. Thermal toughening involves the use of heat. The glass is cut to the required size and shape, and all the processing, such as edge polishing, drilling holes, and the like is then carried out. The glass is then placed in a furnace and heated to a temperature above its annealing point (approximately 600 degrees). It is then cooled rapidly in a forced draught of air. This induces compressive stresses in the glass surface, balanced by tensile stresses in the centre. Thermally toughened glass cannot be broken unless sufficient force is applied, which will first overcome the compression in the surface and then introduce enough tension for breakage. Toughened glass is approximately four times stronger than annealed glass and should it be broken, it will break safely into very small particles which have dulled edges. Toughened glass will also withstand temperature differences of up to approximately 280 degrees Celsius within one panel, as opposed to 50-60 degrees Celsius for annealed glass.
Laminated glass is produced in many combinations of clear, toned and reflective glass consisting of two sheets bonded together by an opaque plastic interlayer, usually polyvinyl butyral (PVB). The glass and PVB sandwich is passed through an oven which raises the temperature to approximately 70 degrees Celsius. It is then passed through a set of rollers, a process that eliminates excess air and foms the initial bond. The laminate is placed in an autoclave which applies heat and pressure and when removed from the autoclave, the laminate is permanently bonded together.
The performance and benefits of laminated glass can be summarised under the following four categories:
- Solar Control
- Sound Control
If someone or something strikes laminated glass with sufficient energy to break it, the energy is dissipated by the resilient effect of the interlayer. The visco-elastic properties of the PVB act to cushion the glass and absorb impact energy. If the glass is broken, the fragments, which form a web-like pattern tend to ‘give’ and not fall out by being retained in place by the interlayer to provide a barrier against weather or future impact.
The safety characteristics of laminated glass are as follows:
- To minimise injury from contact with broken glass
- To minimise injury from falling pieces of broken glass
- To minimise injury from flying pieces of glass
- To minimise injury by falling through the glass
NOISE REDUCING GLASS
Glazing and windows play a critical role in the insulation of buildings against noise. Windows are frequently highlighted as the most vulnerable element in a building’s ability to resist noise intrusion due to the difference in mass of the window glass relative to the other building materials such as concrete or brick walls. By making an informed choice, an appropiate glass product, which is capable of providing substantial insulation against the transmission of noise can be selected. If some of the basic fundamentals relating to glass and sound are understood, it becomes possible to choose windows that will significantly reduce the transmission of excess noise. Selection of thicker glass, laminated glass or double glazing, each of which is capable of reducing noise to some extent, will improve noise insulation of a window.
Around the Window
Before considering the specific noise reducing properties of the various glass products available, it is necessary to realise that the best noise-reducing glass will not be effective when there are gaps or cracks around the frame and sash through which sound can travel. Naturally the construction type of the wall itself can negate any benefit from the properties of the windows and glass. A timber frame with light weight cladding and gyprock will let levels of noise pass through that a cavity brick wall will not, irrespective of the window the wall may include.
Generally speaking, the thicker the glass the better, but the insulation performance depends very much upon the frequency of the noise source. The insulation performance of the glass at low frequencies is of distinct significance as most road traffic noise is at a low frequency. So that increasing the mass by increasing the glass thickness of single glazing has value.
The PVB interlayer in laminated glass provides some internal damping of energy and this has the effect of moderating the loss of noise insulation at the coincidence frequency. As road traffic noise is predominantly low frequency, laminated glass is only marginally better than glass of the same thickness. Laminated glass does, however, also provide improvement in the high frequency range. This makes laminated glass a preferred product for reducing noises such as the whine of jet engines and sharp speech sound.
One of the most common misconceptions relating to glass and noise reduction is the belief that double-glazed glass is the most effective of all products. This can be true but not if the double glazing is of the type designed for thermal efficiency, i.e. two pieces 3, 4 or 5mm thick glass separated by airspaces of 6, 8 or 12mm. A unit of this type is designed to reduce heat loss through windows and will only reduce noise by perhaps 3 or 4 dB; a level of reduction that is noticeable but hardly sufficient to justify the cost. Double glazing of this type is designed for thermal efficiency with some noise reduction as a secondary or lesser benefit. However if noise is the major concern, far greater improvement can be gained for similar or even less cost by using thicker single glass or laminated glass. For double glazing to be effective in noise reduction, it needs to consist of thick glass and/or a wide cavity.
THERMAL EFFICIENT GLASS
With increasing emphasis on thermally efficient dwellings it is now possible to specify glass that cannot only enhance the appearance of your residence but reduce heating and cooling costs. Generally, clear glass is a relatively poor barrier to solar energy and not a good thermal insulator. It is however an excellent transmitter of visible light. When choosing a glass it is necessary to be aware of the multitude of options available but most importantly recognise that all glass types involve a trade off between their functions. For example, it is possible to select a glass that dramatically limits the amount of solar heat transmitted through the glass but the reflective nature of this same glass gives the appearance of a mirror. Likewise, a glass that limits solar heat gain in the summer (when it is required) will also do the same in the winter (when it is not required). The overall energy performance is not only dependant on the glass but also the overall building & design, use of eaves, overhangs, awnings, blinds, drapes and curtains. There have been many recent improvements in thermally efficient glass offered to the market. A BASIX’s statement form from your designer should specify the parameters of the glass required. We suggest a visit to our showroom where we have on display a large selection of Thermal Efficient Glass and can discuss the characteristics best suited to your project.