Many might think you only have to choose between single-pane, double-pane, or triple-pane glass when deciding which window style is right for your needs. But there are even more choices with focus on energy efficiency and high performance as well as aesthetics. Review this information to learn how different Low-E coatings, panes, and other enhancements affect the performance of your windows. Glazing is the main part of the total surface of a window, constituting averagely around 70% to 80% of its surface. Thus, its effect on energy efficiency, noise reduction and security is huge.
Basic natural properties of glass
Density: 88 χ 10-7m/°C
Linear expansion coefficient: 88 χ 10-7m/°C
Thermal permeability: (u-value) 5,2 – 5,8 watt/m2*°C (depending on thickness)
Thermal conductivity: (Κ-value) 1,05 watt/m*°C
Resistance to temperature differences: a 6mm glass that has been artificially heated will break from thermal shock if immersed in water with a temperature difference above 55 ° C.
Resistance to compression: 1000 MPa. It is extremely high and means that to crush a 1cm3 glass volume cube a weight of 10 tons must be applied.
Tensile strength: 40 MPa (this number rises to thermally amplified glass at 200 MPa) clearly reduced in relation to resistance to compression.
Melting point: At about 730°C.
Elasticity: (coefficient Young) 70 GPa . This factor expresses the theoretical power (per unit area) that should be applied in a glass sample, in order to double its original length.
Transverse contraction coefficient: (Poisson ratio μ) 0,22
Hardness: 6.5 Moh scale (diamond has 10,sapphire 9 and plaster 2)
Dielectric constant: for 6mm glass at 21°C
Resistance to compression: 1000 MPa. It is extremely high and means that to crush a 1cm3 glass volume cube a weight of 10 tons must be applied.
Tensile strength: 40 MPa (this number rises to thermally amplified glass at 200 MPa) clearly reduced in relation to resistance to compression.
Melting point: At about 730°C.
Elasticity: (coefficient Young) 70 GPa . This factor expresses the theoretical power (per unit area) that should be applied in a glass sample, in order to double its original length.
Transverse contraction coefficient: (Poisson ratio μ) 0,22
Hardness: 6.5 Moh scale (diamond has 10,sapphire 9 and plaster 2)
Dielectric constant: for 6mm glass at 21°C
- 1 GHz 6,0
- 10 MHz 6,5
- 1 KHZ 7,4
- 10 Hz 30,0
Refractive factor: 1,52 (this factor varies depending on the wavelength of the light under consideration)
Bright reflection: (Loss of light due to reflection) about 8% to 10%. The light, when passing through a dividing surface of 2 optical media with different refractive indices, (eg air-glass) is always reflected. The percentage of light reflected, depends on the refractive indices of the materials and the angle of incidence of light.
Bright permeability: About 85% for 6mm glass.
Translucent ultraviolet radiation: in wavelength 340nm 41% , to 315nm 1%.
Chemical strength: Glass is extremely resistant to most acids in addition to hydrofluoric, at high temperatures and in phosphate. However, it is sensitive to alkalis. Also soluble sulfide impurities from metal corrosion tend to adhere permanently to the free surface of the glass which must be removed as soon as possible.
Bright reflection: (Loss of light due to reflection) about 8% to 10%. The light, when passing through a dividing surface of 2 optical media with different refractive indices, (eg air-glass) is always reflected. The percentage of light reflected, depends on the refractive indices of the materials and the angle of incidence of light.
Bright permeability: About 85% for 6mm glass.
Translucent ultraviolet radiation: in wavelength 340nm 41% , to 315nm 1%.
Chemical strength: Glass is extremely resistant to most acids in addition to hydrofluoric, at high temperatures and in phosphate. However, it is sensitive to alkalis. Also soluble sulfide impurities from metal corrosion tend to adhere permanently to the free surface of the glass which must be removed as soon as possible.
Types of glazing
Glass is available in many types, thicknesses, patterns and finishes. The glass is selected for reasons of safety, appearance and the way it controls the internal environment of the building.
Glass may be grouped into categories by considering:
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Strength and safety
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Appearance
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Environmental control
Strength and safety
Glass in a building will be subject to mechanical loads in the form of wind load and impact. It may also be subject to stresses due to environmental conditions such as temperature changes. The strength properties of glass can be varied by increasing the thickness, heat treatment and combining the glass with other materials to form composites. The strength of glass must be sufficient to resist the loads it is likely to be exposed to. Safety of glass is related to its strength but also takes into account the risk of injury from the failed glass.
Annealed glass
Annealed glass is the basic form of glass produced in float glass plants. It has no special properties of strength or safety and on breaking it forms large shards with sharp edges.
Heat strengthened glass
Annealed glass may be strengthened by controlled heating and cooling. Heat strengthened glass is not a safety glass but is roughly twice as strong as annealed glass. When broken it behaves like annealed glass and breaks into large shards with sharp edges.
Toughened glass
Annealed glass is toughened by heating it to 650”C and rapidly cooling the surfaces. This compresses the surfaces and increases the strength of the glass. Toughened glass is roughly five times as strong as annealed glass. An important property of toughened glass is the way in which it breaks. Any cracking of the glass leads to a rapid release of the surface compression and toughened glass always breaks into small pieces of glass. Toughened glass complying with EN 12600 is a safety glass. Toughened safety glass should be CE-marked and installed with the CE mark visible. Toughened glass cannot be cut or drilled after toughening and must therefore be cut to size before toughening.
Heat soaked toughened glass
Toughened glass may fail due to the presence of nickel sulfide crystals in the glass. To reduce the risk of nickel sulfide failure, the glass may be subjected to a process known as heat soaking. To be effective the heat soaking process must be strictly controlled.
Laminated glass
Annealed, heat strengthened or toughened glass can be laminated in any combination to make a safety or security glass. Two or more pieces of glass are laminated together to give the required properties. The glass may be laminated as a sandwich with a layer(s) of polyvinyl butyral (PVB) between the sheets of glass. Glass can also be laminated by pouring a resin between two sheets of glass. PVB laminates are best suited to flat glass while poured resins are best suited for curved glass. Laminated glass is not as strong as a single pane of glass of the same type and thickness but after failure the broken pieces of glass will be held together by the interlayer. The performance of a laminated glass depends on the type of interlayer used. Some are designed to resist penetration and others solely as safety glasses.
Tempered glass
Tempering is the American term for strengthening and toughening. Tempered glass is roughly equivalent to heat strengthened glass and is not a safety glass. 0nIy fully tempered glass has similar properties to toughened glass. Fully tempered glass used as a safety glass should conform to EN 12600.
Appearance – Designs of glazing
Glass may be patterned by rolling a relief onto one surface while it is still hot and soft. This is done to obscure vision or to change the appearance of the curtain wall. Patterned glass has the same strength and safety characteristics as annealed glass and is not normally a safety glass however it is possible to toughen patterned glass. Some patterned glasses - those that do not have deeply embossed surfaces - may also be laminated.
Printed
It is possible to print patterns on to glass. This may be done to make people aware of the glass for safety reasons. In this case the patterning has to be in the correct position. Note that company logos and other signage may be used for this safety purpose.
Flitted and etched
The surface of glass may be etched or otherwise altered to achieve the same effect as printing. Again this may be done for safety reasons.
Environmental control
Environmental control glasses are used to limit the heat and light passing through a window.
Tinted
Glass may be tinted to reduce light transmission and prevent glare within a building.
Coated
Glasses are coated to change the properties of the glazing. Coatings are used to reflect light and/or heat. Increasing the amount of reflected light may be required for aesthetic reasons (giving a mirror effect) or to restrict the view into the building. Reflection of heat may be required to reduce solar gain or to retain heat within the building. The type of coating will differ depending on its purpose. Low emissive (low-E) coatings are among the most widely used and are provided to reflect heat from inside the building back to the inside and therefore reduce heat loss. They do not reflect solar radiation in the same way due to the different wavelength. They are not visible to the eye.
Printed
Patterns may be printed or etched onto the surface of the glass to obscure vision or prevent glare.
Double and multiple glazing
Glass is frequently used as insulated glazing units (double glazing). This is normally done to reduce heat loss from the building but it can also help to reduce noise levels inside a building. In some cases triple glazing is used to reduce noise levels or further reduce heat loss. Insulated glazing units may be made using any of the glasses described above and different glasses may be used for the inner and outer panes. The panes are separated by a spacer bar. The units may be constructed with a primary airtight seal between the spacer bar and the glass and a secondary structural seal outside the spacer bar holding the glass panes together. Alternatively a single structural and air tight seal may be used.
Gas filled
Insulated glazing units may be gas filled to reduce energy loss through the window. Any units that are broken or damaged should be replaced with equivalent units.
Safety glazing and fire rated glazing
The Building Regulations make specific requirements for the use of safety glazing and fire rated glazing under certain circumstances. The design of the facade will have taken account of these requirements. It is essential that safety glazing and fire rated glazing are installed as specified.
Safety glazing
Glass in critical locations (adjacent to doorways and pedestrian areas and in windows with low sills) has to comply with Building Regulations. The glass has either to break in a safe manner or resist impact. It is normal to use toughened, laminated or wired glass in these locations. Plain annealed glass may be used provided no single pane exceeds 0.5 m2 in area, the smaller dimension is no more than 250mm and the thickness is not less than 6 mm.
Substitution with glass of different performance in a critical zone may be unsafe and should only be approved by the specifier.
Fire rated glazing systems
Fire rated glazing systems will have been tested to show that they can resist fire for the required period of time. The performance of a fire rated screen depends on the exact replication of the test sample on every contract. No substitution of any framing, glazing or other components is permitted. Alumil maintains a register of trained installers and approved specialist contractors.
Thermal transmittance U-value glass (or coefficient K)
Heat transfer through a surface in all three ways (contact, mixing and radiation), is attributed by the U-value factor (also known as coefficient K). It is about the degree of heat loss (in Watt) through a surface of 1m2, for 1 degree Kelvin heat difference, between the internal and outernal space.
The total thermal insulation of a window depends on the thermal insulation of the frame, the thermal insulation of the glazing and thermal break spacers and is attributed by the Uw coefficient while the thermal insulation of the glazing alone is attributed to the Ug factor.
A glazing with a single glass with thickness of 6mm, has a U-value of 5,7W/(m2.K).
A glazing with double glasses, consisting of two non-thermal insulation glasses with 5mm thickness and 12mm air space, between them, has a U-value of 2,8W/(m2.K).
A glazing with double glasses, consisting of a non-thermal insulation glass with thickness of 5mm, and a low-E 4mm glass, with 14mm space filled with argon gas, has a U-value of 1,1W/(m2.K).
To better understand the gravity of the above Ug glass degrees, we can compare them with the coefficient of thermal conductivity of a wall without insulation inside, which has about a U-value of 1,5W/(m2.K), while a wall with insulation, has a U-value less than 0.6W / (m2.K).
Condition of glazing
The performance of glass is highly dependent on its condition. The use of damaged glass or insulated glazing units will impair the performance of the facade.
Glass should be inspected for:
Size
Glass that is undersize will not have sufficient cover in the glazing rebate. This can lead to an inadequate seal at the gasket and in the extreme loss of glass retention. Glass that is oversized will reduce the clearance between the glass and frame which will limit the accommodation of relative movement of the glass and frame. If thinner glazing blocks are used to accommodate oversized glazing panels, the rebate in the lower frame may not be large enough to have proper water egress. This may lead to the breakdown of seals of glazing units. Ultimately the glass may not fit into the frame if it is too large.
Surface defects
Surface defects are uncommon with float glass. However when they do occur they are clearly visible. Surface defects are an obvious source of irritation to the client. It is good practice to check all glass for surface defects at the time of installation. It is far easier to replace glass at this stage while the access scaffold is still in place. Toughened glass may have a slightly rippled surface as a result of the toughening process. This is generally accepted but if particularly bad it may be unacceptable and the glass may have to be replaced. If the cavity of an insulated glazing unit is at a different pressure to the surrounding air, the glass will dish and give distorted reflections. Pressure differences can be caused by sealing the units at too high a temperature or at a different altitude from the site. This results in dishing of the glass as the cavity volume changes. Visual effects can be quite pronounced and unacceptable.
Changes in weather conditions will have a smaller effect that is normally acceptable.
Edge defects
Edge defects include:
- Feathering where the edge of the glass is not exactly square to the face and may not be plane
- Venting where the edge of the glass is clearly chipped to leave sharp edges around a depression
Feathering of the edge is acceptable up to a point. Venting is never acceptable. Edge defects cause stress concentrations which weaken the glass if it is subject to load. Thermal fracture of glass takes place if there is a large temperature difference between different parts of the glass. This can occur when most of the glass is heated by solar radiation but the edge is kept cool by shadows or the insulation of the frame. Stress concentrations at edge defects increase the risk of thermal cracking. An edge tape may be used but this is not recommended as it provides little protection, hides edge damage, prevents inspection of the seal(s) and can even trap moisture causing breakdown of the seal.
Laminated glass
Laminated glass should be visually and optically acceptable. There should be no damage to the edge of any sheet of glass in the laminate.
Edge seals
Sealed units are made with either single or double edge seals to comply with EN 1279.
Double edge seals are used to give a longer life to the unit. Any units replaced on site due to breakage or the presence of defects should be replaced with units of the same construction.
Edge seals should be free of any visible air bubbles.
Identification
Identification of glass on site can present difficulty if it is part of a glazing unit, has invisible coatings or particular strength properties. The main methods of identifying glass are:
Visual inspection with a gauge card held against the surface will identify the glass thickness using the reflection from the back face of the glass. A reflected flame will appear differently on coated surfaces.
Visual inspection with a gauge card held against the surface will identify the glass thickness using the reflection from the back face of the glass. A reflected flame will appear differently on coated surfaces.
Marking of glazing units at the time of manufacture assists identification. Labels should show: type of glass, size, manufacturer, glazing position and orientation. Toughened safety glass should be CE-marked according to EN 12150. Glazing units must be CE-marked according to the appropriate European standard.
Gauges or meters may be used to determine glass thickness. Several commercial systems are available.
DSR (differential surface refractometer) equipment can be used to determine the surface stresses in glass and the degree of toughening. This equipment is expensive and is unlikely to be available on site.
Ultrasonic test equipment can be used to identify laminated glasses. These also sound differently when tapped.
Suitable methods of identification:
Glasstypes Methods
Clearfloat Visual or meter
Patterned Visual
Wired Visual
Tinted Visual
Coated Visual or meter
Heat-strengthened DSR
Toughened Mark, DSR or polarised light
Bent Visual
Laminated Mark, ultrasonic
Glazing unit Mark on spacer
Printed Visual
0ff-line coated Visual, meter, reflections
Glass installation
The following standards apply to glass installation:
EN 1279 Parts 1, 2,3, 4,5 and 6 Glass in building: Insulating Glass Units
ENISO 9001 Quality management systems - Requirements
EN 13830 Curtain walling - Product standards
Glazing materials should be installed in accordance with the manufacturer’s instructions.
EN 1279 gives general guidance applicable to most windows. Where manufacturer’s instructions differ from EN 1279 the manufacturer’s instructions should be followed.
Positioning
It is important that glazing units are correctly positioned. Units that include safety glass should be used in the correct openings and not swapped with non safe units. Units that have different glasses for the inner and outer panes should be positioned with the correct face outermost. This may be required for reasons of safety, appearance or the effectiveness of energy efficient glazing. Each glazing unit will contain two or more pieces of glass that will be of slightly different size due to manufacturing tolerances. Good quality glazing units are constructed with all glass aligned on two edges of the unit that are labelled ’bottom’. Glass should be installed with the correct edge resting on the glazing blocks so that all sheets of glass are equally supported.
Glazing blocks and spacers
Glazing blocks are used to support the glass and must support both panes of a glazing unit. They prevent glass to frame contact and centralise the glazing in the frame. Glazing blocks should support the glazing clear of any water that enters the glazing rebate. Glazing blocks should not block any drainage paths. Some systems require glazing blocks that bridge the drainage channel. Use of sealant to locate setting and location blocks may also block drainage paths.
Glazing blocks may be made from the following materials:
- Neoprene with Shore Hardness 80 to 90
- Plasticized PVC with softness of 35 to 45
- Extruded non-plasticized PVC
Hammered lead is sometimes used in non-drained systems and sealed hardwood may be encountered in some windows but should not be used in curtain walls. Location blocks are used to prevent lateral movement of the glazing and give rigidity to opening lights and factory glazed products. They are made from the same materials as glazing blocks. Distance pieces are used to maintain the distance between the glass and the frame when using wet applied sealants. They are made from the same materials as glazing blocks.
Glass and frame support
The glazing material stiffens the frame of opening lights and doors and prevents them distorting or sagging in use. The glazing block positions are selected to correctly stiffen the frames as well as support the glass. For windows that pivot on a horizontal axis the glazing blocks at the top of the frame also support the glass. The recommended positions for glazing blocks for windows are shown here but the manufacturer’s instructions should also be read.
Glazing blocks should be at least 30mm and no more than 100mm from the corner of the glazing frame. Curtain walling and glazing screens have to move to accommodate movement of the primary structure. Location blocks in curtain walling should be placed near the bottom of the glass to prevent lateral movement of the glass but allow racking of the frame.
Edge clearance
Glass should befitted into the frame with adequate edge clearance. This is necessary so that:
- The glass and frame can move without stressing the glass
- Water entering the frame can drain freely
Minimum edge clearances for glass are:
- 3mmfor glass sizes up to 2m
- 5mm for glass sizes over 2m
- 6mm for all drained systems
Minimum edge clearances for plastic glazing materials are:
- 3mm for plastic sizes up to 1m
- 5mm for plastic sizes between 1 and 2m
- 7mm for plastic sizes between 2 and 3m
Drainage
Drain holes in the bottom or face of the frame must not be blocked by glazing blocks, burrs or sealants.
Storage and Handling Glassweight
Typical glazing units are heavy and larger units require special consideration. It is always preferable to glaze windows at the factory. However for larger windows the completed weight is too great to be lifted manually and these windows have to be site glazed. Some windows have to be deglazed for fixing into the opening.
Glass weighs 2.5kg/m2/mm. Weights of typical glass products are shown below:
6mm glass 15 kg/m2
6 -12 -6 glazing unit 30 kg/m2
7,3 -12 -6 laminated glazing unit 32,5 kg/m2
15mm glass 37,5 kg/m2
Consideration should be given to mechanical handling and lifting of larger glazing units and complete glazed windows.
Glass storage
Glass should be stored:
- In a dry covered area
- Out of direct sunlight
- Stood on edge
- Protected from impact
- Protected from dirt
Glass should be stored on site in a protected location where it will not be damaged and does not become marked or unduly dirty. If glazing seals become wet, particularly if water becomes trapped behind edge tapes, the seals will start to break down. If water is trapped between two pieces of glass for too long then the glass surfaces may be permanently marked. If glass is stored in direct sunlight then heat passes into the stack and cannot escape. The glass within the stack can become very hot causing fracture. Glass should be stored stood on edge and inclined against a rest to prevent it from falling. With glazing units both edges should be supported to reduce the risk of edge damage.
