Types of glass
Double glazing for thermal insulation.
What is Low-e Glass?
Glass used in double glazing window for thermal insulation is known as Low E, or low-emissivity glass. It has a transparent metallic coating that works in two ways to economise heating energy. The dual action coating reflects heat back into the room, whilst allowing heat and light from the sun (known as passive solar heat gain) to pass through. Thermal insultion glass should be used on face 2 or 3 of a double glazing unit (see double glazing units)
Window energy ratings
As government building regulations become more stringent and heating costs rise, it is becoming ever more important to choose the most effective thermal insulation glass. The British Fenestration Ratings Council (BFRC) has set out an official window energy ratings table, ranging from A to G, with A being the most energy efficient. For more detailed information see Window Energy Ratings section). Windows with a C rating or higher are endorsed by the government-backed Energy Saving Trust.
The energy efficiency of a thermal insulation glass is measured by its U (formerly K value) and its Solar Heat Gain Coefficient (g).
In addition to the low E glass used, energy ratings can be further enhanced by the variety of Spacer Bar, Cavity and Edge Spacing used in the double glazing unit.
The ‘U’ value of a double glazing window is the measure of its ability to transfer heat – so double glazing windows with the lowest U value are the most efficient insulators against heat loss from a room (see Window Energy Ratings).
Solar Heat Gain
The Solar Heat Gain Coefficient (SHGC), which measures a thermal insulation glass' ability to transmit solar energy into a room, is measured in value from 0 to 1. The SHGC is commonly referred to as the g-value, or solar factor. The higher the g value, the greater a thermal insulation window’s ability to transmit solar heat (see Window Energy Ratings) and thus increase its energy efficiency.
Types of Low E Solutions
The most efficient thermal insulation glass use a unique manufacturing process which builds up microscopic layers coating, using a technology known as sputtering, under vacuum conditions. (See online and offline coatings). This advanced process builds up a highly resistant, but imperceptively thin coating which gives it a much clearer appearance than other thermal insulation glass. The coating also allows maximum daylight and heat into the room for optimised solar gain. Some products have been shown to reduce heat loss by 24% more than traditional online coated thermal insulation glass, and by 40% compared to standard double glazing window. Further energy savings can be made by using warm edge ‘thermal break’ spacer bars. These can reduce heat lost around the edge of the window by up to 65%.
Combining thermal insulation glass with other solutions
Some ranges of thermal insulation glass can be toughened and laminated and used in double gloazing window in various combinations with solar control, self-cleaning, decorative and acoustic glass. (see Which Combination).
No more noise pollution!
Noise pollution is a growing environmental concern and nuisance, especially in urban areas. Even the background noise of 70 decibels caused by outside traffic can cause disturbance and stress over a continued period of time. Acoustic glass technologies have been developed to combat levels of unwanted noise, whether it be for a translator’s booth, a house located near a busy road or an office next to the airport.
What is acoustic glass ?
Acoustic glass considerably reduces outside noise, especially near busy areas such as motorways, main roads and airports. When used in overhead or roof glazing, double glazing window also provides insulation from rain impact noise. It can be used for interior sound insulation needs, such as office partitions and meeting rooms.
Acoustic glass consists of two or more sheets of glass, bonded together with one or more acoustic interlayers: Double glazing window. The interlayers act as a noise dampening core, weakening the sound as it travels through the glass. Interestingly, by varying the thickness of the sheets of glass, even better sound insulation can be achieved. Acoustic laminated glass also benefits from all the safety and security properties of standard laminated glass. For more information see Acoustic Insulation (how does it work).
Combining acoustic glass with other solutions
Laminated acoustic glass can be combined in double glazing window with a variety of Low E, solar control, self-cleaning and decorative glasses (see Which Glass Combinations).
With Self Cleaning Glass you can forget to clean your windows forever
Double glazing window cleaning can be a time-consuming, costly and environmentally polluting process. Difficult to reach areas - such as glass roofs, upper windows and conservatories can even be dangerous to clean. The latest developments in selfcleaning glass mean that windows can now be kept clean all year round, with minimum maintenance, even in areas of high pollution.
How does it work?
Self cleaning glass, or easy clean glass, has a transparent coating which works in two ways to break down dirt and spread water evenly across the surface. The special coating, which is a applied online during the making of the float glass, is both photocatalyic (reacts with UV rays) and hydrophilic (water-loving). The self cleaning glass coating reacts with the sun’s rays to break down dirt and grime which form on the outside of the double glazing windows. When it rains, the decomposed dirt naturally just rinses away. As the surface is also hydrophilic it attracts water over its entire surface so rain forms a sheet of water over the glass, rather than droplets which can make rain marks. If there is no rain, a simple hose down or rinse with clean soft water is sufficient to activate the selfcleaning mechanism.
Important Points for Maintenance and Installation
- Due to the composition of self-cleaning glass, non silicon joints must be used and the self-cleaning glass should not come into any contact with silicon objects as they will damage the efficiency of the special coating,
Sharp objects and abrasive substances should not be used to remove stubborn dirt on easy clean windows as they can damage the coating. If manual cleaning is required, warm soapy water or standard window cleaning products should be used with a soft, clean cloth or sponge as these will not in any way damage the coating.
Self cleaning glass must be installed in an upright or sloping position of at least 15 degrees to the vertical, to allow water to naturally run down the surface and wash away the broken down dirt.
The self-cleaning coating of the double glazing window should be installed on face 1 – the exterior of the outside facing pane. (See double glazing units)
Visual of four faces of a double glazing window with easy clean glass on face 1
Some self cleaning glasses can be used in conjunction with Low E and solar protection glasses for maximum comfort. They can also be laminated for security or combined with a soundproofing acoustic interlayer. See Which Glass Combination for more possibilities.
Avoid risk of injury caused by broken glass with toughened and laminated forms!
Glass can be a dangerous material
When standard glass breaks, it can form dangerous shards and splinters. To avoid risk of injury caused by broken glass, high tech safety glasses are available in toughened and laminated forms – most of which can be used in double glazing window. Wired glass also qualifies as a safety glass. Less effective, it is only awarded the class C level of the British Safety (BS) standards scale.
Usage of safety glass
Safety glass is now mandatory in some areas of the home (see Building regulations), as well as throughout some types of buildings such as schools. Glass used in roofs must also meet safety requirements in case of an object falling on to the double glazing window which could cause fragments falling below.
Standard requirements for safety glass
Safety glass must conform with the BS 6206 standard requirements in order to qualify as safety glass. Three levels of impact resistance exist under the BS specifications – A, B and C - with class A being highest. For glass to be awarded the class A safety standard, it must be able to resist an impact of 45kg without breaking, or break safely. However those with projects for safety glass should be prepared for the more stringent European safety standards that are currently coming into effect for the UK (for more information see Standards)
Laminated glass is comprised of one or more interlayers, sandwiched between two or more sheets of float (or annealed) glass. The interlayers are composed of a PVB (polyvinyl butyal) laminate, or cast resin.
Cast resin laminated glass is manufactured by pouring liquid resin into the cavity between two glass sheets. This method of laminate is often used where heavily textured or patterned glass is to be used.
PVB laminated glasses tend to have a higher safety performance and are the more commonly used variety. One or more sheets of glass are bonded together with one or more PVB interlayers. If the glass suffers an impact, it may eventually fracture, but the broken fragments will remain bonded to the interlayer.
Other benefits of laminated glass
Safety grade laminates also act as a deterrent against burglary and vandalism. Depending on the composition of the laminate and thickness of the glass, different levels of performance are also possible for acoustic insulation and protection against firearms and explosions. (Also see Security Glass).
Toughened, or tempered, glass is four to five times stronger than ordinary float glass. If it is broken relatively harmless fragments, known as ‘dice’, are formed which are unlikely to cause injury. Toughened glass that meets with the highest British Safety Standards, Class A, is suitable for use in structural double glazing window where the glass is to be fixed by bolts or clamps.
Toughened glass is made by heating prepared and processed glass sheets to a temperature of 700°C (just above the softening temperature of glass) The sheets are rapidly cooled by blasts of cold air to both surfaces, resulting in the outer surface contracting and solidifying before the interior of the glass. This process increases the tensile strength of the glass and gives it safe breakage characteristics.
Toughened and laminated safety glass can be combined with other double glazing windows such as decorative, easy clean, thermal insulation and solar protection glass within the same double glazing window. (See Which Glass Combinations).
Be protected from flames and smoke!
Fire glass needs to resist against impact, provide a physical barrier against gases, flames and smoke, control radiated heat and limit surface temperature. Fire resistant glasses are manufactured using a range of toughening, laminating[Susan1]and coating technologies. Traditional wired glass solutions are also available, but they offer lower heat resistance and can cause injury by shattering.
Thermally toughened fire glasses
Thermally toughened fire resistant glasses offer protection from flames and smoke, with the option to reduce radiated heat. The toughening process, which involves heating float glass to a temperature of 600°C, also means the glass will not break, (or will break safely without causing harm) in the event of extreme temperatures caused by fire.
Types of toughened fire resistant glasses
Thermally toughened fire resistant glasses provide a barrier against smoke, flames and toxic gases for a given amount of time (depending on the specific glass product used). They can also incorporate a heat reflective coating for reducing radiated heat. Some of the most advanced fire glass are filled with a transparent gel which reacts when exposed to fire. The gel absorbs energy from fire, acting as a heat shield and keeping temperatures on the non-fire side of the window much lower.
Fire resistant glasses are required to meet with British Safety standards and the most effective fire glasses attain a class A rating on the BS scale. New, more demanding CE standards will also be coming into effect in the UK shortly.
Laminated/Interlayer fire protection glasses
Laminated/interlayer fire glasses offer protection against conducted heat produced by fire, limiting the temperature rise within a room for a particular length of time. Laminated fire resistant glasses are composed of one or more sheets of float glass, bonded together by one or more interlayers. Thanks to the laminate interlayer, risks of breakage and injury are also reduced, giving this type of fire protection glass security and safety properties. Laminated fire glasses can also be filled with a transparent gel which reacts when exposed to fire, turning opaque. The gel acts as a heat shield, keeping temperatures on the non-fire side of the window much lower.
Combinations with other types of double glazing window solutions
Fire resistant glasses can be combined with other glazing solutions such as decorative glass and Low E glass, within the same double glazed unit. (See Which Glass Combinations).
A glass against vadalism and infraction.
Security glass offers protection against aggression and can act as an important deterrent against vandalism and infraction. As opposed to safety glass, there are no statutory requirements from the British Standards association concerning where security glass must be used. Risk should be assessed on a case-by-case basis and must take any insurance specifications into account.
How does it work?
Security glass is normally laminated glass, comprising two or more sheets of glass with one or more PVB interlayers. The whole assembly is bonded together using heat and pressure, resulting in the complete adhesion of interlayer and glass. In the event of breakage, the glass fragments remain attached to the plastic interlayer and the window retains a residual strength while awaiting replacement. Laminated security glass can be positioned on the inner or outer pane of a double glazing window, but must be used with an appropriate framing system.
Laminated glasses with different levels of safety and security can be obtained by varying the number and/or thickness of each of the fire glass and interlayer components. There are BS standard requirements for security glass capable of giving protection against different types of possible hazards. The three main types of security glass are:
Glass resistant to manual attacks
Double Glazing window resistant to manual attacks, such as a person armed with an axe or crowbar, must conform with the BS EN 1063 specifications for vandalism and BS EN 356 for burglary. This kind of security glass is laminated for safety and generally range in thickness from 6.5 to 25 mm. They can also be supplied with integrated security alarms.
Bullet resistant glass
Ballistic rated glass must conform with BS EN 1063 specifications. Within this category different levels of security glass are available to protect against the passage of bullets from various types of firearms (from pistols to shotguns) - and to prevent against flying shards of glass resulting from bullet impacts.
Explosion-pressure resistant glazing
Any premises located in a potentially hazardous risk area for explosions should use explosion-pressure resistant double glazing window which complies with British Safety standards BS EN 13541. Explosion resistant glass is an effective way to minimise damage and injury that can be caused by an accidental or criminal explosion
Colorful glass with decorative effects.
Decorative glass for glazing can be manufactured in a variety of ways to give a wide choice of patterns, textures, colours and opacity. Decorative glass effects can be created on double glazing window panels by silvering, tinting, acid-etching or applying ceramic paints. The design can offer partial or total coverage, for improved aesthetics and privacy.
Many decorative glasses can be thermally toughened or laminated for compliance with Document N safety standards in the Building Regulations. Decorative glass can be incorporated into double glazing window and combined with Low E and solar insulation double glazing window.
Which properties for the best thermal insulation glass and windows?
With new legislation to reduce energy consumption being introduced, Thermal insulation glass has become the norm - but the latest advances in double glazing window mean that countless combinations of solutions for comfort, security and design are possible. For conservatories for example, selfcleaning glass can be combined with solar control glass for an easy to maintain solution which avoids excess heat build up in the summer.
Below is a table of possible double glazing window combinations possible with Low E glass and up to four other special performance features:
|Thermal insulation glass(Low E)||X||X||X||X||X||X||X||X||X||X|
Solar Control Glass Filters the sun's rays and lower solar reflect!
What is solar control glass ?
Excess heat and glare caused by the sun can be a major source of discomfort in some indoor environments, especially those with glass roofs, conservatories or with large glazed areas.
The latest double glazing window solutions for solar control reflect and filter the sun’s rays, allowing natural daylight into the room, but without uncomfortable visual glare. Rooms can be kept cooler during sunny periods, reducing the need for air-conditioning.
A variety of solar control glass solutions are available, depending on the level of solar control needed. The glass is manufactured by tinting and/or applying a metallic coating (see online and offline coatings). Online coating offers a greater range of performance and a more neutral appearance for solar protection glass.
Solar Heat Gain Coefficient or ‘g’ value
The Solar Heat Gain Coefficient (SHGC), which measures a window’s ability to transmit solar energy into a room, is measured in value from 0 to 1. The SHGC is commonly referred to as the g-value, or solar factor. The lower a window’s g-value, the greater its ability to insulate against solar heat build up.
Combinations with other double glazing window solutions
Solar control glass can be incorporated into double glazing window with combinations of selfcleaning, Low E and decorative glass solutions (see Which Glass Combination). Combining solar control glass with Low E thermal insulation in one double glazing unit, offers optimum temperature comfort all year round.