Select-a-Glaze : glass options for your secondary glazing

Though liquid in nature, don’t push me too far for then I will break, and the damage may scar… What am I?

Glass is a bit of a riddle; at high temperatures, it is structurally like liquids, yet at ambient temperatures, it behaves like a solid. It is hard enough to provide protection yet shatters with incredible ease. It is made by melting sand, soda ash and limestone, opaque in their natural form at temperatures of c. 1700⁰, which then becomes an amorphous solid: glass, a transparent material. 

Glass is glass, right? – why so much choice!

Well not really; there are many different types that can be produced for various applications made by either using different processes or varying levels of the raw materials. Each have specialist properties providing a benefit yet can also have a drawback. This piece is by no means exhaustive in terms of the options there are, but the main glass types that we use in our secondary glazing systems help to meet specifications requiring standard and high-performance levels whether that is U-values, dB levels or to provide security. Buildings, old or new, also have to adhere to many building regulations whether that’s Part L to do with window performance, Part K – protection from falling and guarding, Part Q – security of dwellings and meeting World Health Organisation targets for acceptable noise levels. Using the correct glass in secondary glazing may help you meet these planning or regulatory stipulations without changing the existing primary window glass.

Basic forms of glass

The three main types of standard glass we use in our secondary glazing are:

• Annealed/float glass – this is standard glass that has been slowly cooled, relieving any internal stresses.
• Tempered/safety/toughened glass – this goes through the process of extreme heating followed by rapid cooling which makes it much harder and around 4-5 times stronger than standard glass. Toughened safety glass is now the standard option for all our products. Whilst toughened glass can still be broken it will shatter into small blunt pieces making it much safer than annealed glass.
• Laminated glass – two or more pieces of glass are adhered or bonded to a clear vinyl layer, which holds the glass together if it is broken, keeping the glass in place rather than breaking into dangerous shards. In addition, it can provide good acoustic qualities, helping reducing dB levels and also provides UV light protection – important in settings such as art galleries and museums.

In the interests of safety during manufacturing and user handling, operation and maintenance all our products are manufactured as standard with toughened glass satisfying the standard safety glazing regulations, however, these can all be further enhanced by a wide variety of specialist glass depending on your specific needs.

Glass for thermal performance enhancements

Low-E glass

With an estimated 70% of energy loss occurring in windows and doors and 90% of the window heat loss via the glass, it is imperative to make improvements, especially as we seek to meet carbon targets (as well as to bring energy bills down). The use of low emissivity glass can substantially improve U-values, lessening the amount of heat loss and solar gain, which helps create a more even internal temperature. Low-E glass has a microscopic coating minimising the amount of infrared and UV light entering the building whilst also reflecting the interior heat back into the space. 

• Hard Coat (passive low-E coating) – this is created by fusing the coating to the hot glass surface during its manufacture, which makes it very durable and the only option for use in single glazed secondary units.
• Soft Coat (solar control low-E coating) – this is applied to pre-cut glass in a vacuum chamber at room temperature using the Magnetron Sputtering Vapour Deposition (MSVD) process. Although it provides higher U-values and better solar control, it oxidises and degrades over time when exposed in air, so must be protected, and used as part of a sealed unit.

Sealed units 

There are a wide variety of options for sealed units from having different glass thicknesses or types, one or 2 panes with a low-E coating, filling the space between the panes with inert gasses, or a more recent innovation is a vacuum-sealed unit, whereby the two panes of glass are held apart by microscopic beads during the evacuation of the air – this is a relatively thin option and achieves U-values of 0.7 kW/m²K.

Glass for enhancing acoustic performance

In general, the noise reductions required on many projects can be met by using our units with standard glass. Ideally, the secondary glazing should be glazed with a different glass thickness to the primary glazing, and by introducing a cavity of 100mm or more will provide significant levels of acoustic insulation. Noise transmission via a window is by direct vibration of the glass. The cavity created by the introduction of a fully independent secondary window system decouples the movement of the inner and outer panes, allowing them to act as separate barriers, thereby reducing the resonance of the sound. Where higher levels of dB reduction are required, thicker glass and/or an acoustic laminate glass can be used. Acoustic laminated glass uses a special clear acoustic interlayer sandwiched between the 2 pieces of glass, instead of a standard interlayer, which helps absorb and therefore reduce even more noise transmittance.

How acoustic laminate glass works

High security levels of glass

Most security applications require laminate glass to ensure the integrity of the glazing is matched to the integrity of the frame.

• Glass polycarbonate – this is generally used in secondary glazed units when high levels of security against physical entry are required to meet LPS1175 SR2 and SR3. It is created by resin bonding a sheet of glass to a sheet of polycarbonate.
• PVB laminated glass – when protecting against a potential blast attack the glass needs to absorb the shock or energy, so it must have the ability to move or flex rather than remain rigid. By bonding layers of glass and PVB interlayers, in effect creating a sandwich; the interlayer not only holds the glass together but also allows the glass to flex and stretch to absorb the shock waves.
• Fire glass - this needs to do 2 things; withstand the heat and maintain its integrity in the most severe of circumstances. This laminate glass incorporates a clear intumescent layer in the sandwich which foams and expands at high temperatures creating a rigid insulating barrier to the heat and flames.

Some other specialist glass types:

• Low-iron – also known as extra clear glass or optically clear glass. It is manufactured by reducing the amount of iron in the molten glass composition. This can increase light transparency by 5-6% and reduces the greenish tint that thicker panes of standard glass can have.
• Anti-reflective – this is used to reduce the amount of surface reflection seen from the face of the glass, which can make it appear almost invisible. It is created by dipping low iron glass in metal oxide solutions, which coats all its faces preventing visual reflectance.
• Obscure – varying degrees of opacity can be obtained reducing visibility through the pane; from patterned glass created by embossing a design onto the glass with rollers, by acid etching or sandblasting the surface area, or by introducing an opaque interlayer into a laminate glass.
• Heated glass – this is sometimes specified for use in secondary glazing to zones of buildings with high moisture content, such as swimming pool areas. A sheet of glass is coated with a semiconductive material, which is activated when a current is passed through it, generating a radiant heat source. By heating the glass in these areas to the ambient room temperature, helps to eliminate condensation on the windows.
• Switchable glass – can provide several benefits. It is a sophisticated laminated glass which when a current is passed through it, can change its opacity, create a black-out or provide solar The units can be controlled by wall switches, remote controls, and motion sensors to name a few methods.

Switchable glass shown on and off in a meeting room at Selectaglaze head office Selectaglaze has nearly 60 years’ experience working in all building types, each having its own particular demands. Secondary glazing, when approached correctly, with a trusted specialist, will ensure you get a discreet performance led installation which will provide years of service. We have highlighted the most used glass types, but there are far too many options to cover them all in this piece. If you have a specific requirement that is not covered contact our technical advisory team, they will be happy to help.