Importance of getting the right double glazed glass specification

When fitting replacement double glazing or triple glazing, be the frames aluminium, composite, steel, UPVC or timber, it is important to ensure that you get the right type of double glazing glass or indeed triple glazed glass.

In the case of triple glazing glass, ensure your sealed unit installer fits each triple glazed sealed unit with a toughened middle pane of glass, as this reduces the likelihood of the middle pane cracking as a result of temperature differences between the inner and outer panes, which are usually made of float glass, unless toughened required throughout under Building Regulations.

The current England and Wales Building Regulations state that toughened safety glass must be installed in entrance doors, conservatories, lantern lights, French doors, bi-folding doors, sliding patio doors, in bathrooms where the window is adjacent to the bath or shower and in all windows that are within 300mm of doors, windows within 800mm of the internal floor level. Consequently, in these situations all the panes of glass in both double glazed sealed units and triple glazed sealed units are required by law to have toughened safety glass. In replacement sealed units and in new build or replacement fenestration, there are three main types of glass 1) float glass, 2) toughened safety glass and 3) laminated glass.

Float Glass

Wikipedia states “Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces. Modern windows are made from float glass. Most float glass is soda-lime glass, but relatively minor quantities of speciality borosilicate and flat panel display glass are also produced using the float glass process. The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington, which pioneered the technique (invented by Sir Alastair Pilkington) in the 1950s.”

Toughened Glass

Wikipedia states “Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury. As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.”

Laminated Glass

Again, according to Wikipedia “Laminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral (PVB), between its two or more layers of glass. The inter-layer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic “spider web” cracking pattern when the impact is not enough to completely pierce the glass. “Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered.”

Toughened safety glass costs more to make and buy than float glass, and laminated glass costs more than toughened safety glass. Consequently, do ensure your double glazing installer or replacement sealed unit supplier actually fits a) what you expect and b) what is required under Building Regulations, as liability is joint. If the glass is toughened or laminated it should have a Kite Mark in one or more of the corners of the sealed unit. Most residential double glazing utilises 4mm thick float glass, which usually does not have a Kite Mark.

As they are made by a different type of manufacture, toughened and particularly laminate glass do reduce ever so slightly noise pollution, as sound does not travel through these types of glass as easily as it does via float glass. More important than the glass type when it comes to reducing exterior noise is size of the gap between the glass panes. Basically, the wider the gap, the greater the noise reduction will be, hence why fitting secondary glazing 100mm inside of your existing windows will deliver the optimum sound reduction. Any more or less than this 100mm gap, the noise reduction is not as significant.

For safety, quality, legal and living environment reasons, it is important to check that any replacement windows and doors that you have had supplied and fitted, or indeed replacement sealed units, do in-fact have the right glass specification. Not only that, installing the correct double glazed or triple glazed windows brings peace of mind, and ensures your property is compliant, plus, and most important of all, it prevents you, your family or visitors having a nasty accident with the wrong type of glass in the wrong location.


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