Fire Rated Glazing

Thermal Fracturing

Thermal fracturing, commonly referred to as thermal shock, is when a glass panel breaks due to a temperature difference across its surface. This is common in float glass installations which are not advised for use in architectural glazing.

The use of toughened glass is the first step to ensuring a durable glass installation. There are also different processes and types of glass you can specify to reduce the risk of thermal fracturing and it is important to understand which glass types are more likely to present problems.

Float Glass

Float glass is the base glass material of glass, from which all panes are created from. IQ do not use float glass, due to the fact it is not a safety glass. Many glaziers will sell float glass or float laminated glass as standard, without revealing the lesser quality. These base types of glass cannot withstand harsher conditions and show reduced strength and durability.

Due to the reduced quality, thermal shock or fracturing is more likely to occur in glass installations using float or float-laminated glass panes.

This will present as a single ‘crack’ through the glass unit. If part of a float glass panel is in sun and part in the shade, it is likely that thermal fracturing will occur.

Float glass

Toughened Glass

IQ use toughened glass as our base glass material on all projects, qualifying as a safety glass, for increased strength and durability. Glass panes that have been through the toughening process is up to five times stronger than float glass of the same thickness.

Toughened glass is made using standard float glass, cut to size and shape, before putting them into a toughening furnace. During the process, the panes of glass are heated upwards of 600 degrees, before being cooled rapidly with cold air. This produces tensile stresses on the glass surface, forcing the corresponding glass elements to contract to introduce the stresses into the glass panel and therefore increasing strength.

One of the key factors to consider when specifying large elevations of glazing is the safety of the homeowners if thermal shock did occur. Toughened glass ensures that if the glass panes break, they shatter into small, harmless pieces of glass that would reduce damage to the building and homeowners in the event of a breakage.

Building Regulations document K states that safety glass is legally required in areas of a building that are classed as ‘critical areas’. These include: any glazed area within a window below 800mm from floor level, any glazed area within a window that is 300mm or less from a door and up to 1500mm from floor level and within any glazed door up to 1500mm from floor level.

Despite safety glass only being a legal requirement in specified areas, the IQ group use safety glass on all applications due to the inherent strength and safety issues that float glass presents.


Laminated Glass

Laminating glass is the process of laminating panes together, to form a safety glass. Float glass can be laminated to ensure any breakages are held in place, as well as breaking into small harmless pieces. When float glass is laminated, this is technically a safety glass and is used by many other glaziers, but this is not as secure as toughened glass due to the likelihood of thermal shock remaining high.

At IQ we use toughened glass as a base and then laminate the glass to form a laminated glass pane, to achieve better performance values. Toughened laminated glass can achieve better strength, wind load, acoustic and security performances.

For homes striving to meet particular or specific performance requirements, toughened laminated glass would be the optimum glass specification.

Structural Glass Roof

Thermal Shock/Stress

The risk of thermal shock, occurring when there is a temperature difference across the surface of a glass pane, is more likely to affect float glass. This is because float glass can succumb to thermal shock at a temperature difference of just 40 degrees. IQ’s standard toughened glass can withstand a temperature difference of up to 200 degrees across its surface.

Typically, thermal fracturing would occur in the morning time, when the chilled glass is exposed to direct sunlight. The centre of a pane of glass heats up much more quickly than the edges, putting a strain on the glass.

Cambridge Road Sliding glass doors

Types of Glass Breakages and How To Identify Them

If you encounter a breakage in your glass and you’re not sure if this is due to thermal shock, the pattern of the breakage can reveal a lot. The breakage can reveal the type of glass used as well as the reason.

If there is a single crack within a glass unit with no shattering, this indicates the glass is float glass. This type of breakage would be due to thermal shock or fracture in float glass and would never occur in any of IQ’s installations.

A glass shatter with a butterfly pattern radiating away from a single identifiable point indicates a breakage due to nickel sulphide inclusion. This can occur in toughened glass and is a naturally occurring phenomenon in glazing. To greatly reduce the risk of nickel sulphide inclusion you can specify heat soaked glass. Heat soaking puts the glass through a rapid heating and cooling process to reveal which panes are weak, to ensure the weaker glass is not used on our projects.

A shatter pattern that radiates from a single central point shows us an impact breakage in toughened glass. This occurs when something directly hits the glass.

a nickel sulphide breakage in structural glass

Your Project

To gain the best results for your project, make sure your chosen glazier is providing high-quality safety glass. IQ Glass have never, and will never, use float glass and endeavour to provide high specification glass units for luxury results. Our products and systems are designed to a high specification and the glass used will always have a reduced risk of thermal shock, aiding the longevity of your project.

The team at IQ are on hand to discuss technical specification, design, and any specific project requirements you may require. You can get in touch with the team at to start the process.

Structural glass box extension