How to reduce the impact of thermal stress.

Traditionally, tinted glass was the best way to reduce the amount of heat and light entering your home through your windows.  But with the use of tinted glass came increased risk.

Today, high-performing Low-E double glazing provides exceptional solar control without the reduction in daylight that comes with tinted glass.  Tinted glass works by absorbing thermal energy.  SuperTherm Cool™ works by reflecting it away.

Tinted glass is still a great option if privacy is a larger concern for you, but there are some factors to be considered if you wish to include tinted glass in your double glazing, such as the associated risk of thermal fracture with 5mm grey and 5mm green-tinted glasses.

Thermal stress causing crack in glass

What is thermal stress?

Glass expands or contracts as temperatures change.  This is important to consider when dealing with darker tints such as grey tint as the darker the tint, the more heat energy that is absorbed into the glass.

If the window-pane has nothing blocking it from the sun, then the glass will expand and contract consistently. However, If a window is partly shaded and partly exposed to the sun, the part exposed to the sun will expand as it heats up whilst the shaded area won’t.  This causes thermal stress where the glass is under stress due to differing temperatures which can result in a thermal break.

Conditions that may cause thermal stress:

  • Overhanging eaves providing partial shading on a sunny day
  • Partial shading of a tree or nearby structure such as a screen, carport, or garage
  • Extreme temperature shifts from hot days to very cold nights
  • Window treatments (curtains/blinds) especially if placed too close to the glass surface as they prevent the movement of air over the glass surface
  • Some films or sign writing
  • Using tinted and reflective glass that absorbs and reflects heat

How to reduce the risk of thermal stress.

Tempering or toughening glass reduces the impact of expansion and contraction within the pane of glass.

  • Annealed glass can withstand a temperature difference of around 40°C
  • Toughened glass is around 5 times stronger than annealed or float glass and has a higher thermal strength which enables it to withstand up to around 243°C
  • Heat-strengthened glass is around 3 times more resistant to thermal stress than annealed glass and is able to withstand a temperature difference of 100°C (in the range of 50°C to 150°C)

By toughening glass, the risk of thermal breakage is greatly reduced.  Toughening is highly recommended for all tinted, Low E and reflective glass due to the amount of heat energy held or reflected by this glass.

What is toughened glass?

Toughened (or tempered) glass is annealed glass that processed to become five times stronger than it’s original state.

First the edges of the glass are arrissed or polished to remove the sharp 90° edges.  Any other processing required is carried out prior to the glass reaching the furnace.  Next the glass is moved into a special heated furnace where it is heated to around 650°C where it is heated to a plastic state.  It is then moved into an area called the quench, where it is snap cooled.

This rapid cooling puts the surface of the glass into a state of compression.  The inner core of the glass remains in a state of tension.  This combination of tensions provide the increased strength of toughened glass.  In addition when toughened glass is broken, the stored up energy is released and the glass breaks into small pebbles around the same diameter as the glass thickness keeping you safer than annealed that can break in a large sheet.

Toughened glass can still break, and thermal breaks can still happen however the process of toughening reduces the risk.  “Tempering increases tensile strength, thermal shock resistance, and safety of normal glass. It also increases its heat withstanding capacity. Because of these properties, tempered glass is preferred in applications where strength, safety and thermal resistance are important considerations.” (1)

Introducing SuperTherm Cool™:

Where tinted glass absorbs the thermal energy coming from the sun, high-performing SuperTherm Cool™ Low-E double glazing works by reflecting some of that thermal energy away.  This difference means that SuperTherm Cool™ does not have the associated risk of thermal breaks caused by thermal stress that 5mm grey or 5mm green tinted glasses do.

Not only will your home have superior thermal insulation performance, but you can keep that beautiful natural light from the sun without the risk of unexpected glass breakages due to unevenly heated glass..

SuperTherm Cool Low E Double Glazing

(1) Tempered Glass. (2016, September 6). Retrieved from Glass Academy: