Glass and temperature resistance – what you need to know

Glass seems resistant to everything, but temperature can destroy it in seconds. Cracks caused by thermal shock are a common problem in glass structures – from shower enclosures to sauna glazing. Understanding how glass reacts to heat and sudden temperature changes helps avoid costly mistakes and keeps users safe. Below we discuss the key aspects of the thermal resistance of glass.

What is the thermal resistance of glass?

Thermal resistance is the ability of glass to withstand temperature differences without cracking. It is not about high temperature as such, but about how quickly it changes. Glass has low thermal conductivity – the heated side expands while the cold side stays as it was. This unevenness creates stresses that can exceed the strength of the material.

Ordinary float glass withstands temperature differences of up to about 70°C. This means that pouring hot water on a cold pane can crack it. In installation practice this is a common problem when fitting elements in bathrooms or wellness areas.

The coefficient of thermal expansion describes how much a material changes its volume with temperature. The lower the coefficient, the better the thermal properties. Float glass has a relatively high coefficient, which makes it vulnerable to thermal shock.

How does toughening affect thermal strength?

Toughening is a process in which glass is heated to near its softening point and then rapidly cooled with jets of air. This creates a characteristic stress pattern – the outer layers are in compression and the core in tension. As a result, toughened glass is four to five times stronger than ordinary glass.

Thermally, toughening allows glass to withstand temperature differences of 150 to 200°C – a huge difference compared with float glass. That is why toughened glass is always used where there is contact with hot water, for example with pull handles fitted near showers.

In extreme conditions, such as Finnish saunas where temperatures reach 90°C, toughening is absolutely essential. Accessories such as sauna hinges are only ever fitted to toughened glass, for safety and structural stability.

Which glass works at high temperatures?

Borosilicate glass contains boron oxides, which drastically lower the coefficient of thermal expansion. It withstands temperature differences of up to 300°C and is used in laboratory equipment and ovenware. It is used less often in construction because of its higher cost.

Glass-ceramic is another material with exceptional thermal properties. It withstands temperatures above 700°C and practically does not react to thermal shock. It is used in stove and fireplace doors, where other types of glass would not survive.

For standard building applications, toughened glass is enough. Shower hardware fitted to such glass safely withstands contact with hot water and steam. Toughening provides a sufficient safety margin in everyday use.

Where do thermal cracks come from?

Climatic cracks are a phenomenon specific to insulated glass units. Gas-filled cavities change volume with temperature and atmospheric pressure. When the temperature rises, the gas expands and pushes the panes outwards. In winter the opposite happens – negative pressure pulls the panes inwards.

These cyclical deformations create stresses at the edges and fixing points. In extreme cases they lead to characteristic cracks – usually starting at the edge and running across the whole pane. A climatic crack has a typical appearance: a line starting at a point near the edge of the pane.

The problem is worse when the glass is made at a different altitude from where it is installed. The pressure difference can be large enough for the pane to deform during transport. Installers must take this into account when planning deliveries.

How to avoid thermal shock problems

The first rule is to choose the right type of glass for the conditions of use. Where there is contact with hot water or steam, use only toughened glass. Never fit ordinary float glass in shower enclosures or near heat sources.

The second point is proper preparation of holes and cut-outs. All mechanical processing must be done before toughening – afterwards the glass cannot be drilled or ground. Plan the fixing positions carefully and make sure the glass manufacturer has precise dimensions.

The third aspect is the quality of installation. Fixing points must have suitable spacers to minimise stress concentration. Overtightening screws can create local stresses which, combined with thermal shock, lead to cracking.

Does glass thickness matter for thermal resistance?

Glass thickness affects thermal behaviour in a non-obvious way. A thicker panel has greater thermal inertia – it heats up more slowly but also releases heat more slowly. This means gentler temperature gradients between layers, which in theory reduces the risk of cracking.

On the other hand, thicker glass generates greater absolute stresses for the same temperature difference. In practice the differences are small, and the type of processing – toughened or not – matters far more than the thickness itself.

For typical building applications using 6 to 12 mm glass, the differences in thermal resistance are marginal. Toughening and choosing the right material for the operating conditions remain key.

What to look for when choosing glass

Define precisely the conditions in which the glass will work. Is it an interior with controlled temperature, or a wellness area with a sauna and high humidity? This information allows the manufacturer to choose the right material and processing.

Check the certificates and standards that apply to the glass. Toughening must be confirmed by the relevant documents. In public buildings, additional requirements for thermal strength may apply.

Allow enough lead time. Toughened glass needs special processing, which cannot be rushed without losing quality. Place orders well in advance, especially for larger projects.

The thermal resistance of glass is not an abstract property but a parameter that determines the durability and safety of a structure. Choosing the right material, professional installation and knowing the limits of each type of glass help avoid problems and ensure long, trouble-free service.