The causes of thermal stress fractures and how to avoid them
Thermal stress cracks in glass can be source of concern for homeowners. These fractures compromise the energy efficiency, safety, and aesthetics of a home.
Here, we’ll explore what thermal stress fractures are, how thermal stress in glass occurs, the conditions that lead to it and, importantly, how to prevent it.
Identifying a glass thermal stress fracture
There are some common characteristics associated with glass thermal stress cracks:
Appearance
Stress fractures usually start as small cracks and gradually expand across the pane.
Location
Typically, a thermal stress glass breakage pattern initiates from the edges of the pane and expands inwards.
Sound
Some homeowners report hearing a ‘pop’ when the crack initially forms.
How do thermal stress fractures occur?
Several factors contribute to thermal stress fractures:
Sudden temperature changes
Thermal stress or ‘shock’ occurs when one area of a pane of glass gets hotter than an adjacent area. For example, this can occur when sunlight hits a cold window, causing uneven expansion and contraction in the glass.
Whilst mid-summer is the time of year when the sun is at its hottest, as the sun’s path is higher in the sky, there is less direct heat onto windows.
During spring and autumn, when the sun’s trajectory is at a lower angle and the nighttime temperature is lower, thermal stress fractures can be more common.
Window treatments
Curtains and blinds, especially dark ones, can trap heat against the glass. If one section of the window is exposed to heat while the other remains cool, this increases the risk of cracking.
Simple changes to a window such as posters or vinyl signs being stuck to it, or solar film applied to reflect heat, can cause thermal inconsistency across a pane of glass. This places it at greater risk of fracture.
Glass type and thickness
Toughened glass is very resilient to thermal stress.
Laminated glass is constructed of two or more sheets of glass bonded together by an inner plastic layer. Laminated glass thermal stress is less common, as is tempered glass thermal stress, as the glass has gone through a process of heat-strengthening.
Thicker glass can be less tolerant to thermal stress, and glass containing wire is also more vulnerable.
Because stress fractures often begin the edge of a glass pane, the quality of the glass edge is important. A clean and polished edge will withstand the greatest amount of thermal stress.
Solar control glass will either reflect the sun’s heat or absorb it to reradiate heat outwards. The correct solar control glass will depend on the function and aspect of a particular building or room.
Glass thermal stress analysis
Glass manufacturers, Pilkington, use a checklist of factors that form a glass thermal stress calculator.
By carefully considering these aspects together, this risk of thermal stress can be eliminated:
- The building’s location and aspect
- Type of glass used
- Internal window blinds or external shutters
- Window frame material including frame colour
- Window size including angle/aspect of opening
- Details of internal heating systems
- Details of nearby buildings or trees creating shadow onto glass panes
How to avoid thermal stress fractures
If a window has a stress fracture, then the whole unit must be replaced. This is not usually covered under guarantees provided by window installation companies.
The good news is that there are several ways to prevent thermal stress fractures. Whether you are considering replacing your windows, or simply want to maintain the comfort and efficiency of your home:
Heat-resistant glass
Both toughened and tempered glass are designed to withstand greater temperature fluctuations.
Double-glazed windows
With two glass panes separated by insulating argon gas, double-glazed windows distribute temperature more evenly across the glass, reducing the risk of thermal fractures.
Window treatments
Dark coloured blinds and curtains absorb more heat, which can increase the risk of thermal stress. Light colours reflect sunlight, helping to keep a consistent temperature.
Air circulation
With modern replacement windows, trickle vents allow adequate airflow to create a healthy, ventilated environment.
Keeping curtains or blinds a short distance from the window also increases airflow, preventing heat from being trapped against the glass.
External shading
Depending on your home’s aspect, you may wish to consider exterior shading, such as an awning, which can block direct sunlight and minimise temperature extremes.
Talk to the experts
If you are considering replacement windows, we’ll guide you through all the considerations to help you make the best decision for a secure, well-insulated home.
Please visit our showroom or call us on 01444 45 71 45 to chat through your requirements.
Further reading about thermal stresses on glass ceramics:
Thermal stress resistance on glass ceramics.