How can I tell the crack is from thermal shock?

It ALWAYS starts from the pane edge (inside the frame) at a 90° angle for the first 2-3 cm, then wanders irregularly like a snake. There's no impact point.

How do I prevent thermal shock?

Replace plain glass with Securit (Tempered), which withstands temperature differentials over 200°C. Avoid dark stickers, furniture pressed against the pane and AC vents blowing directly on the glass.

Thermal Shock Glass Breakage: Why Does the Window Crack "On Its Own" and How to Prevent It

It's one of the biggest mysteries a homeowner can face. You wake up on a sunny morning, head to the living room with your coffee, and see a huge, strange crack running across the balcony door glass.

Your first thought is that someone tried to break in. You check the area, but there's no impact mark. No piece is missing, no stone lying around, and the house was locked. You start wondering if your house has "ghosts" or if the building has settled.

The truth is far more mundane and hides in the laws of physics. Your window fell victim to Thermal Shock (Thermal Stress). Let's see in plain language why a seemingly healthy pane "self-destructs" and how you can ensure it never happens again.

1. What Is Thermal Shock? (The Physics Behind the Crack)

Like almost all solid materials, glass expands (grows) when heated and contracts (shrinks) when cooled.

Uneven glass heating - sunny and shaded areas creating stress

☀️ Uniform Heating

When your entire window is bathed in sunlight, it heats and expands uniformly. No problem.

⚡ Uneven Heating

The problem starts when the glass heats unevenly. Imagine that half your window is hit by the burning midday sun while the other half sits in shade (e.g. from an awning, a tree or the building opposite). The sunny section tries to expand; the shaded section resists. This "tug of war" creates enormous mechanical stress. When the temperature difference exceeds 30°C – 40°C (for plain glass), the pane tears.

2. The Thermal Shock "Signature"

Characteristic thermal shock crack - starts perpendicular from the pane edge

How can you tell the crack came from thermal shock rather than, say, a tennis ball?

📐 The Characteristic Crack

A thermal crack has a very distinctive "signature": It ALWAYS starts from the pane edge (where it's hidden inside the aluminium), forming a perfectly perpendicular angle (90 degrees) with the frame for the first 2-3 centimetres. It then begins to wander irregularly (like a snake) towards the centre. There is no "impact point" (the spider-web pattern you see when something strikes glass).

3. The 4 Most Common "Culprits"

If it happened to you, one of the following is most likely to blame:

Thermal shock causes - awning, curtain, stickers, air conditioning

🏖️ 1. Partial Shading

The most classic cause. An awning lowered to the middle of the window, a tree, or even the deep recess of the wall itself casting a "hard" shadow diagonally across the pane.

🛋️ 2. Interior Obstructions & Curtains

Have you pushed a sofa, a large cushion or a heavy, dark curtain right against the glass? These objects trap the sun's heat between the fabric and the glass, locally overheating the area (Heat Trap).

🏷️ 3. Stickers & Posters

Any dark-coloured sticker (even large alarm stickers) absorbs solar radiation, creating a localised "hot" spot (hotspot).

❄️ 4. AC Ducts / Radiators

If hot air from the air conditioner or heat from a radiator blows directly onto the ice-cold (in winter) interior pane, thermal shock is just a matter of time.

⚠️ Low-E & Tinted Glass

Modern Low-E and tinted panes are more susceptible to thermal shock if not manufactured correctly, because by nature they absorb/reflect more thermal energy.

4. The Definitive Solution: Thermal Toughening (Securit)

The biggest mistake you can make when a pane breaks from thermal shock is to call a technician and ask for "exactly the same one". If the conditions (shade, sun) remain the same, the new pane will crack again with mathematical certainty!

Securit glass - withstands temperature differentials over 200°C

🛡️ Securit (Tempered Glass)

The only definitive solution is to replace the plain glass with Securit (Tempered Glass). As we saw in the previous article, Securit is baked at 700°C. This process doesn't just make it resistant to mechanical impacts - it also gives it staggering resistance to temperature differences. While plain glass breaks at a ~30°C differential, Securit glass can withstand temperature differences exceeding 200°C! For all practical purposes, it is "immune" to thermal shock.

5. Summary

✅ The Warning Sign

A spontaneous window crack is not cause for panic, but a warning sign that plain (float) glass cannot handle the local sun-and-shade conditions around your home. Avoid pressing furniture and dark curtains against the glass and, when it's time for replacement, invest in the right technology.

Triplex (Laminated) vs Securit (Tempered) Glass: What Are the Differences Safety Regulations: Where Is Safety Glass Mandatory? What Are Low-E Energy Glass Panes and How Do They Work Fire-Rated Glass (E, EW, EI): How It Withstands Fire

Glass Panes & Glazing Units: 80% of the Window

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