What Causes Thermal Stress Cracks in Windows?

Noticed a sudden crack in your window glass — but there’s no sign of impact? You might be dealing with something called a thermal stress crack, and it’s more common than you think. These types of cracks often catch homeowners off guard because they seem to appear out of nowhere.

Let’s break down what thermal stress cracks are, what causes them, and how to prevent them — especially if you’re considering new windows or planning a replacement.

What Is a Thermal Stress Crack in a Window?

A thermal stress crack is a specific type of glass fracture caused by a significant difference in temperature between different parts of the glass surface. When one area of the window heats up (say, from direct sunlight) while another part stays cooler (like in the shade), the glass expands at different rates. That internal tug-of-war leads to stress — and if the stress exceeds the tolerance of the material, the glass cracks.

These cracks typically start at the edge of the glass and extend inward. You might not hear it happen, but the result is a long, jagged line that grows slowly — and won’t stop until the glass is replaced.

Poor edge quality or chips from handling during installation significantly increase the risk of cracks. That’s why heat-strengthened glass and precision-manufactured window systems are critical for long-term performance.

Do You Need Window Replacement After a Stress Crack?

Yes — unfortunately, once a thermal stress crack forms, there’s no safe or effective way to fix it. Glass doesn’t “heal”. Even minor cracks will continue to spread due to ongoing temperature changes and structural stress.

If your window is cracked, replacement is the only viable option — and it’s an opportunity to upgrade to a better-performing system.

Common Causes of Thermal Stress Cracks in Windows

Thermal stress cracks don’t happen at random — there’s usually a perfect storm of conditions behind them. Here are several factors that increase the risk:

• Rapid temperature changes: If the interior stays cool and the outside glass surface is suddenly hit with sunlight or hot air, a temperature difference forms. This is especially common in spring and fall.
• Direct sunlight: South- or west-facing windows receive strong sun exposure, which creates high thermal gradients between sunlit and shaded areas.
• Low-emissivity coatings or films: While they improve energy efficiency, these coatings can intensify heat absorption if not properly matched to your glass type.
• Partial shading: If part of a window is shaded by a wall, tree, or eave, while the rest is in full sun, that contrast in exposure creates uneven expansion.
• Improper installation: A poorly supported window frame or misaligned sash can concentrate stress at specific points on the glass edge.
• Interior heat sources: Placing a space heater or candle near the window can cause the inner pane to heat rapidly, especially in double-pane windows.

How to Identify a Thermal Stress Crack vs. Other Cracks

Not all cracks in windows are thermal. Here’s how to tell them apart:

If the crack starts at the edge and there’s no obvious point of impact, there’s a good chance it’s thermal.

Why Do Thermal Stress Cracks Occur More Often in Certain Windows?

Some windows are more vulnerable than others. For example, tempered glass is stronger and more resistant to breakage from both impact and thermal stress. However, not all windows — especially older ones — are tempered.

Thermal stress crack vulnerability can be found in:

• Annealed glass (standard float glass) is more likely to crack under thermal stress. It’s less heat resistant and commonly found in older homes.
• Large panes have more surface area and therefore more potential for temperature variation — especially if part of the window is in shade while another is exposed to sun.
• Fixed windows, like picture windows, don’t allow airflow, which means heat can build up quickly on the surface.
• Double-pane windows can experience temperature differences between the inner and outer panes, especially in extreme weather.

Preventing Thermal Stress Cracks: What You Can Do

You can’t stop the sun from shining or the wind from blowing, but you can reduce your window’s vulnerability to thermal damage.

Here are a few ways to prevent thermal stress cracks:

• Use tempered or heat-strengthened glass for windows that get intense sun exposure.
• Avoid placing heat sources near your windows (space heaters, candles, etc.).
• Choose energy-efficient frames that expand evenly and don’t create hot spots.
• Install proper shading (awnings, overhangs, or even sheer curtains) to reduce harsh sunlight.
• Ensure high-quality installation so the window frame provides even support and doesn’t put pressure on the glass.
• Work with a trusted manufacturer that delivers consistent edge quality and proper glazing.