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- Thermal stress cracks start at the glass edge and run perpendicular for 20–30 mm before branching — this pattern is the diagnostic fingerprint that distinguishes them from impact cracks.
- A temperature differential of just 30–40°F (17–22°C) across the glass surface is enough to crack standard annealed glass — a sunny winter morning easily produces this.
- Tempered glass withstands temperature differentials up to 200°C — it is virtually immune to thermal stress fracture.
- Edge damage (chips, nicks, rough cuts) reduces glass strength by up to 50%, making thermal stress cracks far more likely.
- Thermal stress cracks cannot be repaired — the glass must be replaced.
Answer First: That crack that appeared overnight on your window — starting at the edge, running straight inward, no sign of impact — is a thermal stress fracture. It happens when one part of the glass heats up (sun hitting the centre) while the edges stay cold (buried in the frame). A temperature differential of just 30–40°F across the glass surface is enough to crack standard annealed glass. Toronto winters produce this exact scenario every sunny morning from November through March. The glass cannot be repaired. It needs to be replaced.
You wake up on a January morning, sun streaming through the south-facing living room window, and there it is. A crack running from the bottom edge of the glass, angling upward and then curving away. No baseball. No tree branch. No storm. The glass cracked itself.
It feels wrong. Glass should not just break for no reason. But it did not break for no reason. It broke because of physics.
How Thermal Stress Cracks Happen
The Expansion Problem
Glass expands when heated and contracts when cooled. This is true of all glass, all the time. The issue is not expansion itself — it is uneven expansion.
On a sunny winter morning in Toronto, the centre of a south-facing window pane absorbs solar radiation and heats up. The temperature of the glass at the centre might reach 15–25°C. Meanwhile, the edges of the glass — embedded 10–15 mm into the vinyl or aluminum frame — stay close to the outdoor ambient temperature, which could be -10°C or colder.
That is a 25–35°C differential across the surface of a single pane.
The heated centre expands outward. The cold edges resist. Stress builds at the boundary between the warm zone and the cold zone. When the stress exceeds the tensile strength of the glass at its weakest point — usually the edge — it cracks.
Standard annealed (float) glass can fracture from a temperature differential as small as 22°C (40°F) across its surface. A sunny January morning in Toronto easily exceeds this threshold on south and west-facing windows.
Why It Starts at the Edge
Glass edges are the weakest part of the pane. During manufacturing and cutting, the edge is scored, snapped, and sometimes ground. Every nick, chip, or micro-fracture along the edge acts as a stress concentrator. When thermal stress builds, the crack initiates at the weakest point on the edge and propagates inward.
Edge damage from manufacturing, handling, or installation reduces the thermal stress tolerance of glass by up to 50%. A pristine edge can handle a 40°F differential. A chipped edge might fail at 20°F.
The Crack Pattern
Thermal stress cracks have a distinctive signature:
- Origin at the edge — the crack starts within 5 mm of the glass perimeter
- Perpendicular start — the first 20–30 mm runs at roughly 90 degrees to the edge
- Curve or branch — after the initial straight run, the crack curves or forks as it follows the stress field
- No impact point — there is no starburst, bullseye, or radial pattern at the origin. The edge simply splits
If you see a starburst pattern, that is an impact crack — a rock, ball, or pressure point. If you see a clean line starting from the edge with no impact mark, that is thermal stress.
Risk Factors in Toronto/GTA Homes
Partial Shading
The number one risk factor. A window that is half in sun and half in shade (from a roof overhang, adjacent wall, tree shadow, or partially closed blind) has the highest temperature differential. The sunny half heats up; the shaded half stays cold. The boundary between the two zones is where the crack forms.
Partially open blinds are worse than fully open or fully closed. Fully open lets the entire pane heat evenly. Fully closed blocks solar gain entirely. Half-open creates a sharp thermal boundary across the glass.
South and West Exposure
South-facing windows in Toronto receive the most intense winter sun — low angle, direct radiation, for 4–6 hours per day. West-facing windows get intense afternoon sun when the outdoor temperature is still low. Both orientations are high-risk for thermal stress.
North-facing windows almost never suffer thermal stress cracks because they receive no direct sun.
Low-E Coatings
This is counterintuitive. Low-E coatings can increase thermal stress risk in certain configurations. A Low-E coating on surface 2 (the inner face of the outer pane) reflects heat inward, which is great for energy efficiency — but it also causes the outer pane to stay colder while the inner pane heats up. The temperature differential across the sealed unit increases.
This does not mean Low-E is bad. It means the glass specification must account for the thermal stress. In high-risk locations, the outer pane should be heat-strengthened or tempered when Low-E is specified.
Dark Window Films and Tints
Aftermarket window films — especially dark tints and solar control films — absorb heat and raise the glass temperature significantly. A dark film on a south-facing window can increase the glass temperature by 20–30°C above what uncoated glass would reach. This pushes the thermal differential past the breaking point.
If you are adding solar film to windows with standard annealed glass, confirm with the film manufacturer that the glass can handle the added thermal load. Many cannot. For more on window film applications, see our guide on anti-graffiti and solar film.
Single-Pane vs. Sealed Units
Single-pane glass is more vulnerable to thermal stress than sealed units. In a double-pane IGU, the insulating air gap moderates the temperature distribution across each pane. The inner pane never gets as cold as the outer pane, and neither pane experiences the full temperature swing alone.
Single-pane windows in older Toronto homes — common in pre-1980 construction — are the most frequent victims of thermal stress cracking.
Glass Types and Thermal Stress Resistance
| Glass Type | Max Safe Temperature Differential | Thermal Stress Risk |
|---|---|---|
| Annealed (float) | ~40°F / 22°C | High — the most vulnerable |
| Heat-strengthened | ~200°F / 110°C | Low — 3x stronger than annealed |
| Tempered (fully) | ~400°F / 200°C | Minimal — virtually immune |
Tempered glass is the solution for windows in high-risk thermal stress locations. It is 4–5 times stronger than annealed glass against thermal stress. The trade-off is that tempered glass cannot be cut or modified after tempering — it must be ordered to exact size.
Heat-strengthened glass is a middle ground: twice the thermal stress resistance of annealed, but without the shattering-into-small-pieces behavior of tempered. For windows that do not require safety glazing but are at thermal stress risk, heat-strengthened is the right spec.
What to Do When It Happens
Step 1: Confirm It Is Thermal Stress
Check the crack pattern. Edge origin, perpendicular start, no impact point. If the crack matches this pattern, it is thermal stress.
Step 2: Assess Urgency
A thermal stress crack does not mean the glass will fall out. The crack weakens the pane structurally, but the frame holds it in place. For sealed units (double or triple pane), the intact pane maintains the thermal barrier while the cracked pane is replaced.
That said, do not leave a cracked pane through another winter. Water enters the crack, freezes, and the crack grows. What was a single line becomes a web.
Step 3: Replace the Glass
For sealed units, this is a glass-only replacement — the cracked IGU is removed and a new sealed unit is installed in the existing frame. No need to replace the frame if it is in good condition.
For single-pane windows, the glass is re-glazed into the existing sash.
If the window has had a thermal stress crack, the replacement glass should be heat-strengthened or tempered to prevent recurrence. Replacing annealed with annealed in the same location invites the same failure next winter.
Cost
- Glass-only replacement (sealed unit, standard size): $150–$400 per window
- Tempered upgrade (sealed unit, custom): $250–$600 per window
- Single-pane reglazing: $75–$200 per pane
These are a fraction of the cost of a full window replacement ($500–$1,200+). Our residential window replacement service handles both glass-only swaps and full replacements across the GTA.
Prevention
- Keep blinds consistent. Fully open or fully closed. Never half-mast on sunny winter days.
- Inspect edges. If you can see chips or rough spots along the glass edge (visible where the glass meets the glazing bead), consider preemptive replacement with heat-strengthened glass.
- Avoid heat sources near windows. Portable heaters, radiators, and heating vents directed at glass amplify the temperature differential.
- Specify correctly for new installations. South and west-facing windows in new builds or renovations should use heat-strengthened or tempered glass as standard practice. The cost premium is 15–30% — cheap insurance against thermal stress failure.
- Be cautious with window film. Check the manufacturer's thermal stress guidelines before applying dark or reflective films to standard annealed glass.
Frequently Asked Questions
What does a thermal stress crack look like?
It starts at the edge of the glass and extends straight inward for 20–30 mm at a roughly 90-degree angle before curving or branching. There is no impact point (no starburst or bullseye). The crack appears to come from nowhere.
Can cold weather alone crack a window?
Cold alone does not crack glass. The crack happens when part of the glass is cold while another part heats up — for example, when winter sun hits the centre of a pane while the edges stay cold behind the frame. The differential expansion creates the stress.
Does insurance cover thermal stress cracks?
Most home insurance policies do not cover thermal stress cracks because they are classified as gradual deterioration or wear, not sudden accidental damage. Check your specific policy — some comprehensive plans include glass coverage as an add-on.
Can you repair a thermal stress crack?
No. Unlike windshield chips, thermal stress cracks in window glass cannot be injected with resin or patched. The internal tension remains. The glass must be replaced — either as a glass-only swap (if the frame is fine) or as a full unit replacement.
How do you prevent thermal stress cracks?
Keep blinds or curtains fully open or fully closed — partial coverage creates hot and cold zones. Avoid placing heat sources near windows. Ensure glass edges are clean-cut with no chips. For high-risk windows (south-facing, partially shaded), specify tempered or heat-strengthened glass.
Thermal stress crack on your window?
We replace cracked glass across the GTA — glass-only swaps for sealed units, or full window replacement if the frame is also at end of life. We can also assess your other windows for thermal stress risk and recommend upgrades to heat-strengthened or tempered glass where needed.
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