Why Did My Window Crack? Thermal, Impact and Pressure Cracks
Most windows that crack without being hit are thermal stress cracks: a single line that starts at the edge of the glass at a right angle, caused by the centre of the pane warming while the edge stays cold. A starburst from one point means an impact, and a curved, hourglass-shaped crack in double-pane glass points to pressure. None of these cracks can be mended, but the fix is glass, not a new window: the pane or sealed unit is replaced and a sound frame stays.
Thermal stress cracks start at the edge of the pane, run at roughly 90 degrees to it, and have no impact point.
In a GTA winter the sunny centre of a pane can reach 15–25 °C while the edge in the frame stays at −10 °C or colder, which is enough to crack ordinary glass.
Impact cracks radiate from a single point in a starburst; pressure cracks in double-pane units curve like an hourglass.
A cracked pane can't be mended, so the pane or sealed unit is replaced in the existing frame; one GTA installer lists $200–$650 for a sealed-unit swap.
Keep blinds fully open or fully closed and don't aim heaters or vents at glass to cut the risk of thermal cracks.
On this page
If your window cracked without anything hitting it, the most likely cause is thermal stress: the sunlit middle of the pane expanded while the edge, held cold in the frame, didn't. That crack starts at the edge and runs inward at close to a right angle. A starburst from one point means something hit the glass, and a curved crack in a double-pane unit usually points to pressure.
The fix is the same for all three. Cracked glass can't be repaired, so the pane or the sealed unit is replaced. If the frame is sound, it stays where it is.
Read the crack: a quick identification guide
Look at where the crack starts and what shape it takes. That tells you most of what you need.
| Crack type | What it looks like | Usual cause | Fix |
|---|---|---|---|
| Thermal stress | A single line that starts at the edge, runs at about 90° to it, and goes straight for at least 1/2 in (13 mm) before it may branch. No impact point. | Uneven heating: cold edge, warm centre | Replace pane or sealed unit |
| Impact | A starburst that radiates from one central point. Harder hits add rings around the point | A ball, stone, hail, a tool or a break-in attempt | Replace pane or sealed unit |
| Pressure | Curved, like the outline of an hourglass, in double-pane glass, with no impact point | Large pressure or elevation differences acting on the sealed air space | Replace sealed unit |
| Edge knock | A meandering single or branching line from the edge | A chipped edge, a slammed sash, frame movement | Replace pane or sealed unit, and check the frame |
| Tempered glass failure | The whole pane breaks into small, similar-sized granular pieces | Edge damage, impact, or rarely a nickel sulfide inclusion | New tempered glass made to size |
Thermal stress cracks: the classic GTA winter crack
Glass expands as it warms. When the middle of a pane heats up faster than its edges, the edges are pulled into tension. Glass picks up roughly 50 psi of stress for every °F of centre-to-edge difference, and the risk of breakage becomes significant once edge stress passes 2,000–3,000 psi.
A Toronto winter creates that gap easily. Morning sun can warm the centre of a pane to 15–25 °C while the edges, held in the frame, stay at −10 °C or colder, a difference of 25–35 °C. At Pearson, the January mean is −6.3 °C and the mean minimum is −10.5 °C (1971–2000 normals), so clear, freezing mornings are routine from December to March.
What makes thermal cracks more likely
- A freezing night followed by a sunny morning, and panes that are partly shaded.
- Half-drawn blinds and insulating shades, which leave a sharp line between warm and cold glass.
- Heat vents or space heaters aimed at the glass.
- Heat-absorbing and some low-E glass, which runs warmer in the sun.
- Large panes on shaded or recessed walls.
- Damaged edges. A chipped or rough edge fractures at lower stress than a clean-cut one.
How to lower the risk
- Keep blinds either fully open or fully closed on cold, sunny days rather than half-drawn.
- Redirect floor registers and keep space heaters away from the glass.
- When you replace glass on a big south- or west-facing opening, ask about heat-strengthened or tempered glass, which resists thermal stress better. It adds 15–30% to the cost of the unit.
For the rest of your cold-weather prep, see our GTA winter checklist for windows and doors.
Impact cracks
An impact crack has an obvious origin: a small crushed spot with lines radiating out of it. A hard hit on ordinary annealed glass produces many radiating lines with circular rings around the point of impact.
Sudden damage from storms, vandalism and break-ins is generally what home policies cover, subject to the deductible. Our guide to home insurance and broken windows in Ontario explains when a claim is worth it. Before you tidy up, take photos of the crack and whatever caused it.
Pressure cracks in double-pane windows
A sealed unit traps a fixed pocket of air or argon between two panes. When the pressure outside changes a lot compared with the pressure sealed inside, the panes flex. These cracks happen when units are installed at a very different elevation from where they were made, or during drastic weather-system pressure changes. They typically show as a curved, hourglass-like crack with no impact point.
If you find a curved crack in a fairly new unit, keep the paperwork and look for the manufacturer's mark on the spacer bar between the panes. Certified sealed units carry a permanent mark identifying the manufacturer and year of manufacture.
When tempered glass "explodes"
Tempered glass is used in patio doors, storm doors and shower enclosures. When it breaks, it breaks all at once into small granular pieces. Occasionally that happens with no obvious cause.
Nickel sulfide stones, tiny inclusions 0.003–0.015 in across, can cause spontaneous breakage in tempered glass only. It is extremely rare: edge or surface damage, frame movement and vandalism are far more common causes of an unexplained break. Heat-soak testing (EN 14179-1, at 260 °C for two hours) reduces the risk but can't eliminate it.
For where tempered glass is required and how it compares with laminated, see tempered vs laminated glass in Ontario.
Is a cracked window urgent?
| Situation | How urgent | What to do now |
|---|---|---|
| One crack, pane intact, no loose pieces | Book a glass replacement soon | Run clear tape along the crack on the inside to keep it together, and keep people from leaning on the glass |
| Crack in one pane of a double-pane unit | Book soon | The unit's seal is compromised and it may start to fog; the whole sealed unit gets replaced |
| Crack is spreading day by day | Book this week | Close blinds fully or open them fully; don't aim heat at the glass |
| Loose shards, a hole, or a ground-floor window that no longer locks | Same day | Cover or board the opening; see what to do in the first hour |
| Shattered tempered door glass | Same day | Keep everyone away, clear the pieces, and secure the opening |
Epoxy crack fillers are a visible, temporary patch, and published US price guides put them at $25–$50 USD. They don't restore strength, and they don't reseal a double-pane unit.
New glass, same frame: what the fix involves
Thermal and impact cracks can't be repaired; the glass has to be replaced. The good news is that you rarely need a whole new window: the glass is replaced and the frame stays. The exception is a frame that is itself rotted, cracked or warped; that's a window-replacement job, not a glass repair.
In a double-pane window, the two panes and the edge seal form one sealed unit. A crack in either pane means replacing the full unit, because it can't be half-repaired. A glass-only swap removes the glazing beads, lifts out the old unit, sets the new one on fresh setting blocks and snaps the beads back. That's about 20–30 minutes on site per opening. Sealed units are made to measure, so expect roughly 1.5–3 weeks between measuring and installation.
Typical GTA costs
| Job | Typical range (CAD, before tax) |
|---|---|
| Single-pane reglaze | $100–$250 |
| Sealed-unit swap after a thermal crack | $200–$650 |
| Upgrade to heat-strengthened or tempered | +15–30% |
| Full window replacement (what a glass-only fix avoids) | $700–$1,500+ |
Check the warranty first
Some manufacturers cover thermal stress cracks on new windows. Sealed-unit warranties, on the other hand, often exclude cracked panes and usually cover labour for only the first year or two. If the window is less than a few years old, find the purchase paperwork and the date mark on the spacer before you book.
What we'd do
When you send us a photo or we come out, we check four things:
- Where the crack starts. An edge origin at a right angle points to thermal stress. A central point means impact.
- Whether it's a sealed unit and whether it's fogging. A cracked sealed unit is replaced as a whole.
- Whether the glass is tempered. We look for the stamp in the corner. Tempered glass can't be cut, so it has to be ordered to size.
- Whether the frame and sash are sound. If they are, we measure for a glass-only replacement and keep the frame. If they aren't, we'll tell you it's a window-replacement job rather than a glass repair.
If the opening isn't safe or secure in the meantime, we cover or board it until the new glass arrives.
Next step
A crack rarely gets smaller, and in a double-pane unit it lets moisture in. Send us a photo through our contact page for a free quote, or read more about our broken window repair service.