Every January we get the same call. Nobody threw anything, nobody slammed a door, and there is a crack running across the living room window that was not there at bedtime. It is almost always thermal stress.
What thermal stress looks like
You can usually tell it apart from impact damage without touching the glass.
- It starts at an edge, roughly at a right angle to the frame, then curves gently across the pane.
- There is no chip, no starburst and no centre point.
- The line is smooth rather than shattered into pieces.
Impact cracks radiate out from a point. Stress cracks wander.
Why a cold morning does it
Glass expands when it warms and shrinks when it cools. On a clear morning after a cold night, low winter sun hits part of the pane while the rest sits in shade from a soffit, a mullion, a screen or a snowbank. The warm part grows, the cold part does not, and the tension has to go somewhere.
Three things make it more likely on Hamilton homes:
- Edge damage. A tiny nick in the edge from the original install is where the crack starts, sometimes twenty years later.
- Shade lines. Deep sills, awnings and Mountain-brow homes with morning shadow across half the window.
- Heat trapped at the glass. A blanket over the window, furniture pushed tight against it, or a rad blasting straight up the pane.
Big south and west facing panes take the worst of it, which is why we see more of these on newer builds with wide picture windows than on old double-hungs.
What to do the same day
- Tape both sides of the crack with clear packing tape so the pieces stay put.
- Keep the heat away from that window. Move the space heater, open the curtain evenly.
- Do not push, wash or force the sash. A cracked pane in a slider or crank window can let go while it moves.
- Take a photo and measure the visible glass, width then height.
A cracked pane is not an emergency unless the glass is falling out or the opening is not secure. If it is, we board it up the same day and glaze it after.
What it costs to fix
The crack does not shrink and it does not stop. Once glass has split, the pane is finished, so the job is a replacement rather than a repair.
| Situation | Cost |
|---|---|
| Single pane | $150 to $350 installed |
| Sealed unit, standard size | $350 to $650 installed |
| Large picture window or tempered | $650 to $900+ |
| Board up first, glass after | quoted on the call |
Stock sizes often go in the same day. A made-to-measure sealed unit is usually three to seven business days once we have the measurements.
Stopping the next one
You cannot change the sun, but you can remove the tension. Keep heat sources off the glass, avoid dark film on an existing sealed unit, and get failed caulking and blocked weep holes sorted before winter so water is not sitting at the edge of the pane. Caulking runs $75 to $150 per window and often gets done at the same visit.
If a crack turned up this morning, send us a photo and the size and you will have a price on the call.