Yes, you can temporarily seal a hairline crack in a window pane using a few common products — but it's important to understand this is a short-term measure only. Glass with any crack is structurally compromised and can fail suddenly, especially in wind or if the crack spreads. A permanent fix always means replacing the glass, and for double-glazed or safety-glass units, that job is best left to a qualified glazier.
Why Temporary Sealing Makes Sense (and Its Limits)
A hairline crack — one that hasn't yet spread and where the glass is still held firmly in the frame — can be sealed temporarily to:
- Stop draughts and moisture getting into your home
- Slow the spread of the crack while you arrange a proper repair
- Reduce the risk of the crack worsening quickly from wind pressure or temperature changes
However, no sealant restores the structural strength of the glass. If the crack is in a safety-critical location (a door panel, low-level window, or anywhere subject to impact), treat it as urgent and don't delay calling a glazier. NZ Building Code clause F4 (Safety from Falling) and the requirements around safety glazing mean certain locations legally require safety glass — a cracked pane in those areas is a compliance and safety issue.
What You'll Need
Step-by-Step: Temporary Sealing Method
1. Clean the area thoroughly
Wipe both sides of the crack with isopropyl alcohol or methylated spirits and a lint-free cloth. Remove all dust, grease, and moisture. Any contamination will stop the sealant bonding properly.
2. Apply clear tape as an immediate stop-gap
For a very quick fix — especially if rain is coming — press a strip of clear packing tape firmly over the crack on the inside of the window. Smooth out all air bubbles. This won't last long but will keep the weather out overnight.
3. Use a clear epoxy or glass repair resin for a better temporary seal
- Mix the two-part epoxy according to the manufacturer's instructions, or use a ready-to-apply glass repair resin.
- Using a toothpick or fine applicator, carefully work the resin into the crack from one end to the other.
- Wipe away any excess immediately with a clean cloth.
- Allow to cure fully before exposing the window to rain or direct sunlight (check the product's cure time).
- Some resins are designed to be applied and then cured with UV light — follow the specific product directions.
4. Seal the edges
If the crack reaches the edge of the pane or the glazing rebate, apply a thin bead of clear silicone sealant (such as a product from Sika) along the frame junction to prevent water ingress.
What to Avoid
- Don't use super glue (cyanoacrylate) — it can cloud glass and is very difficult to remove.
- Don't apply heat to try to fuse the crack — this can cause the glass to shatter.
- Don't ignore a crack that is spreading — if you see the crack growing, the pane needs immediate replacement.
- Don't attempt this on double-glazed (IGU) units — once the seal of an insulated glass unit is compromised, condensation will form between the panes and the unit must be replaced professionally.
Double Glazing and Laminated Glass: Special Considerations
If your window is a double-glazed unit, a hairline crack in the outer pane can allow moisture into the cavity, causing fogging that cannot be reversed without replacing the entire unit. Temporary sealing of the outer surface may slow moisture entry, but replacement is the only real solution.
Laminated safety glass (common in doors, low-level windows, and some modern homes) will hold together even when cracked, but it still needs replacing — the interlayer degrades over time once the glass is broken.
For guidance on what glazing is required where in NZ homes, BRANZ and MBIE Building Performance are reliable references.
Safety Note
Always wear gloves when working near cracked glass — even a hairline crack can have sharp edges. If the crack is large, jagged, or the glass feels loose in the frame, do not attempt to seal it yourself. Stand clear and call a glazier immediately.
For workplace situations, WorkSafe NZ has guidance on managing broken glass hazards.
When to Call a Professional
Call a qualified glazier if:
- The crack is longer than about 30 cm or is spreading
- The window is in a safety glazing location (door, sidelight, low-level panel, or bathroom)
- The glass is double-glazed, laminated, or toughened
- The crack reaches the frame or the glass feels loose
- You're a landlord — under the Healthy Homes Standards, rental properties must be weathertight, and a cracked window may be a compliance issue (see Tenancy Services)
- You simply want the job done properly and safely
Glazing work itself is not legally restricted work in the same way as electrical or plumbing, but working with glass carries real injury risk. A professional glazier will have the right tools, safety equipment, and access to correct replacement glass. You can find qualified local glaziers on tradedirectory.co.nz.
Window & Glass Association
