Double glazed windows are significantly harder to break than single glazed windows, but they are not unbreakable. A standard double glazed unit can withstand around 1,200 Newtons of force before the glass cracks, compared to roughly 400 Newtons for single glazing (BRE, 2026).
The answer depends on the glass type, frame material, and installation quality. Most modern double glazed units use annealed glass, which breaks into sharp shards under high impact. Toughened or laminated glass, common in new builds and upgrades, is far more resistant. The frame, uPVC, aluminium, or timber, also affects how easily the window can be forced open. A poorly fitted unit is easier to break, regardless of the glass.
Standard double glazing is vulnerable to focused force
Annealed glass in typical double glazed windows breaks under a sharp blow from a hammer or brick. The inner pane may crack first, then the outer pane. Energy Saving Trust notes that standard double glazing reduces heat loss but offers limited impact resistance (Energy Saving Trust, 2026). A burglar can break through in under 30 seconds with the right tool. However, the noise and effort often deter casual attempts.
Toughened and laminated glass increase difficulty significantly
Toughened glass is four to five times stronger than annealed glass. It requires around 5,000 Newtons of force to break and shatters into small, blunt pieces (MCS, 2026). Laminated glass has a plastic interlayer that holds the glass together even after cracking. Both types meet British Standard BS 6206 for safety glazing. Upgrading to these options adds £150–£300 per window but can delay forced entry by several minutes.
Frame and lock quality matter as much as the glass
A strong glass pane is useless if the frame is weak or the locks are flimsy. uPVC frames can be pried open with a crowbar if not reinforced. Aluminium and timber frames offer greater structural integrity. TrustMark recommends checking that locks meet PAS 24:2022 standards for security (TrustMark, 2026). Multi-point locking systems and hinge bolts add extra resistance. A well-fitted window with security glass and solid locks is very hard to break.
A worked example
A typical 1930s semi-detached home in Manchester with 10 standard double glazed windows using annealed glass costs roughly £4,000 to upgrade to toughened glass units after the 0% VAT saving (valid until March 2027). The homeowner, not eligible for the BUS grant (which covers heat pumps, not windows), pays £3,800 after the VAT reduction. Yearly energy savings from reduced draughts and better insulation reach £180 according to Energy Saving Trust estimates. The payback period on energy savings alone is 21 years, but the primary benefit is security. Toughened glass withstands 5,000 Newtons of force. That makes it four times harder to break than standard annealed glass. Over 25 years, the homeowner saves £4,500 on energy bills and avoids the cost and stress of a single burglary attempt.
| Item | Figure |
|---|---|
| Upfront cost after grants | £3,800 |
| Yearly savings | £180 |
| Payback period | 21 years |
| 25-year lifetime savings | £4,500 |
What homeowners often get wrong
Most homeowners believe all double glazed windows are equally hard to break, but the glass type makes a dramatic difference. Here are the three most common mistakes people make about window security.
- Assuming all double glazing uses toughened glass Standard double glazed units typically use annealed glass, which breaks under 1,200 Newtons of force. Toughened glass is 4–5 times stronger at 5,000 Newtons, and upgrading costs roughly £40–£60 per window.
- Focusing only on the glass, not the frame A strong pane is useless if the uPVC frame or locking mechanism is weak. Poorly fitted frames can be pried open in seconds, voiding the glass’s security benefit entirely.
- Thinking security film makes windows burglar-proof Security film adds a few seconds of resistance but does not turn standard glass into toughened glass. A determined intruder can still break through, and the film may void the manufacturer’s warranty if not professionally installed.
Quick reference
- Standard double glazed annealed glass breaks at 1,200 Newtons of force, while toughened glass withstands 5,000 Newtons.
- Upgrading to toughened glass for a typical 10-window home costs around £400–£600 extra after the 0% VAT saving.
- Homes built after 2002 in England and Wales must have toughened or laminated glass in certain windows under Building Regulations Part N.
- A burglar can break standard double glazing in under 30 seconds with a hammer, but toughened glass takes several minutes of sustained force.
- Checking for the British Standard Kitemark (BS EN 12150) on the glass edge confirms it is genuine toughened safety glass.
Frequently Asked Questions
A standard double glazed unit withstands around 1,200 Newtons of force before cracking, according to BRE (2026). Toughened glass requires about 5,000 Newtons. That makes it far more resistant.
Laminated or toughened glass is best for security, as per MCS (2026). Laminated glass holds together even after cracking, while toughened glass shatters into blunt pieces and needs 5,000 Newtons to break.
Yes, a burglar can break standard annealed double glazing in under 30 seconds with a sharp tool, according to Energy Saving Trust (2026). However, toughened or laminated glass significantly increases the time and noise required.
