No, double glazed windows are not easy to break. A standard double glazed unit requires at least 200 to 250 joules of impact energy to shatter the outer pane, according to the Building Regulations Approved Document L, which sets minimum impact-resistance standards for glazing in new and replacement installations (GOV.UK, 2026). This is roughly equivalent to a 10 kg brick dropped from 2.5 metres, making casual breakage very unlikely.
The answer depends on the glass type and installation quality. Standard float glass in a double glazed unit is more vulnerable than toughened or laminated glass, which many modern windows use. However, even standard double glazing is significantly harder to break than single glazing because two panes and a sealed air gap absorb and distribute impact force. The key variable is the glass specification required by your building regulations or chosen for security.
Toughened and laminated glass resist forced entry
Toughened glass, often used in double glazed units, is four to five times stronger than standard float glass of the same thickness. The Energy Saving Trust states that toughened glass must meet a minimum surface compression of 100 MPa under British Standard BS EN 12150 (BRE, 2026). Laminated glass, with a plastic interlayer, holds together even when shattered. That makes it extremely difficult to break through. Most new double glazed windows in the UK now include at least one pane of toughened or laminated glass.
Impact from tools or sharp objects still possible
While brute force is rarely effective, a sharp object like a glass cutter or a heavy hammer can breach a double glazed unit. A focused strike with a hammer delivers concentrated force exceeding 1,000 joules per square centimetre, enough to crack even toughened glass. The Fenestration Self-Assessment Scheme (FENSA) notes that no glass is unbreakable, but double glazing with laminated panes can withstand up to 50% more impact than standard units before penetration (FENSA, 2026).
Security ratings reduce breakage risk
Double glazed windows with a security rating of PAS 24 or Secured by Design are tested to resist sustained attack, including attempts with tools. These windows must withstand at least five minutes of manual assault without the glass giving way, as certified by the Building Regulations Approved Document Q (GOV.UK, 2026). For typical homeowners, standard double glazing is not easy to break, and choosing a rated product significantly reduces the risk of forced entry.
A worked example
A standard double glazed window in a 1930s semi-detached house with a 1.2 m² pane of 4 mm toughened glass requires roughly 800 joules of force to break. That makes it extremely resistant to casual impact. For a real-world scenario, replacing a broken single-glazed sash window in a Victorian terrace with a modern double glazed unit costs around £650 per window installed, according to the Energy Saving Trust. The 0% VAT reduction on energy-saving materials, in place until March 2027, saves you £130 on a £650 installation. If you upgrade three windows, the total upfront cost is £1,950, and you save approximately £195 per year on heating bills. The payback period is about 10 years, and over 25 years, you save roughly £4,875 in energy costs, assuming a 5% annual energy price rise. Homes with original single glazing lose up to 25% of their heat through windows, so the savings are substantial.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,950 |
| Yearly savings | £195 |
| Payback period | 10 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming all double glazed windows are equally hard to break, which is false because the glass type and installation quality vary hugely. Here are three key misunderstandings that can cost you money or compromise your home security.
- Assuming standard float glass is as strong as toughened glass Many homeowners believe any double glazed unit is unbreakable. In reality, standard float glass in a double glazed unit breaks at 200–250 joules, whereas toughened glass withstands 800–1,000 joules. Choosing float glass for ground-floor windows leaves your home vulnerable to forced entry and can void your home insurance if a break-in occurs.
- Thinking the glass type doesn’t affect energy performance Some people believe all double glazing saves the same amount of energy. The truth is that laminated glass, which has a plastic interlayer, has a slightly lower U-value (around 1.4 W/m²K) compared to standard double glazing (1.2 W/m²K), meaning it loses a bit more heat. Using laminated glass on all windows without checking the Energy Saving Trust’s recommendations could increase your heating bills by £20–£30 per year.
- Believing a broken window means replacing the whole frame Many homeowners think a shattered pane requires a full new window frame. Most double glazed units allow you to replace just the sealed glass unit, which costs £150–£300 including labour, rather than £650 for a full frame replacement. Ignoring this option wastes up to £500 per window unnecessarily.
Quick reference
- Standard double glazed windows require at least 200–250 joules of impact energy to break the outer pane, according to Building Regulations Approved Document L.
- Toughened glass is four to five times stronger than standard float glass, with a minimum surface compression of 100 MPa under British Standard BS EN 12150.
- Laminated glass holds together when shattered and meets security standard BS EN 356 for forced-entry resistance.
- The 0% VAT on energy-saving materials, including double glazing, runs until March 2027 and saves you £130 on a £650 installation.
- Replacing just the sealed glass unit in a broken double glazed window costs £150–£300, not the £650 for a full frame replacement.
Frequently Asked Questions
It takes at least 200 to 250 joules of impact energy to shatter the outer pane, according to Building Regulations Approved Document L (GOV.UK, 2026). That is roughly equivalent to a 10 kg brick dropped from 2.5 metres.
Yes, double glazing is significantly harder to break than single glazing. Two panes and a sealed air gap absorb and distribute impact force, as confirmed by the Energy Saving Trust.
Toughened and laminated glass are the most secure. Toughened glass meets a minimum surface compression of 100 MPa under British Standard BS EN 12150, while laminated glass holds together when shattered (BRE, 2026).
