The most common overall thickness for sealed double-glazed units in UK homes is 24 mm to 28 mm, with the cavity between the two panes typically 12 mm to 16 mm wide, according to the Energy Saving Trust (Energy Saving Trust, 2026). The exact figure depends on the glass type, frame design and whether the unit is standard or acoustic.
The total thickness is the sum of the two glass panes plus the cavity. Standard double glazing uses 4 mm glass on each side and a 12 mm to 16 mm argon-filled gap, giving 20 mm to 24 mm. With 6 mm glass on each side and a 16 mm gap, the unit reaches 28 mm. Replacement units must match the original frame’s rebate depth, typically 18 mm to 28 mm in uPVC and timber frames.
Standard double glazing thickness range
A typical sealed unit uses two 4 mm panes of float glass separated by a 12 mm to 16 mm air or gas cavity. This gives an overall thickness of 20 mm to 24 mm. The most common off-the-shelf size is 24 mm (4 mm glass + 16 mm cavity + 4 mm glass). For retrofit replacements, many UK homes built from the 1990s onwards use 24 mm units in uPVC frames (UK Building Regulations Approved Document L, 2026). Thinner 20 mm units were common in older aluminium or timber frames.
Thicker glass for acoustic or security glazing
If you need sound reduction or enhanced security, glass thickness increases. Acoustic double glazing often uses a 6.4 mm laminated pane on one side and a 4 mm pane on the other, with a 12 mm to 16 mm cavity. This gives a total thickness of 22.4 mm to 26.4 mm. Security glazing may use 6 mm toughened glass on both sides, producing a 28 mm to 32 mm unit. The glass must be certified to British Standard BS EN 12150 for toughened glass or BS EN 356 for security glazing (BRE, 2026).
Cavity width and thermal performance
The gap between the panes is the key variable for energy efficiency. A 12 mm cavity filled with argon gives a U-value of around 1.8 W/m²K, while a 16 mm argon-filled gap can achieve 1.4 W/m²K or lower. Cavities wider than 20 mm do not improve performance due to increased convection inside the gap (MCS Certified, 2026). For new installations, Building Regulations require a centre-pane U-value of 1.2 W/m²K or better, which typically demands a 16 mm to 18 mm cavity with low-emissivity coating.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates annual heating savings of roughly £195 for a gas-heated home of this size, with carbon savings of 330 kg CO₂ per year. Payback on the investment lands at approximately 23 years based on current energy prices, though comfort gains and reduced condensation are immediate. Over a 25-year lifespan, the total saving reaches about £4,875, meaning the windows pay for themselves and then some. If the household qualifies for ECO4, the upfront cost could drop further or even be fully covered, though eligibility depends on benefits and property energy performance as assessed by gov.uk.
| Item | Figure |
|---|---|
| Upfront cost after grants | £4,500 |
| Yearly savings | £195 |
| Payback period | 23 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming a thicker double glazed unit always performs better for insulation. This leads to three specific errors that cost homeowners money and comfort.
- Ordering the thickest unit possible Many homeowners think 28 mm units are automatically superior to 24 mm ones. In reality, if your frame rebate is designed for 24 mm, fitting a 28 mm unit can cause the glass to bulge, break the seal, and void the warranty within a year.
- Ignoring the cavity width People focus on total thickness but forget the gas-filled gap between panes is what matters for insulation. A 20 mm unit with a 12 mm cavity can perform worse than a 24 mm unit with a 16 mm cavity, wasting up to £50 per year on extra heating compared to the better option.
- Assuming all 24 mm units are equal Homeowners buy the cheapest 24 mm unit without checking the glass quality. A standard float glass 24 mm unit has a U-value around 1.4 W/m²K, whereas a low-E coated 24 mm unit with warm-edge spacers can drop to 1.0 W/m²K, saving an extra £30 annually according to Energy Saving Trust.
Quick reference
- Standard UK double glazed units are 24 mm total thickness, using two 4 mm panes with a 16 mm cavity.
- Thinner 20 mm units were common in older aluminium or timber frames from the 1980s and are less energy efficient.
- For ECO4 eligibility, your property must have an EPC rating of E, F, or G and you must receive a qualifying benefit.
- Replacing single glazing with 24 mm double glazing saves a typical semi-detached home around £195 per year on heating bills.
- Never order a thicker unit than your frame rebate depth allows, or the seal may fail and the warranty will be void.
Frequently Asked Questions
The standard overall thickness is 24 mm to 28 mm, according to the Energy Saving Trust. Most homes built from the 1990s use 24 mm units.
Replacement units must match your frame's rebate depth, typically 18 mm to 28 mm for uPVC and timber frames. Measure the cavity plus glass thickness.
Acoustic double glazing uses a 6.4 mm laminated pane on one side, giving a total thickness of around 24 mm to 28 mm. Thicker glass improves sound and security.
