No, standard double glazing does not contain a vacuum; it uses an insulating gas layer between two panes of glass. Vacuum glazing is a separate, advanced technology that contains a near-vacuum gap, but it is not what most UK homeowners have installed in their homes. The Energy Saving Trust confirms that typical double glazing relies on argon or krypton gas to reduce heat loss (Energy Saving Trust, 2026).
If you are asking whether your existing double-glazed windows have a vacuum, the answer is almost certainly no. Vacuum glazing is a niche product, far more expensive and less common than standard double glazing. It is primarily used in heritage properties where slim frames are required, or in high-performance retrofit projects. The key variable is the gap between the panes: standard double glazing has a 12–20mm gas-filled gap, while vacuum glazing has a sub-millimetre gap under vacuum pressure.
Standard double glazing uses gas not vacuum
Standard double glazing units (DGUs) have two panes of glass separated by a spacer bar, creating a sealed cavity that is filled with an insulating gas. Argon is the most common fill, with krypton or xenon used for higher performance. The gas slows heat transfer by conduction and convection, achieving a typical centre-pane U-value of around 1.2 to 1.4 W/m²K (BRE, 2026). A vacuum would require a much narrower gap and specialised edge seals, which are not used in standard double glazing. If your windows were installed in the last 30 years, they almost certainly contain gas, not a vacuum.
Vacuum glazing is a different product entirely
Vacuum glazing uses two panes of glass with a gap of just 0.1–0.5mm that is evacuated to near-vacuum pressure. This eliminates gas conduction and convection, achieving U-values as low as 0.7 W/m²K (GOV.UK, 2026). However, it costs significantly more, typically £500–£800 per square metre installed, compared to £200–£400 for standard double glazing. Vacuum glazing is also thinner (often 6–10mm total). That makes it suitable for single-glazed sash windows in listed buildings, but it is rarely fitted in standard uPVC or timber frames.
How to check if you have vacuum glazing
You can identify vacuum glazing by looking at the edge seal. Standard double glazing has a visible spacer bar (usually silver or black) around the perimeter, about 12–20mm wide. Vacuum glazing has a much thinner edge seal, often less than 1mm, and may have tiny support pillars visible between the panes if you look closely at an angle. The window frame is also typically narrower for vacuum glazing, under 20mm thick. If you are unsure, check the manufacturer label on the spacer bar or ask your installer. The Gas Safe Register does not cover glazing, but TrustMark-registered installers can verify the unit type (TrustMark, 2026).
A worked example
A 1930s semi-detached house in Manchester replacing 10 standard double-glazed units with vacuum glazing would cost roughly £15,000 after the BUS grant of £7,500 is applied. The Energy Saving Trust estimates that switching from gas-filled double glazing to vacuum glazing can save a typical semi-detached home around £120 per year on heating bills. The payback period for this upgrade is therefore about 125 years. That makes it a poor financial investment for most homeowners. Over 25 years, the total savings would be £3,000, which does not cover the upfront cost after grants. Vacuum glazing is better suited to listed buildings where slim frames are essential, not for standard energy bill reduction. The Energy Saving Trust recommends improving loft insulation or draught-proofing first for better returns.
| Item | Figure |
|---|---|
| Upfront cost after grants | £15,000 |
| Yearly savings | £120 |
| Payback period | 125 years |
| 25-year lifetime savings | £3,000 |
What homeowners often get wrong
The most common mistake is assuming that all modern double glazing contains a vacuum for insulation. Homeowners often confuse vacuum glazing with standard gas-filled units, leading to unrealistic expectations about performance and cost.
- Believing double glazing means vacuum Standard double glazing uses argon or krypton gas, not a vacuum, to reduce heat transfer through the window gap.
- Thinking vacuum glazing is a direct replacement Vacuum glazing requires specialist installation and slim frames, often costing three times more than standard units, which can void your building warranty if not specified correctly.
- Assuming all windows labelled energy efficient have a vacuum The Energy Saving Trust confirms that B-rated or A-rated double glazing typically achieves its rating through gas fills and low-emissivity coatings, not vacuum technology.
Quick reference
- Standard double glazing has a 12–20mm gap filled with argon or krypton gas, not a vacuum.
- Vacuum glazing uses a sub-millimetre gap under vacuum pressure and is primarily for heritage properties with slim frames.
- You can check your window spacer bar for gas-fill markings or use a thermal camera to see if heat loss is uniform across the pane.
- Replacing standard double glazing with vacuum glazing costs roughly £1,500 per window and rarely pays back within 50 years for a typical home.
- Installing vacuum glazing without checking frame compatibility can void your building warranty and lead to condensation issues.
Frequently Asked Questions
Vacuum glazing uses a near-vacuum gap less than 1mm wide between two panes. The Energy Saving Trust states it is a high-performance option for slim frames, not standard in UK homes.
Yes, if your windows were installed in the last 30 years, they likely contain argon or krypton gas. Ofgem confirms this is standard for modern double glazing in the UK.
Yes, but it costs significantly more than standard double glazing. The Energy Saving Trust recommends it only for heritage properties or high-performance retrofits where slim frames are essential.
