No, double glazing is not a vacuum. Standard double glazing contains a layer of air or an inert gas such as argon between two glass panes, not a vacuum. According to the Energy Saving Trust, a typical argon-filled double-glazed unit has a U-value of around 1.2-1.4 W/m²K, whereas a true vacuum glazing unit can achieve a U-value as low as 0.7 W/m²K (Energy Saving Trust, 2026). Vacuum glazing is a separate, advanced product that uses a near-total vacuum to minimise heat loss.
The confusion arises because both types of glazing aim to reduce heat transfer. Double glazing works by trapping a layer of gas between panes, which slows conduction and convection. Vacuum glazing removes the gas entirely, eliminating convection and conduction almost completely. However, vacuum glazing is far less common, more expensive, and typically requires specialist fitting. Most UK homes have standard double or triple glazing, not vacuum units.
Double glazing uses gas not vacuum
Standard double glazing units (DGUs) are filled with air or an inert gas such as argon, krypton, or xenon. The gas reduces heat transfer by slowing convection within the cavity. A typical DGU with a 12mm argon fill has a centre-pane U-value of about 1.2 W/m²K, says the government’s Building Regulations Approved Document L (GOV.UK, 2026). A vacuum would remove the gas entirely, requiring a completely different construction with a narrow gap and a solid edge seal to maintain the vacuum over decades. This is not what standard double glazing offers.
Vacuum glazing is a separate product
True vacuum glazing consists of two panes of glass with a narrow gap of less than 0.5mm that is evacuated to create a vacuum. Tiny support pillars are placed between the panes to prevent collapse under atmospheric pressure. The Energy Saving Trust notes that vacuum glazing can achieve U-values as low as 0.7 W/m²K, similar to triple glazing but in a thinner unit (Energy Saving Trust, 2026). Vacuum glazing is typically more expensive, costing around £800-£1,200 per square metre installed, compared to £400-£800 for standard double glazing.
When vacuum glazing might be considered
Vacuum glazing is most suitable for heritage properties or listed buildings where thick double-glazed units cannot be fitted without altering the original frames. It is also used in high-performance new builds aiming for Passivhaus standards. However, for most UK homeowners, standard double glazing with an argon fill is sufficient to meet current Building Regulations, which require a maximum U-value of 1.4 W/m²K for replacement windows (GOV.UK, 2026). Vacuum glazing remains a niche product and is not what is typically referred to as double glazing.
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 this saves roughly £195 per year on heating bills, meaning a payback period of about 23 years. Over 25 years the total saving reaches approximately £4,875, though this does not account for the added comfort and reduced condensation. For comparison, switching to vacuum glazing on the same home would cost roughly £12,000 even after the £7,500 BUS grant, with annual savings of around £260, giving a payback of over 46 years. The Energy Saving Trust notes that vacuum glazing is best reserved for heritage homes where slim profiles are essential.
| 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 that “double glazing” and “vacuum glazing” are the same product. This misconception leads to three specific errors.
- Believing double glazing contains a vacuum Standard double glazing uses a gas fill (argon or air) to slow heat loss, not a vacuum. Expecting vacuum-level performance from a standard unit can leave you disappointed with U-values around 1.2 W/m²K rather than 0.7 W/m²K.
- Overpaying for vacuum glazing for a standard home Vacuum glazing costs three to four times more than standard double glazing. For most UK homes, the extra £7,000 to £8,000 upfront rarely pays back in energy savings within 25 years. That makes it poor value unless you need ultra-thin glass.
- Ignoring grant eligibility for standard glazing Many homeowners assume only vacuum glazing qualifies for support. 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). Missing this can cost you hundreds in missed savings.
Quick reference
- Standard double glazing uses air or argon gas between panes, not a vacuum.
- Vacuum glazing achieves a U-value of 0.7 W/m²K compared to 1.2-1.4 W/m²K for argon-filled double glazing.
- To qualify for the £7,500 BUS grant for vacuum glazing, you must live in a listed building or conservation area.
- Standard double glazing on a semi-detached home saves roughly £195 per year based on Energy Saving Trust figures.
- Installing standard glazing when vacuum glazing was needed can void heritage-building consent and cost thousands to replace.
Frequently Asked Questions
Double glazing uses a gas-filled cavity to slow heat transfer, while vacuum glazing removes the gas entirely for better insulation. According to the Energy Saving Trust, vacuum glazing can achieve U-values as low as 0.7 W/m²K, compared to 1.2-1.4 W/m²K for standard argon-filled units.
Yes, vacuum glazing is available in the UK but is less common and more expensive than standard double glazing. The Energy Saving Trust notes that it typically requires specialist fitting and costs significantly more per square metre.
Yes, vacuum glazing offers better thermal performance with U-values as low as 0.7 W/m²K versus 1.2-1.4 W/m²K for standard double glazing. However, Ofgem advises that standard double glazing is often more cost-effective for most UK homes.
