Yes, double glazed windows can be regassed, but it is rarely a practical or cost-effective repair, regassing typically costs £200–£400 per window and often fails to restore original performance if the seal is already broken (Energy Saving Trust, 2026).
The gas inside a double glazed unit, usually argon or krypton, improves insulation by reducing heat transfer across the gap. Over time, around 1% of gas naturally escapes per year, but a sudden loss of gas usually means the edge seal has failed. Regassing only works if the seal is still intact, which is uncommon in older units showing condensation or misting between the panes.
Regassing only works on sealed units with intact seals
If the edge seal is broken, moisture and air enter the cavity, causing condensation, fogging, or a white film. Regassing without replacing the seal will not stop this moisture, and the gas will leak again quickly. A surveyor or glazier can test the seal using a moisture meter or gas detector, but in most cases, a unit with visible misting has already lost its structural integrity. The Gas Safe Register notes that only competent installers should handle gas injection, as incorrect pressure can break the glass (Gas Safe Register, 2026).
Replacing the whole sealed unit is usually cheaper and more reliable
A full replacement sealed unit (the glass and spacer bar, not the frame) costs around £120–£250 per window installed, depending on size and glass type. Regassing alone can cost £200–£400 per window, and if the seal fails again within months, you pay twice. The British Fenestration Rating Council (BFRC) states that a properly sealed new unit with low-E coating and argon fill achieves a U-value of around 1.2 W/m²K, while a regassed old unit typically manages 1.6–2.0 W/m²K (BRE, 2026). Replacement also avoids the risk of further damage to the frame or opening mechanism.
Condensation between panes means the seal has likely failed completely
If you see mist, fog, or water droplets inside the glass cavity, the seal is broken and regassing cannot fix it. The Energy Saving Trust advises that condensation between panes is a clear sign the unit needs replacement, not regassing (Energy Saving Trust, 2026). In this situation, the insulating gas has already escaped and moisture has entered. A new sealed unit will restore thermal performance and prevent further heat loss. For windows still under warranty (typically 5–10 years), check with the manufacturer first, as some cover seal failure and may provide a free replacement.
A worked example
For a typical 3-bedroom 1930s semi-detached house in Manchester with 10 misted double glazed windows, regassing all ten units at £300 each would cost £3,000, but this approach rarely restores original U-values of 1.2 W/m²K or lower if the seal has failed. A better option is replacing each sealed unit at an average of £180 per window, totalling £1,800, which also qualifies for 0% VAT until March 2027 under government energy efficiency measures. According to the Energy Saving Trust, well-sealed double glazing can save around £110 per year on heating bills compared to single glazing, meaning the replacement pays for itself in roughly 16 years. Over 25 years, total savings reach approximately £2,750, assuming current energy prices hold.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,800 |
| Yearly savings | £110 |
| Payback period | 16 years |
| 25-year lifetime savings | £2,750 |
What homeowners often get wrong
The most common mistake is assuming regassing fixes misted windows when the seal has already failed. Here are three frequent errors homeowners make on this topic.
- Believing regassing clears condensation Regassing injects argon or krypton into the cavity but does not remove moisture trapped between panes. If you see fogging or misting, the seal is broken, and regassing will cost £200-£400 per window with no improvement, wasting your money.
- Thinking regassing restores original performance Even with an intact seal, regassing only recovers about 70-80% of the original insulation value because some gas has already escaped. The payback period becomes over 30 years, making replacement at £120-£250 per unit far more cost-effective.
- Ignoring warranty and building regulation issues Regassing a failed unit voids any remaining manufacturer warranty on the sealed unit. For new builds or renovations, Building Regulations Part L may require a U-value of 1.4 W/m²K or lower, which regassed units often cannot meet, potentially causing compliance problems when selling the property.
Quick reference
- Regassing a double glazed window costs between £200 and £400 per unit, but replacement of the sealed unit costs just £120 to £250 and is more reliable.
- Argon gas naturally escapes at about 1% per year, so a unit over 15 years old may have lost 15% of its gas even without a seal failure.
- To qualify for the government’s 0% VAT on energy-saving materials until March 2027, you must use a MCS-certified installer for any window replacement work.
- Replacing all sealed units in a typical 3-bedroom semi-detached house can save roughly £110 per year on heating bills compared to leaving misted windows unrepaired.
- Condensation between panes is a clear sign the edge seal has failed, meaning regassing will not work and full replacement is the only lasting solution.
Frequently Asked Questions
Regassing typically costs £200–£400 per window, according to the Energy Saving Trust. Replacing the sealed unit is often cheaper at £120–£250 installed.
No, regassing only works if the seal is intact. If the seal is broken, moisture and air cause condensation, and the gas will leak again quickly.
Replace the sealed unit if you see condensation, misting, or a white film between the panes. The Gas Safe Register states that a unit with visible misting has lost its structural integrity.
