If your double glazing has failed, you will typically see condensation between the glass panes that does not wipe off. Other clear signs include misting, a visible draught, or a noticeable drop in indoor temperature near the window. The Energy Saving Trust states that failed double glazing can reduce thermal efficiency by up to 50%, costing a typical UK home an extra £100 to £150 per year in heating bills, roughly the same as running a chest freezer continuously.
Check these specific indicators:
- Condensation or mist trapped between panes: this means the seal has broken and argon gas has leaked out.
- A cold draught felt near the window even when closed: this suggests the frame or seal is compromised.
- Visible cracks or chips in the glass: these can breach the seal and cause failure.
- Higher energy bills without other changes: a 20% increase on a £1,000 annual heating bill adds £200, comparable to a full tank of petrol.
- Frost on the outside of the inner pane in winter: this indicates the insulating gap has been lost.
| Test | What to look for | Likely cause |
|---|---|---|
| Visual inspection | Mist or dirt between panes | Failed seal |
| Hand test | Cold air felt on a still day | Leaking frame or seal |
| Condensation test | Moisture that does not wipe off | Broken gas fill |
| Energy bill check | Unexplained 10-20% increase | Reduced insulation |
This matters most during winter when heating costs are highest. If you have single-glazed or very old windows, the difference may be less noticeable because the baseline performance is already poor. It does not matter as much if you are planning to replace the windows soon anyway.
Bottom line: if you see persistent condensation between the panes or feel a draught, your double glazing has almost certainly failed and replacing the sealed unit will typically cost £150–£300 per window, which pays back in lower heating bills within 2–3 years.
Sources
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 that failed glazing can increase heating bills by £100 to £150 per year, so fixing these five windows should save around £120 annually. With no government grants specifically for double glazing replacement under ECO4 or the BUS scheme, you pay the full £1,250 upfront. Over 25 years, the total savings from reduced heat loss reach approximately £3,000, giving you a net benefit of £1,750 after recovering the installation cost within just over a decade.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,250 |
| Yearly savings | £120 |
| Payback period | 10.4 years |
| 25-year lifetime savings | £3,000 |
What homeowners often get wrong
The most common mistake is assuming mist between panes is harmless condensation that will dry out on its own. Here are three errors that cost UK homeowners time and money.
- Thinking the mist will clear Once the seal fails and argon gas escapes, the insulating gap is permanently lost. Wiping the glass or using dehumidifiers does nothing, the moisture is trapped between panes and will reappear, wasting £50 to £100 on ineffective DIY fixes before replacement is needed.
- Ignoring a small draught A slight cold draught near a closed window suggests the frame or seal is compromised, not just a minor gap. Left unchecked, this can void any remaining warranty on the window and add £80 to £150 to your annual heating bill, according to the Energy Saving Trust.
- Replacing the whole window frame Many homeowners assume failed glazing means a full new window, costing £500 to £1,000 per window. In most cases, only the sealed unit needs replacing, at £150 to £300 per window, a 60% saving if you keep the existing frame and ask a glazier for a like-for-like unit.
Quick reference
- Condensation between panes that does not wipe off is the clearest sign your double glazing has failed and the seal is broken.
- Replacing just the sealed unit costs £150 to £300 per window, compared to £500 to £1,000 for a full frame replacement.
- Failed double glazing can increase your heating bills by £100 to £150 per year, per the Energy Saving Trust.
- You do not need planning permission to replace failed sealed units in existing windows in most UK homes.
- Check your window warranty before ordering, many 10-year guarantees cover seal failure, saving you the full replacement cost.
Frequently Asked Questions
You can check for signs like condensation or mist trapped between the glass panes that doesn't wipe off, a cold draught near the window, or a sudden 10-20% increase in your heating bills. These indicate a broken seal or gas leak, reducing thermal efficiency by up to 50%.
Replacing a failed sealed unit typically costs between £150 and £300 per window in the UK. This investment often pays back within 2-3 years through lower heating bills, as failed glazing can add £100-£150 annually to energy costs.
Yes, failed double glazing can increase your heating bills by £100 to £150 per year, according to the Energy Saving Trust. This is roughly a 10-20% rise on a typical £1,000 annual heating bill, comparable to running a chest freezer non-stop.
The most common cause is a broken seal, which lets argon gas escape and moisture enter between the panes. This can happen due to age, weather exposure, or damage like cracks or chips in the glass, leading to condensation and reduced insulation.
There isn't a specific grant for replacing failed double glazing, but the Energy Company Obligation (ECO4) scheme may help low-income households with window upgrades. Check with your energy supplier or Ofgem for eligibility, as funding varies by region.
