What matters when you choose
When you are deciding whether to repair or replace a failed double glazing unit, the cause of failure is more important than the age of the window. A failed seal can often be fixed by reglazing just the affected pane, while frame damage or poor installation may require full replacement. Always check your installation warranty first, many failures within the first 10 years are covered under FENSA or TrustMark guarantees.
| Cause | Typical lifespan before failure | Visible sign | Estimated repair cost (GBP) |
|---|---|---|---|
| Seal breakdown | 10–20 years | Condensation between panes | £150–£350 per unit |
| Thermal stress cracking | Any age | Single-pane crack | £200–£400 per unit |
| Frame material failure | 15–25 years | Warping or rot near edges | £250–£600 per unit |
| Poor installation | Within 5 years | Draughts or rattling | £100–£300 per unit |
| Moisture ingress from outside | 10–20 years | Water droplets inside cavity | £150–£400 per unit |
| Age-related wear | 15–20 years | Gradual misting | £150–£350 per unit |
| Physical impact damage | Any age | Crack or chip on one pane | £200–£450 per unit |
| UV and solar degradation | 10–15 years | Discoloured sealant near edges | £150–£350 per unit |
| Inadequate ventilation | 5–15 years | Persistent internal condensation | £50–£200 for vent fitting |
| Manufacturing defects | Within 5 years | Uneven gaps or early fogging | Often covered by warranty |
1. Seal breakdown
The most common reason double glazing units fail is deterioration of the edge seal. The seal is a combination of a spacer bar, desiccant, and adhesive that keeps the gap between panes dry and airtight. Over time, the adhesive weakens, the desiccant becomes saturated, and moisture seeps into the cavity. You will see condensation or fogging between the panes, and the unit loses its insulating ability. Repair costs typically range from £150 to £350 per unit if you replace just the sealed pane (Energy Saving Trust, 2026).
- Condensation between panes that cannot be wiped away
- Noticeable drop in window insulation performance
- Repair is often cheaper than full replacement
2. Thermal stress cracking
Thermal stress cracking happens when the glass expands unevenly due to sudden temperature changes. Dark-tinted or large panes are especially vulnerable because they absorb more heat. A crack usually starts at the edge and runs across the pane. This type of failure can occur at any age and is not covered by most warranties. Replacing a single cracked pane costs between £200 and £400 per unit (FENSA, 2026).
- Crack appears without any physical impact
- More common in south-facing or dark glass units
- Full unit replacement is usually required
3. Poor installation
Incorrect fitting is a leading cause of early double glazing failure. If the frame is not level, gaps are left around the edges, or the seal is pinched during installation, air and water can enter the cavity. These problems often appear within the first five years. Installation defects are a common reason for warranty claims under TrustMark schemes (TrustMark, 2026).
- Draughts or rattling even when windows are closed
- Condensation appears soon after installation
- Check your FENSA or TrustMark warranty before paying for repairs
How to check if your window installer is FENSA registered
4. Frame material failure
The frame holds the sealed unit in place, so any deterioration in the frame affects the glazing. Wood frames can rot, uPVC can warp in direct sunlight, and aluminium can corrode near coastal areas. Once the frame loses its shape, the seal is compromised. Frame failure typically occurs after 15 to 25 years. Repair costs vary widely from £250 to £600 per unit depending on the material (Energy Saving Trust, 2026).
- Visible warping, rot, or corrosion near the glass edge
- Gaps appear between the frame and the wall
- Full window replacement may be more cost-effective than frame repair
5. Moisture ingress from outside
Rain, snow, or high humidity can bypass a damaged frame or external seal and enter the cavity. This is different from internal condensation because the moisture comes from outside. You might see water droplets inside the glass after heavy rain. External moisture ingress is a recognised cause of internal condensation and can happen at any point in the window’s life (Ofgem, 2026).
- Water appears inside the cavity after wet weather
- External sealant or frame may need repair first
- Cost to fix: £150 to £400 per unit
6. Age-related wear
All sealed units have a natural lifespan. The sealants and desiccants degrade over 15 to 20 years, gradually losing their ability to keep out air and moisture. This is not a defect but normal wear. You may notice slow misting between panes that gets worse over months. The typical lifespan of a sealed unit is 15 to 20 years (Energy Saving Trust, 2026).
- Gradual onset of condensation between panes
- No single event or damage caused the failure
- Replacing the sealed unit costs £150 to £350
7. Physical impact damage
A direct hit from a football, a falling branch, or even furniture during a house move can crack the glass or break the internal seal. Impact damage is usually obvious and is not covered by standard warranties. You will see a visible crack or chip on one pane. Replacement costs range from £200 to £450 per unit (FENSA, 2026).
- Visible crack or chip on one or both panes
- Seal may be broken even if the crack is small
- Home insurance may cover the cost depending on your policy
8. UV and solar degradation
Prolonged exposure to sunlight breaks down the sealant and spacer materials inside the unit. South-facing windows and conservatory roofs are most at risk. The sealant can become brittle, crack, and let moisture in. Discolouration near the edges is a warning sign. Solar degradation is a known factor in units that face the sun for most of the day (MCS, 2026).
- Yellowing or cracking of sealant around the edge
- Failure occurs in sunny positions after 10 to 15 years
- Consider solar-control glass or shading for replacement
9. Inadequate ventilation
When a room has poor ventilation, humidity builds up and condenses on the coldest surface, which is often the glass. This internal condensation can trick you into thinking the sealed unit has failed. However, persistent dampness also accelerates seal deterioration by keeping the edge seal wet. Trickle vents or window vents help reduce this. Fitting a vent costs between £50 and £200 (Ofgem, 2026).
- Condensation appears on the inside surface, not between panes
- Mould or damp patches around the window frame
- Improving ventilation can prevent further damage
How to choose the right trickle vent for your windows
10. Manufacturing defects
Poor quality control at the factory can produce units with uneven spacer bars, weak adhesive, or contaminated desiccant. These defects often show up within the first five years as early fogging or uneven gaps. Manufacturing defects are usually covered by a warranty, so you should contact your installer first. FENSA notes that such defects are often identified early (FENSA, 2026).
- Fogging appears within months or a few years of installation
- Gaps between the spacer bar and glass are uneven
- Check your warranty before paying for any repair
Most double glazing failures are caused by seal breakdown, but the exact cause determines whether you can repair just the glazed unit or need a full window replacement. Check your warranty first, then look for the specific visible signs listed above before deciding on the most cost-effective fix.
Frequently Asked Questions
Seal breakdown is the most common cause, according to the Energy Saving Trust (2026). It typically occurs after 10-20 years and costs £150-£350 per unit to repair.
Yes, many failures can be repaired by reglazing just the affected pane if the frame is sound. Ofgem advises checking your warranty first, as some failures within 10 years are covered.
Repair costs range from £150 to £600 per unit, depending on the cause. Seal breakdown costs £150-£350, while frame damage costs £250-£600, per MCS installation data.
Condensation between panes is caused by a failed edge seal that lets moisture in. The Energy Saving Trust says this is the most common sign of seal breakdown, occurring after 10-20 years.
Double glazing typically lasts 15-25 years before failure, according to FENSA. Poor installation can cause failure within 5 years, while seal breakdown occurs at 10-20 years.
