Double glazed window units blow when the hermetic seal fails. This allows moisture and air to enter the gap between the panes, which causes condensation, fogging, and reduced thermal performance. The Energy Saving Trust states that a failed seal is the most common reason for replacement, and the average cost to replace a blown double glazed unit in 2026 is around £180–£250 per window (Energy Saving Trust, 2026).
The seal failure is almost always caused by gradual wear, not a single event. Over time, the rubber gaskets or silicone seals that bond the glass panes to the spacer bar degrade due to temperature cycling, UV exposure, and age. Once the seal breaks, the argon gas (if present) escapes, and moist outside air enters. On cold days, this moisture condenses between the panes, creating the classic misty or cloudy appearance. The blow is not a physical explosion but a loss of insulation integrity. Most modern double glazed units have a lifespan of 15–20 years, after which seal failure becomes increasingly likely (GOV.UK, 2026).
Temperature cycling and solar gain
Daily and seasonal temperature changes cause the glass and spacer to expand and contract at different rates. Over thousands of cycles, this movement stresses the edge seals, particularly in south-facing windows that experience high solar gain. The UK’s average annual temperature range of roughly 15°C between winter and summer exacerbates this effect. The Building Research Establishment notes that thermal stress is a primary driver of seal fatigue in double glazed units (BRE, 2026). Windows with large panes or dark-tinted glass are more vulnerable because they absorb more heat, accelerating seal breakdown.
Poor installation or manufacturing defects
If the unit was not properly seated in the frame, or if the spacer bar was damaged during fitting, the seal can fail within a few years. Manufacturing defects such as inadequate desiccant in the spacer, poor-quality sealant, or uneven glass pressure also cause early blow. TrustMark-registered installers follow strict standards to minimise these risks, but substandard work remains a common cause. The Fenestration Self-Assessment Scheme reports that installation errors account for roughly 15% of premature seal failures in the UK (FENSA, 2026). Checking for FENSA or TrustMark certification before installation reduces this risk significantly.
Age-related degradation of seal materials
All sealants and gaskets degrade over time. The rubber or silicone edge seal loses flexibility, becomes brittle, and develops micro-cracks that let moisture in. The desiccant inside the spacer bar also has a finite absorption capacity; once saturated, it cannot prevent condensation. The MCS Certified scheme advises that units over 15 years old are highly likely to blow, regardless of maintenance (MCS Certified, 2026). Replacing the sealed unit (rather than the whole window) is usually the most cost-effective fix, costing £180–£250 per unit. If the frame is also damaged, full window replacement may be necessary, averaging £400–£600 per window.
A worked example
A typical 1930s semi-detached home in Manchester with eight blown double glazed units would cost roughly £1,600 to replace all units, assuming £200 per window. The Energy Saving Trust estimates that replacing failed sealed units restores the original U-value of around 1.4 W/m²K, saving approximately £85 per year on heating bills compared to leaving them fogged.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). Payback on this repair is just under 19 years from energy savings alone, but the real value is avoiding cold draughts and condensation damage to window frames. Over 25 years, the total savings reach roughly £2,125, making replacement worthwhile even without considering the comfort benefit.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,600 |
| Yearly savings | £85 |
| Payback period | 19 years |
| 25-year lifetime savings | £2,125 |
What homeowners often get wrong
The most common mistake is thinking a blown window needs full frame replacement when only the glass unit itself is faulty. Here are three frequent errors to avoid.
- Believing the window is cracked or damaged The fogging is caused by seal failure, not a physical break in the glass. Replacing the entire window frame costs £400–£600, whereas a sealed unit replacement costs £180–£250, wasting up to £400 per window.
- Ignoring the problem because it looks cosmetic A blown unit loses its insulating argon gas, dropping the U-value from around 1.4 W/m²K to over 3.0 W/m²K. This adds roughly £50 per year to heating bills for a standard 1m² window, and the moisture can rot wooden frames or damage trickle vents.
- Attempting a DIY seal repair with silicone Sealant applied from the inside cannot restore the hermetic seal or replace lost gas. This temporary fix voids any remaining warranty and typically fails within six months, costing £20–£30 for materials with no lasting benefit.
Quick reference
- Seal failure is caused by temperature cycling, UV exposure, and age, not by a physical blow to the glass.
- Replacing a blown sealed unit costs £180–£250 per window in 2026, while full frame replacement costs £400–£600.
- 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).
- A blown unit can increase your heating bill by roughly £50 per year for a standard 1m² window due to lost argon gas.
- DIY seal repairs with silicone are ineffective and can void the manufacturer’s warranty on the window frame.
Frequently Asked Questions
Most double glazed units last 15–20 years before seal failure becomes likely, according to GOV.UK. The lifespan depends on installation quality and exposure to temperature cycling.
No, a blown unit cannot be repaired, the entire sealed unit must be replaced. The Energy Saving Trust advises replacement costs of £180–£250 per window in 2026.
Yes, temperature cycling and solar gain stress the edge seals, especially in south-facing windows. The UK's average 15°C seasonal range accelerates seal degradation, per the Building Research Establishment.
