What Does "Blown" Double Glazing Actually Mean?
When glaziers say a window is "blown," "misty" or "failed," we mean the same thing: the hermetic seal around the edge of the sealed unit (the sandwich of two glass panes plus a spacer bar) has broken down. That seal exists to trap a layer of inert gas - usually argon - between the panes. Argon conducts heat about 30% less efficiently than ordinary air, which is exactly what makes modern double glazing so insulating.
Once the seal fails, three things happen in quick succession. The argon escapes, atmospheric air takes its place, and crucially that air carries moisture. Over weeks and months, that moisture condenses on the cool inner glass surfaces and slowly etches mineral deposits into them. The window becomes a poor insulator and a permanent eyesore.
The single most important thing to know: a blown sealed unit almost never means you need a new window. The frame stays exactly where it is. We measure, manufacture and fit a brand new sealed unit inside the existing frame - usually in under an hour per window.
The 5 Clear Signs Your Double Glazing Has Blown
1. Condensation Trapped Between the Panes
This is the giveaway. If you see moisture, mist or visible water droplets between the two panes of glass - not on the inside or the outside - the seal has failed. The simple test: wipe both faces of the glass with a dry cloth. If misting persists, it's trapped where you cannot reach.
Many homeowners notice this most clearly first thing in the morning, when overnight temperature drops force any trapped moisture to condense.
2. Cloudy, Milky or Streaked Glass That Won't Clean
Even when the condensation clears on a warm afternoon, a tide-line of mineral deposit often remains - a cloudy, milky or rainbow-tinted film on the interior glass surfaces. This is essentially limescale baked onto the inside of your window. No amount of polishing the outer surfaces will shift it.
3. Cold Spots Around the Glass in Winter
Hold the back of your hand close to the centre of the pane on a cold morning. A working sealed unit feels neutral. A blown one feels noticeably cooler - sometimes downright cold. The lost argon layer is no longer doing its job, and your heating is working overtime to compensate.
4. More Outside Noise Than There Used To Be
The acoustic performance of double glazing depends on the same gas layer. When it goes, traffic, aircraft, neighbours and weather all become more audible. Many of our Essex customers first call us not because of the look, but because the bedroom suddenly feels noisier.
5. Creeping Energy Bills With No Other Explanation
If your bills have crept up despite no change in habits, tariff or thermostat, failed sealed units could be the cause. A typical 3-bed semi with eight blown units can lose 15-20% more heat through the windows than the same house with healthy ones - that adds up fast at current energy prices.
What Causes Sealed Units to Fail?
- Age - most units last 15-25 years before the seal degrades
- Thermal cycling - daily expansion and contraction stresses the seal, worst on south- and west-facing windows
- Poor original installation - units fitted without proper drainage or packing fail much sooner
- Blocked drainage slots - water pooling around the unit attacks the seal from the outside
- Failed external sealant - the silicone bead between frame and brickwork is your first line of defence
How Much Does Repair Cost in 2026?
For a standard sealed unit replacement in 2026, expect £85-£180 per window fitted. That includes the new argon-filled, Low-E coated unit, removal of the old glass, fitting, and a 25-year guarantee. We've broken the prices down further in our 2026 cost guide.
Compare that to £600-£1,000+ per window for a full replacement and the choice is usually obvious.
Don't Wait Too Long
Blown units never recover. Every month you leave it, more moisture enters and the mineral staining worsens. Catch it within 6 months of the first misting and the swap is straightforward; leave it for a couple of years and the surrounding gasket and frame can also deteriorate.
If you're across our Essex or East London service area, send us a photo on WhatsApp - we'll usually diagnose it the same day.