Failed Double Glazing

What Does "Failed Double Glazing" Mean?

When you see condensation, misting, or a milky haze between the two panes of glass, the hermetic seal of your sealed glass unit (IGU) has failed. Moisture from the air has entered the cavity between the panes. This is different from condensation on the inside surface of the glass (a ventilation issue) or on the outside surface (which actually indicates good insulation).

Failed sealed units cannot be repaired. Once moisture is inside, the unit must be replaced. The question is whether you replace just the glass or the entire window.

Why Sealed Units Fail

• Age — sealed units have a typical lifespan of 15–25 years. After this, the seal perishes naturally. If your windows are 20+ years old, failures will become increasingly common.
• Poor original quality — cheap sealed units with thin spacer bars and low-grade sealant fail faster. This is common in budget window installations from the 1990s and 2000s.
• Frame movement — if the window frame has warped, twisted, or settled, it puts stress on the glass unit. Timber frames that have not been maintained are particularly prone to this.
• Water ingress — if the drainage channels in the frame are blocked, water sits against the seal and degrades it prematurely.
• Thermal stress — south-facing windows experience significant temperature swings. Over years, this thermal cycling weakens the seal.

Replace the Glass or Replace the Window?

If the frame is sound and the failure is isolated to one or two units, replacing just the sealed glass unit is the cheaper option. A glazier can swap the unit for £100–£250 per window.

However, if multiple units have failed, the frames are in poor condition, or the windows are 20+ years old, full replacement is usually better value. You will get modern double or triple glazed units with a 10-year warranty, better thermal performance, improved security (PAS24), and frames that will last another 30–60 years.

Signs It Is Time to Replace

• Three or more failed units in the same property
• Frames showing rot, paint failure, or warping
• Draughts around the frame edges
• Difficulty opening or closing windows
• Windows over 20 years old — more failures are coming
• Planning to sell — buyers notice failed glass instantly

If any of these apply, a free site survey will tell you exactly where you stand. We will be honest about whether replacement is necessary or whether a glass-only swap is the smarter move.

How Modern Timber Windows Prevent This

Our sash windows and casement windows use premium sealed glass units with warm-edge spacer bars and dual-seal construction. Combined with engineered hardwood frames that maintain their shape for decades, the risk of premature seal failure is dramatically reduced. We back this with a 10-year warranty covering internal condensation from seal failure.