By Wellington Glass & Mirror
Why Units Fail and How to Fix Them Properly
Double glazing is designed to improve comfort, energy efficiency, and noise reduction. However, like any building system, it does not last forever. Over time, double-glazed units can fail, resulting in reduced clarity and performance.
If you are noticing misting or condensation between panes, water ingress, or declining thermal performance, your double glazing may be failing. The key question is not just can the glass be replaced, but why did it fail in the first place.
This article explains the most common causes of double-glazing failure, what to look for in older systems, and why proper remediation often involves more than simply swapping out the glass.
What Is Failed Double Glazing and Why It Happens
Double glazing fails when the seal between the glass panes breaks down. Once the seal is compromised, insulating gas escapes, moisture can enter the cavity, and performance is lost.
Common signs of failure include:
- Condensation or moisture trapped between panes
- Persistent fogging that cannot be cleaned
- Reduced thermal performance
Condensation trapped between panes is a common sign of seal failure and is different from surface moisture caused by indoor or outdoor humidity and poor ventilation. If you are unsure what type of condensation you are experiencing, our article on why Wellington homes experience condensation explains the difference and what to look for.
Once a unit has failed, it cannot be repaired. The glass unit must be replaced.
Failure typically occurs due to a combination of age, unit quality, installation quality, poor glazing-system design, or blocked drainage in aluminium joinery. Most double-glazed units have a practical lifespan of around 15–25 years. Over time, UV exposure, temperature changes, blocked drainage or natural building movement can result in degraded seals, even in well-installed systems.
In some cases, failure happens earlier due to manufacturing defects or poor quality-control. Depending on the age and documentation, warranty considerations may apply.
Double Glazing is Not Just Glass
Importantly, double glazing is not just glass. It is part of a complete glazing system that includes:
- The insulated glass unit, including spacer bars and seals
- Joinery design
- Drainage and ventilation
- Installation method and quality
Failure can occur if moisture gains access to the seal between the glass panes, if spacers are exposed to UV, or if rebate depth or setting blocks are incorrect. Replacing glass alone without addressing these underlying issues often leads to repeat problems.
At Wellington Glass & Mirror, we assess the entire glazing system, not just the visible failure. This allows us to identify the true cause and ensure the replacement unit delivers long-term performance, not just a short-term fix.
Aluminium Windows: Why Double Glazing Often Fails
In aluminium joinery, double glazing commonly fails due to drainage limitations within the surrounding frame.
Blocked or poorly designed drainage holes prevent moisture from escaping. Over time, trapped moisture degrades seals, stresses the glazing unit, and accelerates failure.
- Drainage holes may be undersized or incorrectly positioned for the climate and wind load
- Poor maintenance can allow insects or debris to block drainage holes and moisture to accumulate
Other reasons for failure in aluminium windows include manufacturing defects or incorrect sizing or glazing of double-glazing units that result in exposure of the spacer and primary seals to UV light.
When these underlying issues are not addressed, simply replacing the glass can result in ongoing or repeat failure.
Timber Joinery: Associated Damage and Hidden Issues
In timber windows, failed double glazing is often linked to the condition or movement of the joinery, not just the glass unit itself.
Common contributing factors include:
- Moisture ingress leading to timber swelling or rot
- Deteriorated paint or sealant systems allowing water penetration
- Natural timber movement placing stress on seals and glazing rebates
If the timber condition is not addressed, replacing the glass alone can trap moisture and accelerate further deterioration.
Remediation, Repair & Replace: Fixing the Root Cause
At Wellington Glass & Mirror, we take a problem-solving approach to failed double glazing. Rather than just replacing glass, we assess the entire glazing system to understand why failure occurred and what is required to restore long-term performance.
Upgrading Performance: More Than a Like-for-Like Replacement
Many homeowners also choose to replace old double-glazing units to significantly improve thermal or acoustic performance.
Over the past 20 years, manufacturing quality and high-performance glass technology have advanced considerably. Modern double-glazed units deliver far better thermal, solar control and acoustic performance than earlier systems, even when installed into existing joinery.
Today’s high-performance glass and glazing options allow us to achieve meaningful improvements in:
- Thermal insulation and indoor comfort
- Solar control and overheating reduction
- Noise reduction
- Condensation resistance
Depending on your home and needs, we can now specify:
- A broad range of Low-E coated glass types that have different thermal and solar control performance characteristics
- Argon or Krypton-filled units for improved insulation
- Laminated glass for enhanced UV and noise control
- Advanced glass combinations tailored to window orientation and room use
As a result, replacement units often outperform the original glazing system by a significant margin, delivering improved comfort, efficiency, and long-term value rather than simple like-for-like replacement.
When Should You Replace Old Double Glazing?
You should consider assessment and replacement if:
- Your units are more than 15–20 years old
- You see condensation between panes
- Rooms feel cold or draughty despite double glazing
- You want to change thermal, solar radiation control and/or acoustic performance in some or all rooms of your home
- You want to repair, refurbish or replace existing timber joinery, timber windows showing signs of moisture damage
- You want to upgrade your aluminium sashes, stays, seals and handles, or resolve persistent drainage problems
As do most components of housing, your joinery also requires regular maintenance and periodic refurbishment to ensure longevity.
Early intervention can prevent more costly joinery repairs later.
Expert Assessment Matters
Failed double glazing is rarely just a glass problem. Understanding why it failed is critical to ensuring replacement units last.
Wellington Glass & Mirror combines glazing expertise with joinery understanding to deliver long-term solutions, not short-term fixes.
If you are experiencing failed double glazing or want to upgrade the performance of existing units, contact our team for a professional assessment.
Contact our team today to discuss your existing double glazing and book an assessment.
Frequently Asked Questions: Replacing Failed Double Glazing
Once a double-glazed unit has failed, it cannot be repaired. Condensation or fogging between the panes indicates seal failure, and the glass unit must be replaced to restore performance.
Most double-glazed units have a practical lifespan of around 15–25 years, depending on exposure, installation quality, joinery condition, and maintenance. Units over 20 years old are more likely to experience seal failure.
In many cases, yes. If the joinery is structurally sound, the glass unit can be replaced. However, the surrounding glazing system should be assessed to ensure drainage, sealing, and joinery condition will support the new unit long term.
Double glazing performance depends on the full glazing system, including the double-glazing units, joinery, drainage, and ventilation. Replacing glass alone without addressing underlying issues can lead to repeat failure or reduced lifespan of the new unit.
Yes. Modern high-performance glass technology has advanced significantly over the past 20 years. Replacement units can deliver better thermal insulation, condensation resistance, noise reduction, and solar control than older systems.
