How to Spot a Failed Double Glazing Seal | Expert Guide

How to Spot a Failed Double Glazing Seal (And What to Do)

Double-glazed windows are designed to keep your home warm, quiet, and secure. However, like any building component, they are subject to wear and tear. Over time, the hermetic seals surrounding the double-glazed units can degrade, allowing moisture to seep into the gap between the glass panes. This is known in the glazing trade as a \"blown\" or \"failed\" unit. Identifying a failed seal early allows you to take action before it compromises your home's energy efficiency.

Expert Reviewed by Marian Gheorghe, Head Window Fitter — Last reviewed: July 2026

What is a Sealed Double-Glazed Unit?

To understand seal failure, it helps to understand how a modern double-glazed window is constructed. A double-glazed unit (often called an IGU — Insulated Glass Unit) consists of two panes of glass separated by a hollow spacer bar. The spacer bar is filled with desiccant (silica gel granules) designed to absorb any residual moisture. The edges of the unit are sealed with a primary polyisobutylene seal and a secondary hot-melt butyl sealant to create an airtight, moisture-proof barrier. The internal chamber is then filled with dry air or an insulating gas like Argon.

Key Indicators of a Failed Double Glazing Seal

The symptoms of seal failure vary depending on the severity of the leak and weather conditions:

1. Internal Mist or Condensation: The most obvious sign. If you see fogging, misting, or actual water droplets between the two panes of glass, the seal has failed. If the condensation is on the room-side surface of the glass, it is a ventilation/humidity issue; if it is on the outside garden-side of the glass, it means the unit is performing well. Moisture *inside* the unit is the red flag.
2. Cloudy or Dirty Glass: When moisture repeatedly enters and evaporates inside the unit, it leaves behind mineral deposits and dirt. This creates a permanent cloudy, white, or streaky film on the internal surfaces of the glass that cannot be cleaned away.
3. Distorted Reflection: When the gas (Argon) leaks out of the unit, a vacuum can form, causing the glass panes to bow slightly inward toward each other. This causes reflections in the glass to look distorted or funhouse-like.
4. Drafts and Cold Spots: If you feel a draft coming directly through the glass panel (not the frame edges), or if the glass feels extremely cold compared to other windows, it suggests the insulating Argon gas has escaped.

What Causes Blown Double Glazing?

Several factors can cause the seal of an IGU to fail prematurely:

• Age: Over 15–20 years, seals naturally degrade due to constant exposure to temperature changes and environmental stress.
• Thermal Pumping: Daily exposure to the sun causes the air/gas inside the unit to expand, putting pressure on the seals. At night, the unit cools and contracts. This continuous cycle, known as thermal pumping, eventually fatigues the sealant.
• Poor Frame Drainage: Window frames have built-in drainage holes at the bottom to channel rainwater away. If these drainage holes become blocked with dirt or debris, water accumulates in the rebate. The bottom of the glass unit sits in standing water, which degrades the secondary sealant.
• Substandard Installation: If the window was installed incorrectly — for example, if the frame was twisted or glazed without the correct packers — it puts uneven physical stress on the glass unit, cracking the seals.

How to Fix a Failed Window Seal

Once the seal has broken, the desiccant inside the spacer bar becomes saturated, and the thermal insulation properties are lost. There are two primary approaches to resolving this issue:

1. Glass-Only Replacement (Recommended)

If your window frame is structurally sound, you do not need to replace the entire window. Our glazing repair team can remove the glazing beads, lift out the failed glass unit, and install a brand-new, A-rated double-glazed unit into the existing frame. This restores the window's transparency, security, and thermal insulation (U-value of 1.2–1.3 W/m²K) at a fraction of the cost of a full replacement, typically saving 60% to 70% compared to a full new window frame.

2. Full Window Replacement

If the window frame itself is rotten, warped, or does not meet modern security requirements, it is far better to invest in a complete replacement frame and glass unit. This allows you to upgrade to modern multi-chambered uPVC or slimline aluminium profiles.

About the Author: Marian Gheorghe

Head Window Fitter & Active Director, Window Fitters Ltd

Marian Gheorghe is the Head Fitter and Director of Window Fitters Ltd. He has diagnosed and replaced thousands of failed double-glazed units across London and Surrey over the last 15 years, specializing in cost-effective glass swaps and draft-proofing.

Connect on LinkedIn → Expert Reviewed: July 2026

Failed Glazing FAQs

Can I drill a hole in the glass to clear the condensation?

Some companies offer \"double glazing defogging\" services where they drill a hole in the glass, inject drying agents, and seal the hole. While this may temporarily clear the moisture, it does not restore the seal or replace the lost Argon gas. The window's insulating properties are gone, and condensation usually returns. Replacing the glass unit is the only permanent solution.

How long do replacement double-glazed units last?

All our replacement glass units come with our standard 10-year guarantee against seal failure. With proper installation and frame drainage maintenance, replacement units regularly last 20 years or more.

Does a misty window affect building control or survey ratings?

Yes. If you sell your house, a surveyor will flag misted double glazing as a defect, and it will lower your property's EPC score due to heat loss. Swapping the failed units before listing your home is a quick, low-cost way to ensure a clean survey report.