Misted Double Glazing: How to Fix Condensation Between Window Panes

If your window looks foggy, heavily misted, or has water droplets trapped physically inside the glass where you can’t wipe them, your double glazing unit has officially “blown.” Here is the truth about how to fix it.

1. What Does Misted Double Glazing Mean?

You have tried wiping the inside of your window. You have gone outside and wiped the exterior. However, the cloudy, foggy mist remains completely stubbornly trapped. Sound familiar?

In the glazing industry, we refer to this as a “blown” or failed sealed unit. Specifically, it means the critical hermetic seal that joins the two panes of glass together has broken down over time. It is no longer air-tight. Consequently, your window has completely lost its thermal efficiency.

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When the perimeter seal fails, heavy water droplets and dense fog become permanently trapped inside the cavity where they cannot be wiped away.

2. The Anatomy of a “Blown” Window Seal

To fundamentally understand why this has happened to your window, you need to look at how modern double glazing is actually manufactured.

A modern Insulated Glass Unit (IGU) is not just two pieces of glass simply shoved into a plastic frame. Instead, the two glass panes are held precisely apart by a spacer bar. The edges are then heavily injected with an industrial sealant. Finally, the cavity between the glass is filled with Argon gas (which acts as a much better insulator than standard air).

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The anatomy of an Insulated Glass Unit (IGU). When the black perimeter sealant perishes, the entire unit fails.

How the Moisture Gets Trapped

Inside the spacer bar, manufacturers hide tiny silica beads (desiccant)—essentially the same moisture-absorbing beads you find in a new shoebox. If a microscopic hairline crack develops in the perimeter seal (usually due to years of natural expansion and contraction from the sun), moist outside air is slowly sucked into the cavity.

Initially, the silica beads absorb this moisture, so you don’t notice a problem. However, eventually, the beads become completely saturated. Consequently, the exact moment they can hold no more water, the moisture rapidly turns into visible condensation on the inside of the cold glass.

3. The DIY Myth: Can I Just Drill a Hole in the Glass?

If you search online for “how to fix misted windows,” you will inevitably find DIY videos suggesting you drill tiny holes into the glass or spacer bar to manually “vent” the moisture out, followed by injecting a chemical drying agent.

As a professional with four decades of experience, I strongly advise against this. Here is exactly why:

• Thermal Destruction: The moment your seal originally failed, the heavy, insulating Argon gas escaped. Drilling a hole actively guarantees that the gas is gone forever. While you might temporarily clear the visual fog, you are left with two pieces of glass that offer vastly reduced thermal insulation. Therefore, your heating bills will not benefit.
• It is Temporary: The original perimeter seal is still broken. Consequently, more moisture will inevitably return.
• Safety Risks: Drilling directly into toughened safety glass will cause it to instantly shatter into thousands of pieces, requiring immediate emergency replacement.

4. The Real Fix: Glass Unit Replacement

Many homeowners immediately panic when they see a blown window, assuming they will have to pay thousands of pounds to rip out their uPVC or aluminium window frames, ruining their wallpaper and plasterwork in the process.

The great news is: You rarely need a whole new window.

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A KJM installer safely removing the internal glazing beads. We simply extract the failed glass and slot a brand new, A-rated unit directly into your existing frame.

Because the window frame and the glass are actually two completely separate components, our installers can perform a direct “glass replacement.” Specifically, we carefully unclip the retaining beads from your existing frame, remove the heavy, water-logged failed unit, and safely slot a brand new, Argon-filled, highly energy-efficient glass unit directly in its place.

Ultimately, it takes a fraction of the time, causes zero mess to your interior decor, and costs significantly less than a full window replacement.

5. Frequently Asked Questions