Condensation Between Window Panes: Causes & How to Fix It

Introduction

If you've noticed a cloudy haze or droplets of water between the panes of your double-glazed windows, you're not alone. Condensation between window panes is one of the most common glazing complaints we deal with at AMB Glass — and the good news is, it's fixable.

The bad news? It's not something that goes away on its own, and it's a sign that your sealed unit has failed. In this guide, we explain exactly what's happening, why it can't be wiped away, and what your options are.

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What Is Causing the Condensation?

Double-glazed windows consist of two panes of glass separated by a spacer bar and sealed together to create a hermetically closed unit. This cavity is filled with argon gas, which acts as an insulating barrier.

Over time — typically after 15–25 years, though sometimes sooner — the seal around the edge of the unit can break down. Once this happens, the argon gas escapes and is replaced by ordinary air. Because air contains moisture, that moisture condenses on the inner glass surfaces when temperatures change.

The result: a persistent haze, fogging or water droplets trapped between the two panes that cannot be cleaned from either side.

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Why Can't I Just Wipe It Away?

The condensation is forming on the inside surfaces of the glass — between the two panes — where no cloth or cleaning product can reach. Even if you could somehow get into the cavity, the root problem is the failed seal, not the moisture itself.

Until the unit is replaced, the condensation will continue to return, and the appearance of the window will typically get worse over time.

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Is It Dangerous?

Condensation between the panes isn't a structural or safety hazard — the window will continue to hold. However, a failed sealed unit does have real consequences:

• Reduced insulation: The argon gas is what makes double glazing thermally efficient. Once it escapes, the window performs closer to single-glazed, meaning higher heating bills and a colder room.
• Reduced visibility: Heavy misting can significantly obscure the view, particularly on windows that face south or west where sunlight accelerates the effect.
• Cosmetic deterioration: Calcium and mineral deposits left by repeated condensation cycles can cause permanent staining on the inner surfaces of the glass over time.

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Surface Condensation vs Failed Sealed Unit — How to Tell the Difference

Not all window condensation is a failed seal. Here's how to tell them apart:

Surface condensation (on the room-side surface of the glass):

• Appears on cold mornings
• Clears as the room warms up
• Usually worse in kitchens and bathrooms
• Not a glazing fault — caused by humidity in the room

Condensation between the panes:

• Trapped between the two glass layers
• Does not clear when the room warms up
• Visible as a milky haze or water droplets inside the unit
• Indicates a failed seal — the unit needs replacing

If you're not sure, look at the window closely on a warm afternoon. If the haziness is still there, it's between the panes.

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Can a Failed Sealed Unit Be Repaired?

Unfortunately, no. There's no way to reseal a failed double-glazed unit once moisture has entered the cavity. Some companies offer 'condensation removal' services using drilling and desiccant injection — but these are temporary at best and don't restore the unit's thermal performance.

The correct solution is to replace the sealed unit.

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Does This Mean I Need to Replace the Whole Window?

Not necessarily. If your window frames are in good structural condition — no rot, cracking, warping or broken locking mechanisms — you can replace just the glass unit and keep the existing frame. This is the most cost-effective solution and avoids the disruption of a full window replacement.

The replacement process typically takes 30–60 minutes per window. A new unit is cut to your exact specifications, the old unit is removed and the new one is fitted into the existing frame. The result looks and performs like a brand-new window.

You would need a full window replacement if:

• The frame is damaged or structurally compromised
• The frame is very old and the new unit doesn't fit the original groove specification
• You want to upgrade to a different frame style or material

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How Long Should Double Glazing Last Before the Seal Fails?

Quality double-glazed units from reputable manufacturers typically last 15–25 years before seal failure becomes common. Several factors affect longevity:

• Quality of the original unit: Cheap units use low-quality sealant and spacer bars that fail sooner
• Installation quality: Poor installation — particularly inadequate drainage — accelerates seal failure
• Exposure: South and west-facing windows are subject to more thermal expansion and contraction, which stresses the seal
• Cleaning habits: Aggressive cleaners applied directly to the edge seals can accelerate breakdown

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How Much Does It Cost to Replace a Failed Sealed Unit?

The cost depends on the size of the window, the glass type (clear, obscure, toughened) and the number of units. As a general guide:

• Small windows (e.g. bathroom obscure): from around £80–£120 per unit
• Standard casement window: from around £100–£180 per unit
• Large patio or bay window: from around £180–£350+ per unit

These are supply-and-fit prices. A local glazier like AMB Glass will give you a firm quote once they have your measurements.

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What to Do Next

If you've confirmed you have condensation between the panes, the next step is to get a quote for sealed unit replacement. A local glazier can typically measure, order and fit the replacement within a few days.

At AMB Glass, we cover Uxbridge and all surrounding areas — Hillingdon, Ealing, Hounslow, Slough and beyond. We'll assess your windows, give you an honest quote and complete the replacement with minimal disruption.