Have you noticed a misty layer forming between your double-glazed window panes that won’t wipe away? Condensation between window panes is more than just an annoying visual problem, it’s a clear sign that your windows aren’t performing as they should. This moisture build-up indicates a breakdown in your double glazing’s sealed unit, compromising both the appearance of your home and its energy efficiency.
What is Condensation Between Window Panes?
Understanding Double Glazing Technology
Before we look into the problems, it’s helpful to understand how modern double-glazed windows work. These energy-efficient windows consist of two panes of glass separated by a spacer bar, creating an insulated gap that’s typically filled with air or an inert gas like argon or krypton. This sealed unit dramatically reduces heat transfer, keeping your home warmer in winter and cooler in summer thus reducing your energy bills.
The spacer bar around the perimeter contains desiccant beads, moisture-absorbing materials that keep the space between the glass panes completely dry. When functioning properly, the entire unit is hermetically sealed, meaning no moisture can penetrate from outside or inside your home.
How It Differs from Normal Condensation
It’s important to distinguish between condensation on your windows and condensation between the panes. Surface condensation on the interior glass is completely normal, especially in bathrooms, kitchens, or on cold mornings. This happens when warm, moisture-laden air inside your home meets the cooler glass surface, causing water vapour to condense into droplets. You can simply wipe this away.
Condensation trapped between double glazing, however, is an entirely different matter. If you see moisture, fog, or water droplets in the space between the two panes that you can’t reach or wipe away, you’re dealing with a sealed unit failure. This internal condensation indicates that the airtight seal has been compromised, allowing moisture to enter the previously sealed space.
Why This Problem Matters
When moisture infiltrates the cavity between your window panes, several problems emerge. The fogging obscures your view and makes your windows look unsightly, which can be particularly frustrating if you’ve invested in quality glazing. More significantly, the presence of moisture means your windows have lost their insulating properties. That gas-filled or air-filled gap, which once provided excellent thermal performance, is now compromised.
The result? Higher heating bills as warmth escapes through the failed units, cold spots near affected windows, and potential damage to window frames if moisture continues to accumulate. In some cases, prolonged exposure to trapped moisture can lead to mould growth, permanently stained glass, or degradation of internal components.
Common Causes of Condensation Between Window Panes
Seal Failure: The Primary Culprit
The most common reason for condensation in double glazing is seal failure. Double-glazed units rely on a perimeter seal, typically made from butyl, silicone, or polysulphide compounds, to keep the space between the panes airtight. Over time, these seals can deteriorate due to several factors.
Age and natural degradation can also lead to internal moisture between window panes. Most quality double-glazed units come with warranties ranging from 10 to 25 years, but the seals can fail earlier depending on various conditions. As sealants age, they become less flexible and can crack or separate from the glass or spacer bar, creating tiny gaps where moisture can enter.
UV exposure from direct sunlight breaks down seal materials over time, particularly on south-facing windows that receive the most intense sunlight throughout the day. The constant expansion and contraction caused by temperature fluctuations places stress on the seals, eventually leading to failure.
Temperature Fluctuations and Thermal Stress
Extreme temperature changes can accelerate seal breakdown. Windows in unheated conservatories, porches, or rooms that experience dramatic temperature swings are particularly vulnerable. When glass heats up, it expands; when it cools, it contracts. This constant movement stresses the edge seals and spacer bars.
Additionally, windows fitted in positions where they receive intense, direct sunlight for extended periods may experience thermal shock—rapid temperature changes that cause uneven expansion across the glass surface. This is especially problematic with dark-framed windows or those with tinted glass, which absorb more heat.
How To Fix Moisture Between Window Panes
Short-term fixes (temporary)
If you need to delay a full repair or replacement:
- Drill-and-vent repair: Some specialty companies offer a defogging service. They drill small holes, dry the space with air or a desiccant, then reseal it. This clears fog and slows future moisture buildup, but it doesn’t restore full insulation.
- Dehumidifiers & ventilation: Reduce indoor humidity (especially in kitchens/bathrooms) to lessen condensation—but this won’t fix the seal itself.
Long-term fix (permanent)
You have two main options:
- Replace the sealed glass unit (IGU)
- Keep the existing window frame (cheaper than full window replacement).
- A glazier can remove the failed unit and install a new, factory-sealed IGU.
- Costs depend on size and glass type.
- Replace the entire window
- Recommended if multiple panes are fogging or the frame is also damaged, inefficient, or hard to remove.
- This ensures new seals, modern insulation, and can improve energy efficiency.
Condensation between window panes can be annoying. If left untreated, however, it can lead to more serious problems.
If you’re having problems with condensation between your window panes and you want a permanent solution, contact Window & Doors 2 You. You can call us on 01952 474 898 or contact us online.