If you've noticed a foggy, misty appearance between the two panes of your double glazing that won't wipe away, you're looking at a failed sealed unit. It's one of the most common double glazing problems we see across Bristol — and understanding why it happens helps you make the right decision about fixing it.
How Sealed Double Glazing Units Work
Every double-glazed window consists of two panes of glass separated by a spacer bar around the edges. This spacer bar contains a desiccant — a moisture-absorbing material — and the cavity between the panes is filled with an insulating gas, typically argon. The entire assembly is sealed airtight with a durable edge sealant. This sealed cavity is what gives double glazing its insulating properties, trapping a layer of low-conductivity gas that dramatically reduces heat transfer.
The desiccant inside the spacer bar plays a crucial role: it absorbs any tiny amount of moisture that may have been present during manufacturing, keeping the cavity bone-dry. When this system is intact, your windows stay clear and provide excellent thermal insulation.
Why Sealed Units Fail — And Why Bristol's Climate Makes It Worse
Over time, the edge sealant that holds the unit together begins to degrade. Constant exposure to UV radiation, temperature fluctuations and moisture gradually breaks down the seal. Once even a microscopic gap appears, moist air seeps into the cavity. The desiccant absorbs this moisture initially, but once it's saturated, water vapour accumulates between the panes — and that's when you see condensation.
Bristol's maritime climate accelerates this process considerably. Our city experiences high average humidity, frequent rainfall and significant temperature swings between day and night — particularly in areas like Clifton and Redland where elevated positions face prevailing south-westerly winds. Homes in Bedminster and Southville, close to the river, often experience even higher humidity levels. These conditions place extra stress on window seals, which is why we see so many failed units across the city.
Internal Condensation vs Between-Pane Condensation
It's important to understand the difference. Condensation on the inside surface of your windows (the room side) is usually caused by high indoor humidity and poor ventilation — this is normal and can be managed with better airflow. Condensation on the outside surface actually indicates your windows are working well, as the outer pane stays cold enough for dew to form.
However, condensation between the panes is entirely different. It means the sealed unit has failed and there is no DIY fix. No amount of ventilation, heating or dehumidifying will clear moisture that's trapped inside the sealed cavity. The only solution is to replace the sealed glass unit itself.
Health and Energy Implications
Failed sealed units aren't just an aesthetic nuisance. The persistent moisture creates ideal conditions for mould growth around window frames, which can trigger respiratory problems, allergies and worsen conditions like asthma — particularly concerning for families with young children or elderly relatives. We frequently see significant mould problems in homes across Fishponds, Eastville and Kingswood where older double glazing installations are reaching the end of their lifespan.
There's also a real financial impact. A failed sealed unit loses its insulating gas fill, meaning your window's thermal performance drops significantly. For homes in exposed areas like Portishead, Clevedon and Thornbury, where winter winds are relentless, this translates directly into higher heating bills. Replacing failed units typically pays for itself within two to three heating seasons through energy savings alone.
The Fix: Sealed Unit Replacement
The good news is that fixing condensation between panes doesn't mean replacing your entire window. At Pane Relief, we remove only the failed glass unit and fit a brand-new sealed unit into your existing frames. This is significantly cheaper than full window replacement, causes minimal disruption, and restores your windows to full performance. Most replacements take just one to two hours per window, and we carry a wide range of standard sizes to ensure a fast turnaround for homeowners across Bristol and the surrounding areas.