If you can see mist or water droplets trapped between the two panes of your double glazing, no amount of wiping will shift it. The moisture is inside the sealed unit itself, and it means the seal has failed. Here is what has actually gone wrong, what your options are, and what a fix realistically involves.
A double glazed unit is two panes of glass bonded around a spacer bar, with the gap filled with dry air or argon gas. Inside the spacer bar sits a desiccant, a drying agent that soaks up any trace of moisture so the gap stays perfectly clear. Over time the perimeter seal degrades, usually after 10 to 20 years, and humid air starts leaking in faster than the desiccant can absorb it. Once the desiccant is saturated, moisture condenses on the inner faces of the glass whenever the temperature drops.
South-facing windows tend to fail first because daily heating and cooling makes the unit expand and contract, a process known as solar pumping that gradually works the seal loose. Poor original installation matters too. If the unit was fitted without proper glazing packers, it can sit in pooled water at the bottom of the frame, which rots the seal from below. We see this regularly in Dunstable and the surrounding villages on windows fitted in the 1990s and early 2000s that are now well past their expected life.
Not really, and it is worth being honest about this. Wiping, dehumidifiers and ventilation only help with condensation on the room side of the glass. Once moisture is between the panes, the sealed unit has failed and cannot recover on its own.
Some companies offer a drill-and-dry service, where small holes are drilled in the glass, the moisture is extracted, and valves are fitted. It can clear the misting for a while and typically costs less than a new unit, but it does not restore the argon gas or the insulating performance, and the misting often returns within a couple of years. For a unit that is otherwise sound it can be a stopgap, but we would not recommend it as a long-term fix.
The good news is that if your frames are in decent condition, you usually only need a new sealed unit, not a whole new window. A glazier measures the existing unit, orders a replacement made to size, and swaps it in. Most installs take under an hour per window, and the frame, handles and locks stay exactly as they are.
Cost depends on size, glass type and spec, but as a rough guide a standard replacement unit for an average casement window tends to fall somewhere between £80 and £200 fitted, with larger units, obscure glass or toughened safety glass costing more. A modern unit with a warm edge spacer and low-emissivity glass will also insulate noticeably better than one made 20 years ago, so you often end up with a warmer window than the original.
The replacement unit should come with a guarantee, commonly 5 to 10 years, so ask before you order and keep the paperwork. Proper installation matters as much as the unit itself. The glass should sit on setting blocks so it can drain, and the frame's drainage slots should be clear so rainwater escapes rather than pooling around the seal.
A little routine care helps too. Clear the drainage slots along the bottom of the outer frame once or twice a year with a thin plastic tool, and check the external seals for gaps or shrinkage each autumn. Bedfordshire winters bring plenty of damp, still days, and standing water against a seal is the main thing that shortens its life.
Usually not. If the frames are sound, a replacement sealed unit can be fitted into your existing frame, which costs a fraction of a full window replacement. New windows only make sense if the frames themselves are draughty, warped or rotten.
Possibly. Many installers guarantee sealed units for 5 to 10 years and some longer, so dig out your original paperwork or FENSA certificate before paying for a repair. If the windows came with the house, the guarantee may still be valid and transferable.
Yes, slightly. A failed unit has lost its insulating gas layer, so it performs closer to single glazing and you may notice that window feels colder and attracts more condensation on the room side too. It is worth replacing before winter rather than after.
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