Condensation and misted double glazing: repair or replace?

Misty or steamed-up windows are one of the most common things homeowners ask about, but "condensation" can actually mean two quite different problems, and it's worth working out which one you've got before calling anyone out. The first is ordinary condensation on the room-facing surface of the inner pane — the same thing that happens on a cold glass of water on a warm day. The second is condensation that forms between the two panes of a sealed double-glazed unit, which is a much more specific issue and generally means the unit itself has failed rather than there being anything wrong with your ventilation or heating habits.

Surface condensation happens when warm, moisture-laden air inside a room meets a cold glass surface and the water vapour turns back into liquid droplets. It's typically worse in winter, first thing in the morning, and in rooms that generate a lot of moisture — bathrooms, kitchens, and bedrooms where windows stay closed overnight. Crucially, this can happen on perfectly good, modern double glazing; it's a sign of humidity and airflow in the room rather than a fault with the window itself, and it usually clears once the room is heated and ventilated.

Condensation between the panes is a different picture. A sealed double-glazed unit has a gap between the two sheets of glass, held apart by a spacer bar that contains a desiccant material designed to keep that internal cavity completely dry. The whole assembly is sealed around the edge, commonly with silicone or a similar sealant. When that seal breaks down, moist air can get into the cavity, and once the desiccant becomes saturated it can no longer absorb it, leaving a persistent fog, staining, or watermarks trapped inside the glass itself. The tell-tale sign is that this misting doesn't go away when you heat or ventilate the room, because the moisture isn't in the room air at all — it's sealed inside the glazing.

Seal failure like this is generally put down to a combination of age and wear, sealant that has dried out and cracked after years of expanding and contracting with the seasons, and sometimes poor original installation — for example a unit fitted slightly out of square, or a frame that wasn't properly sealed against the weather. UV exposure and repeated temperature swings are commonly cited as contributing factors too. Units can and do fail well within what most people would consider the working life of a window, so age alone isn't a reliable guide to whether a particular unit will develop this problem.

The reassuring part is that a failed seal generally doesn't mean the whole window needs replacing. If the frame, hinges and locking mechanism are still sound and it's only the glass that's misted or discoloured, most glaziers can fit a new sealed unit into the existing frame rather than ripping out and replacing the whole window — a job that's typically quicker, less disruptive and considerably cheaper than a full window replacement. Cost guides commonly put a single replacement sealed unit somewhere in the region of £150-£300 depending on size, with small units toward the lower end and larger or specially coated units costing more; a full window replacement, by contrast, is commonly quoted at several hundred pounds upwards per window. These figures vary a good deal by region, glass specification and site access, so it's worth treating them as a rough steer rather than a fixed price and getting a couple of like-for-like quotes.

It's also worth being aware that building regulations for window work aren't uniform across the UK. In England and Wales, replacement of whole windows is typically covered by a competent person scheme such as FENSA, which lets registered installers self-certify compliance; Scotland instead generally requires a building warrant through the local authority and applies its own, separate thermal performance standards; Northern Ireland has its own regulations and its own competent person scheme. A simple like-for-like sealed unit swap within a sound existing frame is generally treated differently from installing an entirely new window, but if you're ever unsure whether particular glazing work needs to be notified, it's worth checking with your installer or local authority rather than assuming the rules are the same everywhere.

For the more everyday surface condensation problem, prevention is mostly about ventilation and moisture control rather than glazing at all. Trickle vents, the small vents built into window frames, are designed to allow a steady trickle of background ventilation and are generally most effective when left open, including through winter, rather than shut to save heat. Extractor fans in kitchens and bathrooms, ideally vented to the outside, make a noticeable difference when moisture-generating activities like cooking and showering are involved, though it's commonly advised not to have windows or trickle vents open in the same room as a running extractor fan, since this can undermine the fan by drawing air straight in from the vent rather than the rest of the room. Simple habits also help: keeping pan lids on while cooking, venting tumble dryers outside or using a condensing model, keeping a steady background level of heating rather than letting rooms go very cold, and leaving a gap between curtains and the glass so warm air can circulate.

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