If you can see a foggy patch or droplets that you cannot wipe away from either side of the glass, the sealed unit has failed. It is one of the most common jobs we are called to across Glasgow and the central belt, particularly from October to March when the temperature swing between inside and out is at its widest. The good news is that in most cases you only need to replace the glass unit, not the whole window.
Not all condensation on double glazing is a fault, so the first job is to find out where the moisture actually sits. Run a finger across the glass. If the mist smears on the room side, it is ordinary surface condensation caused by warm damp air meeting a cold pane. If it is on the outside and disappears by mid morning, that is a sign of a well insulating unit, not a problem.
If the glass is dry on both faces but the misting is still there, it is trapped between the two panes. That is the only one of the three that means the window itself has failed, and it is what most people mean when they search for this problem.
A double glazed unit is two panes of glass bonded to a spacer bar around the edge, with the gap filled with dry air or argon gas. A desiccant inside the spacer soaks up any residual moisture. Over time the edge seal degrades, outside air is drawn in, the desiccant becomes saturated and moisture starts to condense on the inner faces of the glass where you cannot reach it.
Most units last 15 to 25 years, though we see plenty in Glasgow that fail sooner. Frames that face the prevailing south westerly weather take a constant battering from driving rain, and older aluminium spacer bars tend to fail earlier than modern warm edge types. Poor drainage in the frame is another common cause, because the bottom edge of the unit ends up sitting in water for weeks at a time.
Trickle of moisture can also point to a fitting fault. If the unit was installed without proper packers or glazing tape, the edge seal is stressed every time the sash is opened or the wind loads the glass, and it will fail years before it should.
You may have seen firms offering to drill small holes in the unit and flush the moisture out. This can clear the fog for a while, but it does not restore the seal or the argon fill, so the insulation value of the unit is gone for good and the misting usually returns. We do not recommend it as anything other than a short term cosmetic fix.
The proper repair is to replace the sealed unit while keeping the existing frame. On most uPVC and aluminium windows this is straightforward. The glazing beads are removed, the old unit lifted out, a new unit sized to the same dimensions is fitted with fresh packers and the beads are replaced. A typical unit takes a fitter under an hour once it has been made.
If one unit has gone and the frame is sound, replace the glass. If several units have failed within a year or two of each other, the frames are discoloured or warped, the handles and hinges are worn, or the window is a single glazed or early double glazed installation from the 1980s or 1990s, replacing the whole window is often the better value.
A new A rated window with a warm edge spacer and low emissivity glass will typically achieve a U value of 1.2 to 1.4 W/m2K, compared with around 2.8 for an old unit with a failed seal. In a draughty Glasgow tenement or a 1960s semi that difference shows up on the heating bill and in how comfortable the rooms feel. The honest position is that this depends on the age and condition of the frames, and a proper survey should tell you which route is right rather than a sales pitch.
Check any guarantee you already hold too. Reputable installers offer 10 years on sealed units, and if your windows are newer than that, the fix may not cost you anything.
New units will last longer if the frame drains properly, so make sure the drainage slots along the bottom of the frame are clear of debris. Keep the external seals in good condition and have any cracked or shrunken gaskets replaced before water gets behind the glass. Look for units with a warm edge spacer rather than aluminium, as the flexible seal copes better with the repeated expansion and contraction of a Scottish winter.
If your problem turned out to be condensation on the room side rather than inside the unit, the fix is in the house rather than the window. Use extractor fans when cooking and showering, keep trickle vents open, avoid drying washing on radiators and keep a steady low background heat rather than blasting the heating morning and evening.
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