What is actually going on inside the unit
A sealed double glazed unit is two panes of glass bonded to a spacer bar around the edge, with a strip of desiccant inside that spacer to absorb the small amount of moisture trapped in during manufacture. An outer sealant then locks the whole thing together and keeps the argon gas fill in place.
Over years of expansion and contraction from heat and cold, that edge seal gradually breaks down. Once it does, humid air from outside starts working its way in every time the temperature changes, and the desiccant inside the spacer becomes saturated and can no longer soak it up.
Why the desiccant matters
While the desiccant still has capacity, tiny amounts of moisture get absorbed invisibly and you see nothing. Misting only appears once that desiccant is fully saturated and can absorb no more, which is why a unit can seem perfectly fine for years and then start fogging quite suddenly.
How this differs from ordinary surface condensation
Condensation on the inside of the room facing pane, the surface you can actually wipe with a cloth, is a completely different issue caused by warm moist air in the room meeting a cold glass surface. That kind clears when you wipe it or improve ventilation. Misting between the panes never clears with wiping because it is sealed inside the unit itself, which is the simplest way to tell the two apart.
- Surface condensation: on the room side of the glass, wipes away, linked to humidity indoors
- Trapped condensation: between the two panes, cannot be reached, means the seal has failed
Can the unit be replaced on its own?
In most cases, yes. If the surrounding uPVC or timber frame is still square, sound and free of rot or warping, a glazier can measure the opening, order a new sealed unit to the exact size, and fit it into the existing frame without touching anything else.
When the frame changes the decision
If the frame itself has warped, the beading no longer seats properly, or the frame profile is an old style that is difficult to source matching beading for, then replacing just the glass becomes impractical or poor value. In those cases the more sensible route is a full window replacement rather than fighting an ageing frame to save one unit.
- Frame square and sound: replace the glass unit only
- Frame warped, rotten or draughty elsewhere: replace the whole window
- Several units misting across the house: worth reviewing whole house replacement costs
Got a misted window right now?
Call 0116 442 0332 and we will tell you honestly whether it is a unit swap or a full replacement.
Does trapped condensation cause damp or mould?
Trapped condensation stays sealed inside the unit itself and does not release moisture into the room, so on its own it is not a source of damp or mould in the house. What it does affect is how the window looks and how well it insulates, since the moisture and lost gas fill both reduce the thermal performance of that pane.
Where damp or mould does appear near a window, the more likely cause is separate: a failed seal around the frame itself letting water in from outside, or poor ventilation in the room creating high humidity that condenses on cold surfaces nearby. Worth checking both possibilities rather than assuming the misted unit is to blame for a damp patch on the wall.
How a replacement unit is measured and fitted
A glazier removes the internal glazing beading carefully so it can be reused or matched, lifts out the failed unit, and takes precise measurements of the opening rather than the old glass, since the old unit may have distorted slightly with age. The new sealed unit is then ordered to those measurements with matching spacer bar colour and glass specification, so it looks consistent with the rest of the window once it is back in place.
Fitting itself is usually quick once the new unit arrives, typically well under an hour per window, with the beading refitted and any sealant reapplied around the edge to keep the installation weathertight.
The cost logic in simple terms
A single unit swap is a fraction of the cost of a new window because you are paying for glass and labour, not a full frame, hardware and finishing. But if you are already replacing two or three misted units in an old frame that is also draughty and hard to operate, the maths can tip toward full replacement being the better long term spend rather than patching an ageing window piece by piece.
Getting a proper decision, not a guess
Because the right call depends entirely on the condition of your specific frame, this is genuinely worth a short site visit rather than a decision made from a photo. We cover this exact question in more depth in our companion guide on whether to repair or replace misted double glazing, which walks through the decision step by step.