When double glazing mists up, the seal around the sealed unit has failed and moisture has got between the panes. The frame is almost always fine. We replace just the glass unit and leave the frame in place — typically £75–£190 per unit, against several hundred pounds for a whole new window.
A double glazed unit is two panes bonded to a spacer bar, with a desiccant inside the spacer that absorbs the small amount of moisture sealed in at manufacture. The whole assembly is sealed at the edge with a butyl and polysulphide seal.
Over time — usually 10 to 20 years, faster on a south-facing elevation where thermal cycling is harshest — that edge seal degrades. Moist air gets in, the desiccant saturates, and once it is saturated, moisture condenses between the panes every time the temperature drops. That is the fog you are looking at. It cannot be dried out, and it cannot be resealed; the unit has to be replaced.
You will find companies offering to drill the unit, flush it and reseal it. It clears the visible fog. It does not restore the unit: the desiccant is dead, the argon fill is gone, the cavity is now vented to outside air, and the thermal performance is permanently degraded. It will mist again.
Replacing the sealed unit costs a little more and actually fixes the problem, restores the U-value, and comes with a guarantee.
Measure the unit — width, height, overall thickness, and the glass makeup (for example 4-16-4). Match the specification, including low-E coating, warm-edge spacer and argon fill. Make the unit. Return, remove the beads, take out the failed unit, fit the new one and re-bed the beads.
One visit to measure, one to fit, usually 3–7 days apart because the unit is made to order. On a straightforward job we can often measure and fit in the same week.
| Job | Typical cost |
|---|---|
| Misted / blown double glazed unit, replaced per unit | £80 – £190 |
| Same-day glass replacement, standard pane per pane | £120 – £280 |
Ranges include VAT and installation. Final price depends on size, spec, access and site condition — a survey gives you a fixed number.
Typically £75–£190 per sealed unit, replaced. That compares with several hundred pounds for a complete new window — and because the frame is usually sound, replacing the whole window is normally unnecessary.
Not properly. 'Defogging' — drilling and flushing the unit — clears the visible mist but leaves a dead desiccant, no argon and a cavity vented to outside air. The thermal performance is permanently lost and it will mist again. Replacing the sealed unit is the only real fix.
The edge seal on the sealed unit has failed, letting moist air into the cavity and saturating the desiccant inside the spacer bar. It typically happens after 10–20 years, and sooner on south-facing elevations where the unit goes through the harshest daily temperature swings.
Almost never. The frame, the hinges and the locks are usually fine — it is only the sealed glass unit that has failed. If the frame is a pre-1990s aluminium one with no thermal break, that is a different conversation, because it will keep producing condensation on the inside face regardless of the glass.
Replacement sealed double glazed units, made to measure and fitted UK-wide. Low-E, warm edge spacer, argo…
What double glazing repairs actually cost: misted units, broken glass, handles, hinges, locks, patio roll…
Why aluminium windows get condensation, why old non-thermally-broken frames stream with water, and what a…
Same-day and next-day aluminium window and door repairs across the UK. Misted units, broken glass, handle…
Free survey, fixed price, no pressure. Or call us and we'll give you a realistic ballpark in five minutes.
