The fog between the panes never appears on a convenient day. One morning the kitchen window has a grey bloom in the middle of the glass, you wipe it, and nothing happens, because the moisture is not on either surface you can reach. It is inside the unit.
What has happened is simple. A double glazed unit is two panes sealed around a spacer bar, with dry air or gas in the gap. The seal has failed, damp air has crept in, and the desiccant in the spacer bar has given up absorbing it. The misting is the symptom. The failed seal is the fault. And crucially, it is a fault in the glass unit, not in the window.
Why that distinction matters to your wallet
Plenty of homeowners ring the company that fitted the windows, hear the word 'replacement', and assume the whole window has to come out, frame and all. Usually it does not. If the frame is sound, a glazier can measure the failed unit, order a new sealed unit to size, and swap it in an hour or so. The frame stays, the disruption is minimal, and the bill is a fraction of a full window replacement.
Some firms only sell full windows, so a full window is what they will quote you. That is not dishonesty exactly, it is just what they do. Repair specialists exist precisely because of this gap, and a surprising number of the companies listed on this site handle failed units as their bread and butter.
When the whole window really should go
There are honest reasons to replace rather than repair. If the frame is early uPVC that has gone brittle and yellow, if hinges and locks are failing at the same time, if the window is single-glazed with a bolted-on secondary pane from the 1980s, or if half the units in the house are misting within a year or two of each other, you are watching a generation of windows reach the end of its life together. Paying to re-glaze one unit at a time in dying frames is a slow way to spend more.
Age is the useful signal. A failed unit in a ten-year-old window is a repair. A failed unit in a thirty-year-old window is a warning.
Check the guarantee before you pay anyone
Sealed units commonly carry guarantees of five or ten years, and the window installation itself may have a longer one. Dig out the paperwork from the original job before booking anything. If the installer is still trading and the unit is in guarantee, the fix may cost you nothing but a phone call. If the installer has vanished, an insurance-backed guarantee from the original job may still respond, which is one reason that paperwork is worth keeping in the first place.
Getting it looked at
Ask whoever quotes you to state, in writing, whether they are pricing a replacement sealed unit or a full window, and to say why. A firm that can explain the difference clearly is a firm you can work with. You can browse double glazing companies near you, including repair specialists, or describe the misted unit and get quotes without leaving the house.
One practical tip before anyone visits: photograph the misted unit in morning light, when the fogging shows worst, and measure the visible glass. Those two things let a repair firm give you a meaningful price over the phone.
