Misting between the panes means the sealed unit has failed. The perimeter seal holding the two sheets of glass together has broken down, the drying agent inside has saturated, and moist air is now condensing on the inner face of the glass. You cannot dry it out or clean it. The fix is a new sealed unit, and on a frame that is still sound that is often all you need, with no reason to replace the whole window.
Key takeaways
- Condensation between the panes always means a failed seal. Condensation on the room side is a ventilation issue instead.
- A failed unit loses much of its insulating benefit, because the cavity gas has escaped and moisture conducts heat.
- If the frame, hinges and locks are sound, replacing just the glass is usually the right route.
- If frames are distorted or hardware is failing across the house, glass alone is a patch rather than a repair.
- It is also the moment to upgrade the specification, for example to a low emissivity coating or acoustic laminate.
How a sealed unit is actually built
A modern unit is two panes held apart by a spacer bar, with the gap filled by a dry dense gas, normally argon, which conducts heat less readily than air. Inside the spacer sits a desiccant that absorbs the small amount of moisture sealed in at manufacture. A primary and a secondary seal keep the assembly airtight. Once either is compromised, the desiccant starts absorbing outside moisture instead, and when it saturates you see mist.
The upgrade in modern units is often the spacer itself. A warm edge spacer (an insulating strip in place of the old aluminium bar) reduces the cold bridge at the edge of the glass, which cuts edge condensation and helps the whole unit last. We go into the components in our guide to what actually makes a window energy efficient.
Why do sealed units fail?
Age is the headline answer, but it is not the whole story. The biggest single cause we see is water sitting against the bottom edge of the unit.
The bottom edge of a sealed unit is meant to sit on setting blocks with drainage below, so water that gets into the frame runs out through the weep holes. When those slots block with debris, or a unit has been bedded straight onto the frame with no drainage path, water pools against the secondary seal and degrades it. Clearing drainage slots is the simplest maintenance job in glazing and almost nobody does it.
| Where you see it | What it means | What fixes it |
|---|---|---|
| Between the panes, in the cavity | The perimeter seal has failed and the desiccant is saturated | A new sealed unit. Drying cannot work on a sealed space |
| On the room side of the glass | Warm humid room air meeting a cold surface | Ventilation, extraction at source, warmer glass |
| Outside face, early morning | Surface cooling below the dew point overnight | Nothing. It shows the glass is insulating well |
Is replacing just the glass enough?
Often, yes, and the decision comes down to the condition of everything that is not the glass. Glass only replacement makes sense when the frame is square, the gaskets are still pliable, hinges and locks operate smoothly, drainage is clear and the external seal to the brickwork is intact. In that situation the frame has plenty of life left and a new unit restores the performance.
Full replacement becomes the better route when the frame has bowed, gaskets have shrunk and let water into the rebate, hinges are corroding, or failures are appearing across the whole house. At that point you are chasing the same job repeatedly. Our piece on five signs your uPVC frames have reached the end of their life covers the frame side in detail.
How a sealed unit replacement is carried out
What drives the cost of a sealed unit replacement?
The glass specification is the main lever. A plain unit and one with a soft coat low emissivity coating, an acoustic laminate inner pane and toughened safety glass are different products entirely. Size matters next, and not in a straight line, because oversized units need more careful handling. Then come the specifics: whether safety glazing is required in that position, whether an obscure pattern has to be matched to neighbouring panes, whether a leaded or Georgian design has to be reproduced, and what access the elevation needs.
If you are weighing up whether to repair or upgrade, the wider comparison in our article on double versus triple glazing and glass performance is a useful next read, and the BFRC explains how window energy ratings are calculated if you want to understand the labels.
Simple habits that make sealed units last longer
- Clear the drainage slots along the bottom of the outer frame twice a year with a soft brush.
- Never block or seal weep holes, even if you see water at them after rain. That is them working.
- Keep external sills clear so water runs away from the frame rather than sitting against it.
- Check gaskets are still soft and seated rather than shrunken and lifting.
- Report a single misted pane early. One unit is a small job.
Frequently asked questions
Can misted double glazing be cleaned or dried out?
How long should a sealed unit last?
Will one failed unit affect the others?
Does the frame have to come out to change the glass?
Can I upgrade the glass while it is being replaced?
Is toughened glass required in my window?
Should I claim on a guarantee instead?
A single misted pane is a small job, not a whole new window
If one or two panes have gone cloudy and the frames are otherwise sound, you very likely do not need new windows. We will come out, tell you honestly whether the glass or the frame is the problem, and quote for whichever is genuinely the right answer. If you are already thinking about a wider upgrade to double glazing across the house, we will price both so you can compare properly.