Condensation inside double glazing: why it happens and how to fix it
What mist between the panes means, why it will not go away, what a new sealed unit costs in 2026, when the whole window is worth replacing, and the rules for replacing only the glass.
Mist you cannot wipe off, sitting between the two panes of a double-glazed window, means the seal around that glass unit has failed. It will not clear on its own, and ventilating the room will not touch it. We explain how to tell it from ordinary condensation, what fixes it, what a new sealed unit costs in 2026, and when the whole window is worth replacing instead.
What condensation inside double glazing means
The glass in a double-glazed window is a sealed unit: two panes held 16mm to 20mm apart by a spacer bar around the edge, with the gap often filled with argon or krypton gas. The spacer bar holds a desiccant, a drying agent that soaks up the small amount of moisture sealed in when the unit is made. Sealant bonds the glass to the spacer so that no air gets in or out.
When that seal fails, outside air creeps into the gap. The desiccant soaks up its moisture until it is full, and after that the water condenses on the inner faces of the glass. Glaziers call the result a blown, misted or failed unit. All three names mean the same fault.
Is the condensation between the panes or on the glass?
Condensation can form in three places on a double-glazed window, and only one of them is a fault. Wipe the room side with a dry cloth, then the outside. If the mist is still there after both, it is between the panes.
| Where it forms | What it means | What fixes it |
|---|---|---|
| Room side of the glass | Moist indoor air meeting the coldest surface in the room | Less moisture, more ventilation, steady heat |
| Between the two panes | The sealed unit's edge seal has failed | A new sealed unit, or a new window |
| Outside face of the glass | The unit insulates well, so the outer pane stays cold | Nothing: it clears as the morning warms up |
Wet glass on the room side every morning is ordinary condensation on the inside of the glass, and it comes from moisture made in the house by breathing, cooking, showers and drying laundry. A fan, open trickle vents and a dehumidifier all help with that. None of them can reach water sealed between two panes.
Condensation on the outside face shows up on clear, cold mornings, mostly on newer units. The outer pane gets so little heat from the room that it cools below the dew point of the outdoor air, which means the unit is doing its job.
Why double glazing seals fail
Every sealed unit fails in the end. Units usually last 10 to 25 years, and for standard double glazing the seal commonly goes after 10 to 15. These are the things that shorten that:
- Age. Sealant hardens and loses its grip on the glass over the years, and the desiccant can only absorb so much before it is full.
- Heat and cold. The gas in the gap expands on a sunny afternoon and contracts on a cold night, so the seal flexes every day. South-facing windows get the biggest daily swing.
- Water at the edge of the unit. When the drainage slots at the bottom of the frame block, rain sits against the lower edge of the unit and keeps the seal wet for days.
- How it was made and fitted. A unit badly sealed at the factory, or glazed into the frame so that its weight bears on the seal, fails early.
Two things help a new unit last. Keep the drainage slots clear so water runs out of the frame, and ask for warm-edge spacers, which run warmer at the edge of the glass than the old aluminium spacer bars.
Will condensation between the panes go away?
No. The water is sealed in with nowhere to go. It comes and goes with the temperature, thick on a cold morning and faint or gone on a warm afternoon, which is why a failed unit can look clear when a glazier visits at lunchtime.
A hair dryer on the glass clears the mist for a few minutes by warming the pane, and it condenses again as the glass cools. Left for months or years, the water leaves a white, cloudy film of mineral deposits on the inner faces that stays even on dry days, and no cleaning reaches it.
Does misted double glazing need fixing straight away?
It is not an emergency. A failed unit still keeps out the wind and rain, and the window still opens and locks. What it loses is the view, and some of its insulation, because the gas that filled the gap has leaked away and damp air has taken its place.
Fix it before the mineral film sets in, before winter if the room is hard to heat, and before you sell, because a buyer can see misted glass from across the room.
Ways to fix condensation inside double glazing
There are three fixes, and only two of them last.
| Fix | What happens | Does it last? |
|---|---|---|
| Demisting (drill and dry) | Small holes are drilled near the corners of the glass, a cleaning solution is sprayed in, the gap is dried and the holes are sealed with vent plugs | The view clears, but the seal stays broken and the unit never gets back its original insulation |
| New sealed unit | The glazier removes the failed unit and fits a new one made to measure in the existing frame | Yes, with a new guarantee on the glass |
| New window | Frame and glass are replaced together | Yes, and it deals with a failing frame at the same time |
Anti-condensation sprays sold for misted units treat the glass, not the seal. For a frame in good condition, a new sealed unit is the fix.
How much it costs to replace a misted double glazing unit
These are 2026 prices for a new sealed unit, supplied and fitted in the existing frame with the old unit taken away. The size of the pane and the type of glass set the price.
| Unit | Price per unit |
|---|---|
| Small unit, such as a bathroom window or top opener | £60 – £160 |
| Standard casement unit | £80 – £220 |
| Large unit, such as a living room or bedroom window | £120 – £350 |
| Obscure or patterned glass | £140 – £300 |
| Toughened or laminated safety glass | £180 – £400 |
| Georgian bar or leaded | £200 – £450 |
| Patio door or large door glass | £250 – £650+ |
Some glaziers include VAT, the call-out and disposal of the old unit in the price, and some list them separately. Upper-floor windows that cannot be reached from inside may need access equipment on top. Ask for every item in writing before you compare two quotes.
New sealed unit or a whole new window
If the frame is sound, replace the unit. The frame is where the money goes in a new window, and a good frame outlasts more than one set of glass.
Replacing the whole window makes more sense when:
- a timber frame is soft or rotten, or a uPVC frame is warped, cracked or badly discoloured
- the hinges, handles or locks are failing as well as the glass
- the windows are 20 years old or more and several units are misting, since most windows last 20 to 30 years
- you want a different frame material, colour or style anyway
A new uPVC casement costs £400 to £900 fitted, aluminium £700 to £1,600 and timber £1,000 to £2,500. Fitting labour varies by region, and fitting a new window in London and the South East costs 10% to 20% more than the national average.
Is misted double glazing covered by the guarantee?
Often, if the windows are under 10 years old. Most manufacturers guarantee sealed units for 10 years, and many replace a failed unit free of charge within that time. The paperwork from the original installation names the installer and the guarantee length.
Windows fitted by an installer registered with a competent person scheme also come with an insurance-backed warranty and a complaints procedure, which is the next call if the installer will not deal with a failed unit.
Building regulations for replacing a sealed unit
In England, a new window and new glass in an old frame are treated differently. A whole replacement window is a controlled fitting. It needs building regulations approval, which a registered installer can self-certify, and it has to meet a U-value of 1.4 W/m²K or a Window Energy Rating of band B.
Replacing only the glass is not a controlled fitting. Approved Document L defines the controlled fitting as the entire window including the frame, and says that new glazing in an existing frame does not have to meet the energy efficiency requirements.
Safety glass still matters in the risk zones. The safety glazing rules cover any pane up to 800mm above the floor, glass in a door up to 1500mm, and a side panel within 300mm of a door's edge up to 1500mm. A glazier prices toughened or laminated glass for those panes, which is why they sit higher in the price table.
Scotland, Wales and Northern Ireland
In Scotland, the building standards technical handbook lists work to a window that is not a complete replacement, including its glazing, as needing no building warrant, though the work must still meet the standards. Wales publishes its own Approved Documents and Northern Ireland has its own building regulations, so the England rules above are not the test in Cardiff, Swansea or Belfast. The council's building control team can confirm what a glass-only job needs there.
What happens when a glazier replaces a sealed unit
Each unit is made to measure, so the job usually takes two visits.
- The glazier measures each failed unit, notes the glass each pane needs, and checks the frame, hinges and locks.
- The new units are made to those sizes, with safety glass wherever a pane sits in a door or low down.
- On the fitting visit the glazing beads come off, the failed unit comes out and the new one goes in on packers so it sits square in the frame.
- The beads go back on, the frame is cleaned and the old unit is taken away.
- You get the guarantee for the new units in writing.
Common questions
Will condensation between panes go away?
Condensation between the panes of double glazing will not go away, because the water is sealed inside a unit whose seal has failed. It fades on warm afternoons and comes back on cold mornings. The lasting fix is a new sealed unit, or a new window if the frame is failing too.
Should you get condensation inside double glazed windows?
You should never get condensation between the two panes of a double-glazed unit, because a working seal keeps damp air out of the gap. Condensation on the room side of the glass is different: it comes from moisture in the house and is common on cold mornings. A wipe with a dry cloth tells you which one you have.
How do you get rid of condensation inside double glazing?
Replacing the sealed unit gets rid of condensation inside double glazing for good. Demisting, where holes are drilled and the gap dried, clears the view but leaves the seal broken. A hair dryer clears it only until the glass cools again.
How much does it cost to fix condensation between window panes?
A new sealed unit for a standard casement window costs £80 to £220 supplied and fitted in 2026. Small units start at about £60, large ones run to £350, and toughened or laminated safety glass costs £180 to £400. A whole new uPVC window costs £400 to £900 fitted.
Can I replace just the glass in a double glazed window?
You can replace just the sealed unit when the frame, hinges and locks are sound. The glazier measures the glass, has a unit made and fits it in the existing frame. In England, glass-only replacement does not have to meet the energy standard that a whole new window does.
Does a hair dryer get rid of condensation inside double glazing?
A hair dryer clears condensation inside double glazing for a few minutes at most. Warming the glass turns the water back into vapour, but the vapour stays sealed in the gap and condenses again as the pane cools. The seal is still broken afterwards.
Is misted double glazing covered by a guarantee?
Misted double glazing is often covered when the windows are under 10 years old, because most manufacturers guarantee sealed units for 10 years. Find the installation paperwork and report the fault to the installer in writing before paying anyone else to replace the unit.
Do I need building regulations approval to replace a sealed unit?
Replacing only the glass in an existing frame is not a controlled fitting in England, so it does not have to meet the energy rules for a new window. A whole new window does need approval. In Scotland, glazing work short of a full replacement needs no building warrant but must still meet the standards.
Why is there condensation on the inside of my double glazing in the morning?
Condensation on the room side of double glazing in the morning comes from moisture in the house settling on the coldest surface. Two adults asleep breathe out up to half a litre of water a night. Open trickle vents and curtains left open overnight let the air reach the glass and carry it away.