Misted Double Glazing Repair London
A milky haze or trapped droplets between the panes won't wipe away, because the moisture is inside the sealed glass unit. If you've been searching for misted double glazing repair London options, the good news is that the fix rarely means new windows. In most cases the frame is sound and only the sealed unit has failed. This guide explains why it happens, how to confirm it, why quick fixes fall short and what affects the cost of a proper replacement.
Quick answer: Double glazing mists between the panes when the unit's seal fails, letting moist air in and saturating the desiccant inside the spacer bar. Wiping or drilling won't restore it properly. Replacing just the sealed glass unit in the existing frame will, at a fraction of the cost of a whole new window.
Key Takeaways
Mist between the panes means the unit's seal has failed. It can't be wiped away or dried out from inside the room.
Drilling and "demisting" can clear the glass for a while, but the broken seal and the lost argon gas stay broken.
Replacing only the sealed unit restores clarity and insulation while keeping your existing frame, hinges and locks.
A standard clear unit is the most economical to replace. Large, toughened, laminated or decorative glass costs more because it is made to a higher specification.
Glass in doors and low-level panes must be safety glass, so a misted unit in those spots is a good chance to bring it up to standard.
Why Double Glazing Mists Up Between the Panes
How a sealed unit is put together
A double-glazed "sealed unit" is two panes of glass held apart by a hollow spacer bar around the edge. Glaziers also call it an insulated glass unit, or IGU. The spacer is filled with desiccant: tiny beads of molecular sieve that soak up any moisture trapped when the unit was made. Each pane is bonded to the spacer with a primary seal of butyl (polyisobutylene). A secondary seal of polysulphide, polyurethane or silicone then wraps the outer edge. The cavity holds either dry air or argon gas, and argon insulates noticeably better.
While both seals hold, the cavity stays bone dry and you never think about it. The desiccant has a limited capacity, though. Once a seal is breached, humid outside air is drawn into the cavity every time the unit warms and cools. The beads slowly fill up. When they can't absorb any more, moisture condenses on the inner faces of the glass, and that's the fog you're looking at. Glaziers describe this simply as a failed seal, or a "blown" unit.
What makes the seal fail
Age is the main culprit. Most sealed units last somewhere between 10 and 25 years, depending on how well they were made and fitted. Every sunny afternoon the gas inside expands, and every cold night it contracts. The seals are flexing constantly, which glaziers call "breathing". South- and west-facing windows take the most punishment, as do the large panes in bay windows, because they heat and cool the most.
Water is the other big enemy. uPVC and aluminium frames have small drainage slots in the bottom of the glazing rebate. When these clog with grit, leaves or old sealant, rainwater sits around the bottom edge of the unit and gradually breaks down the secondary seal. Several other things speed up failure:
poor original fitting, such as missing glazing packers or a unit resting directly on the frame
pressure washing
harsh solvent cleaners
some aftermarket window films
Across London, a great deal of the uPVC fitted to Victorian and Edwardian terraces in the 1990s and early 2000s has now reached the age where units start to go. That's why misted panes often turn up in clusters along the same street, from Streatham to Walthamstow.
Is It Really a Failed Unit? How to Tell
The wipe test
Not every steamy window has a failed unit. Condensation on the room-side face usually comes from indoor humidity: cooking, hot showers, drying clothes on radiators or closed trickle vents. Condensation on the outside face on a cold, clear morning is actually a good sign. It means the glazing is keeping the heat in, so the outer pane stays cold.
The test is simple. Take a dry cloth and wipe both the inside and outside faces of the glass. If the haze or droplets are still there afterwards, the moisture is between the panes and the unit needs attention. No amount of ventilation, dehumidifiers or extra heating will clear it.
Signs a unit needs replacing
Look for any of the following:
fog that comes and goes with the weather, often worst on a sunny morning after a cold night
droplets or streaks running down inside the cavity
white or grey tide lines left behind as trapped moisture evaporates
a small pool of water sitting along the bottom of the spacer bar
a pane that feels noticeably colder than the windows around it
Act on those tide lines sooner rather than later. They are mineral deposits, and over time they can permanently etch the inner surface of the glass.
Cracks, chips at the edge or a visibly perished seal around the perimeter are also clear signs. A cracked unit has lost its seal by definition, even if it hasn't misted yet. Cracked glass in a door or a low-level pane is also a safety issue.
Things that look like failure but aren't
Rainbow-coloured swirls that shift as you move your head are usually Brewster's fringes. This optical effect appears when two very flat, high-quality panes sit almost perfectly parallel. It's harmless and doesn't mean the seal has gone. A faint neutral tint is also normal: modern low-emissivity coatings give the glass a slight cast that can look different from older units next to it. Neither of these needs any work.
Why Drilling and Wiping Rarely Fix Misted Glass
How "demisting" works
Some firms offer to clear misted units without replacing them. The method is broadly the same everywhere:
one or two small holes are drilled in the outer pane
a cleaning solution is sprayed into the cavity and flushed out
the cavity is dried with warm air
the holes are plugged with small caps or one-way vent valves
On paper it's cheaper than a new unit, and straight afterwards the glass can look clear again.
Why it falls short
The trouble is that the original seal is still broken. Any argon that was in the cavity has gone for good and been replaced by ordinary air, so the unit insulates worse than it did when new. The desiccant in the spacer bar is still saturated. As the unit carries on breathing through its failed edge seal, moisture can creep back in, often within a season or two. The valves themselves can collect dirt and leave small visible circles in the glass. If minerals have already etched the inner surfaces, no amount of flushing removes the cloudiness, because the damage is in the glass itself.
There's a hard limit, too. Toughened glass can't be drilled at all, because it shatters the moment you try. Most glass in doors, side panels and low-level windows should be toughened, so it can't be treated this way.
Home remedies fare even worse. Wiping obviously can't reach the cavity. Running a bead of silicone around the outside of the glass is a common DIY attempt, but it can trap moisture against the edge seal, block drainage and make the eventual reglazing messier.
Misted Double Glazing Repair London: Replacing Just the Sealed Unit
The lasting fix is to replace the failed unit and keep the frame. You may not need to replace the whole window if four things are true:
the frame isn't warped, cracked or badly discoloured
the sash opens and closes properly
the hinges and handles work
the locks engage
A new sealed unit brings back clear views, restores the insulating cavity and removes the weak point a failing seal can become. For most households, misted double glazing repair London work like this costs a fraction of a new window and causes far less disruption.
What the process looks like
It starts with an accurate survey. The glazier measures the unit's width, height and overall thickness. Thickness is commonly 24mm or 28mm in uPVC frames, but it can be 20mm on older installations or 32mm and above on newer ones. They also record the glass type, spacer bar colour, any Georgian bars or leaded detail, obscure patterns for bathrooms, and whether the position calls for safety glass. Sealed units are made to order by a glass processor, so there's normally a gap of around one to two weeks between survey and fitting.
On the day, the glazing beads are carefully unclipped and the old unit is lifted out. On most modern uPVC the beads are on the inside, while some older installations have them on the outside. The rebate is cleaned, the drainage slots are cleared, and the new unit is set on fresh glazing packers. On opening sashes, the glazier uses toe and heel packing, wedging packers at opposite corners so the sash stays square. Get this wrong and the sash can drop and catch on the frame. The beads go back in, the gaskets are checked and the job is done. A typical unit takes 20 to 40 minutes, with no plastering, no making good and no lost window frame.
Upgrading while you're at it
A unit fitted 20 years ago may have had a centre-pane U-value of around 2.8 W/m²K. A modern unit fills the same space and performs far better. With soft-coat low-E glass, argon fill and a warm-edge spacer, a centre-pane U-value of about 1.1–1.2 W/m²K is typical, and that warmer spacer also helps reduce condensation round the edges.
Other upgrades can be fitted in the same space:
acoustic laminated glass, if you're on a busy route like the South Circular or under a Heathrow or City Airport flight path
laminated glass for extra security on ground-floor windows
a fresh obscure pattern for a bathroom
Safety glass and the regulations
Building Regulations Approved Document K requires safety glass in "critical locations". In practice that means:
any glazing in a door, or in a side panel within 300mm of a door, up to 1500mm from floor level
glazing in other windows and screens below 800mm from the floor
Many older units in these positions are ordinary annealed glass. Replacing a misted one is the ideal time to switch to toughened or laminated glass. Toughened glass carries a permanent stamp in one corner showing its standard. Simply swapping the glass in an existing frame isn't generally treated as notifiable window replacement work. Replacing the whole window is, and needs either a competent person scheme certificate or building control sign-off. The government's guidance on Building Regulations approval explains when approval is required.
What Affects the Cost of a New Sealed Unit
Comparing your options
Every window is different, so the only reliable figure is a free, written quote. The comparison below shows why replacing the sealed unit is usually the sweet spot between cost and a result that lasts.
Option | Restores insulation? | How long it lasts |
|---|---|---|
Wiping, ventilating or heating | No | Doesn't touch mist inside the unit |
Drill and demist | Partly cosmetic; argon not restored | Variable; mist often returns |
Replace sealed unit (standard clear, up to about 1m²) | Yes, often better than original | Typically 10–25 years |
Replace sealed unit (large, toughened, laminated, Georgian or decorative) | Yes | Typically 10–25 years |
Replace the whole uPVC window | Yes | Frame and glass renewed |
What affects the cost
Size matters most. A small fanlight unit costs far less than a full-height patio pane. Glass specification is next: toughened, laminated, acoustic, self-cleaning and obscure glass all add cost, as do internal Georgian bars and leaded designs. Access also plays a part. Upper-floor windows in mansion blocks, converted flats or tall townhouses in places like Islington and Clapham may need extra time, or occasionally access equipment, if the unit can't be fitted safely from inside.
Doing several units at once usually brings the cost per unit down, because one survey and one fitting visit cover them all. That makes sense if a few panes are showing early signs. It's also worth asking whether a firm charges for the visit. Choosing one with no call-out charges and a written quote before work starts keeps the total predictable. For lettings, estate agents and property managers often batch misted units across a block or portfolio for the same reason.
For most London households, misted double glazing repair London jobs cost a small fraction of replacing every window in the house.
Frequently Asked Questions
Can misted double glazing be fixed without replacing the glass?
The glass can sometimes be cleared by drilling and flushing the cavity, but that doesn't repair the failed seal or replace lost argon gas, so performance stays reduced and mist often returns. Etched glass can't be cleared at all, and toughened glass can't be drilled. Replacing the sealed unit is the only fix that reliably lasts.
How long does it take to replace a misted sealed unit?
Units are made to measure, so allow roughly one to two weeks from survey to fitting. The fitting itself is quick, usually 20 to 40 minutes per unit, and the frame stays in place. Your window is open to the elements only for the few minutes it takes to swap the units over.
Is a misted unit a security risk?
The mist itself doesn't weaken the window, but it shows the unit is ageing. It's also a sensible prompt to check how the glass is held in. Externally beaded windows can be easier to tamper with than internally beaded ones. A visit to replace misted glass is a good time to check hinges, handles and locks too. If you're in London and want everything looked at in one go, ask for a check of the whole window.
Will a new unit match my other windows?
A good glazier will match the unit's thickness, spacer bar colour, obscure pattern and any Georgian bars so the new glass blends in. Modern low-E glass can have a very slightly different tint from much older units next to it. If that concerns you, ask to see a sample, or replace units in matching pairs on the same elevation.
Are misted units covered by a guarantee?
Many installers guarantee sealed units for a set period, so check your original paperwork first, especially if the windows are under ten years old. If the installer has stopped trading, an insurance-backed guarantee may still apply. Once any guarantee has expired, replacing the individual unit is normally far cheaper than claiming for, or buying, a whole new window.
Misted panes rarely mean the end of your windows. In most cases they mean one tired component has reached the end of its life. Vibrant JDR Windows Limited has been repairing windows and doors across London since 2016, including glass replacement for misted and damaged units in uPVC, aluminium and tilt and turn windows. There are no call-out charges for repairs, and every job starts with a free, no-obligation quotation. If you've spotted fog between your panes, get in touch with the team at Vibrant JDR Windows Limited and arrange a quote to get your clear views back.
Comments