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Misted Double Glazing Repair

Diagnosis: failed sealed unit

Misted double glazing replacement in London: new glass, same frame.

Glazing London replaces misted, blown and failed double glazed units across Central & North London. New sealed units are made to measure in 7-10 days and fitted into your existing frames, usually in under an hour per unit. Free quotes 24/7 by phone or WhatsApp photo, with no call-out fee.

WhatsApp a photo of the mist Call 07399 253464

Misted double glazed unit with condensation trapped between the panes — Same window, same frame, only the sealed glass unit is replaced.

The diagnosis

Why double glazing mists up.

A double glazed unit is two panes of glass bonded to a spacer bar around the edge, sealed airtight, with a moisture-absorbing desiccant inside the spacer. Over 10-20 years, sunlight and thermal movement fatigue the perimeter seal. Once it cracks, humid air is drawn into the cavity every time the temperature swings.

At first the desiccant quietly soaks up that moisture, so you see nothing. When it becomes saturated, the excess condenses on the inner faces of the glass: fog, droplets, and eventually milky staining and mineral tide-marks that etch the glass permanently. That's what glaziers call a failed or blown window. Condensation between panes can't be wiped away, and drilling "demisting" holes only hides it for a season, because the seal and desiccant are still dead.

Fog or droplets inside

Condensation between panes that never wipes off. The classic sign.

Milky or streaked glass

Long-failed units stain white as minerals dry onto the inner faces.

Cold spots & higher bills

The insulating gas has escaped, so the window loses heat like single glazing.

It comes and goes

Early failure shows only on cold mornings, then clears, but it will worsen.

The cost logic

Replace the glass, not the frame.

What we do: new sealed unit only

Keeps your existing uPVC, aluminium or timber frames
Typically 50-70% cheaper than a full window replacement
No scaffolding, no plaster damage, no redecorating
Fitted from inside in under an hour per unit
Modern low-E, argon-filled glass, better than the original

What you rarely need: a whole new window

Only justified when the frame itself is rotten, warped or broken
Days of work and disruption instead of an hour
Costs several times more per window

If your frames genuinely are past saving, we'll say so, and our installation team quotes fairly because we fabricate our own frames.

Not sure which you need? Frame problems like stiff hinges, dropped sashes or leaking gaskets are handled by our window & door repair service, often in the same visit as the new glass.

How it works

From misted to crystal clear in four steps.

Measure the failed unit Free survey

We measure the visible glass and the unit's overall thickness. Most are 24mm or 28mm (e.g. 4-20-4: two 4mm panes over a 20mm spacer bar). Spacer bar size, glass type and any Georgian bars or leading are recorded so the new unit matches exactly. Photos over WhatsApp are often enough for a quote.

Your unit is manufactured 7-10 working days

Each sealed unit is made to measure with low-E glass, argon fill and a warm-edge spacer. Where the window is a door panel, low-level glazing or within 300mm of a door, we specify toughened glass to meet Building Regulations.

Fitted in under an hour per unit No mess

We remove the beads or gaskets, lift out the failed unit, re-pack and square the new one, and reseal. Frames and decoration are untouched; old units are taken away for recycling.

Checked, guaranteed, invoiced

Every unit is checked for squareness and drainage, covered by our workmanship guarantee, and you receive an itemised invoice. Multiple units in one visit cost less per unit, so most customers do the whole elevation at once.

The energy rating benefit

A blown unit has lost its argon fill and insulates little better than single glazing. Today's replacement units combine low-E coated glass, argon-filled cavities and warm-edge spacers, with centre-pane U-values around 1.2 W/m²K versus roughly 2.8 for a failed unit. That means warmer rooms, less condensation on the room side, and lower heating bills, without touching the frames.

Get a free fixed quote

Want the full background first? Read our guide: misted double glazing, what to do. Multiple failed panes on a busy road? Consider pairing new units with secondary glazing for noise, or browse all glass replacement services.

Questions

Misted units, answered

Why has my double glazing misted up between the panes?

The perimeter seal of the sealed unit has failed and the desiccant in the spacer bar is saturated, so moist air entering the cavity condenses on the glass. It cannot be wiped away because it is inside the unit, so the unit needs replacing.

Do I need a whole new window if my double glazing is blown?

No. If the frame is sound we replace only the sealed glass unit, keeping your existing uPVC, aluminium or timber frame. It typically costs a fraction of a full window replacement and each unit is fitted in under an hour.

How long does misted double glazing replacement take?

We measure the failed unit, have a new sealed unit manufactured to the exact size and spacer thickness in 7-10 working days, then fit it, usually in under an hour per unit, with no damage to frames or decoration.

Can drilling or "demisting" fix a misted unit permanently?

Drill-and-vent demisting only clears the visible fog temporarily. The failed seal and saturated desiccant remain, so moisture returns and the insulating cavity is lost for good. Replacing the sealed unit is the only permanent fix and usually costs little more.

Will a new sealed unit improve energy efficiency?

Yes. A failed unit has lost its insulating gas fill. We replace it with a modern low-E, argon-filled unit on a warm-edge spacer, with better performance than the original unit had even when new, and lower heat loss through the glass.

How many misted units can you replace in one visit?

As many as you need. Most homes with one failed unit have others on the way. We survey every window for free while on site, and replacing several units in one visit works out cheaper per unit. Book a free survey or call 07399 253464.

Cracked or smashed rather than misted? That's 24/7 emergency glazing, we make it safe first, then reglaze.