Misted Double Glazing Repair in Liverpool

What does “misted” or “blown” double glazing actually mean?

A double glazed window is not two loose sheets of glass. The glass part is a factory-made sealed unit, sometimes called an insulating glass unit or IGU. Two panes are held apart by a spacer bar around the edge, the hollow spacer is filled with desiccant that soaks up any moisture, and the whole perimeter is sealed. Many units also have argon gas in the cavity to cut heat loss.

When that edge seal breaks down, damp outside air slowly gets into the cavity. For a while the desiccant absorbs it, but once the desiccant is saturated the moisture has nowhere to go. It condenses on the inner faces of the glass when the temperature changes, which is why many people notice it first on cold mornings or when the sun comes round onto the window. Over time it can leave a permanent white haze or streaky tide marks inside the unit. That is a “misted”, “blown” or “failed” unit, and it is the most common job we see as part of double glazing repair in Liverpool.

Is it really a failed unit? Three quick checks

Before you call anyone, it is worth checking where the condensation actually is, because two of the three cases are not faults at all.

A simple test: wipe the inside face with a dry cloth, then go outside and do the same. If the mist is still there after both, it is inside the unit. If you are seeing room-side condensation in a bathroom or bedroom, our trickle vent fitting page explains how background ventilation can help.

Why do sealed units fail?

There is no single lifespan for a sealed unit, and some last far longer than others. The common causes we come across are:

• Age. Edge seals slowly break down. Many homes across Liverpool and Merseyside had uPVC windows fitted from the 1980s through the 2000s, so a lot of first-generation units are now reaching the end of their working life.
• Blocked drainage. uPVC frames have drainage slots in the bottom of the outer frame. If those clog with dirt or leaves, rainwater sits in the glazing rebate and the bottom edge of the unit stays wet, which can attack the seal.
• Poor glazing packers. Units should sit on packers so the weight is supported and the edge seal is not sitting in water or under strain. Missing or badly placed packers can shorten a unit’s life.
• Sun and temperature cycling. South- and west-facing windows heat up and cool down more, and the seal flexes each time.
• Damage to the edge. A knock during fitting, a scraped bead or a solvent-based cleaner on the seal can all start a failure.

When we replace a unit we also check the drainage slots and packers, so the new unit is not set up to fail the same way.

Can misted double glazing be repaired without replacing the glass?

You may see services that drill a small hole in the glass, dry the cavity, add a valve and “reseal” it. This can improve how the window looks for a while, but it does not restore the original sealed, gas-filled unit – the cavity is now open to air through the valve and any argon is gone. We are honest about this: it is a cosmetic option rather than a lasting repair, and it is not one we recommend. Replacing the sealed unit is the fix that brings the window back to proper working order.

What happens when you book a misted unit replacement?

1. Phone call. We ask how many windows are affected, where they are, what the frames are made of and roughly how old they are. A photo helps, especially of any marks printed on the metal spacer bar inside the unit.
2. Measuring visit. We check the condensation is genuinely inside the unit, measure each one accurately, note whether the location needs safety glass and look at the frame, drainage and hardware.
3. Written quote. We explain what we found and quote for the units, with any optional extras such as obscure or low-E glass listed separately.
4. Units made to order. Sealed units are manufactured to size, so there is usually a wait of a few days to a couple of weeks between measuring and fitting.
5. Fitting visit. We remove the beads, take out the old unit, set the new one on fresh packers, re-bead and clean up. A standard unit is often under an hour per window.
6. Final checks. We open, close and lock the window before we leave so you know everything works as it should.

Choosing the glass for your new unit

A replacement does not have to be a copy of what was there. While the unit is being made, you can choose:

• Low-E and argonA modern unit with a low-emissivity coating and argon fill usually keeps more heat in than an old unit from the 1990s. If several of your windows are clear but old, see our energy-efficient glass upgrade page.
• Obscure glassBathrooms, landings and front-door side panels often suit a patterned or frosted pane. Our obscure and privacy glass page shows the usual options.
• Safety glassUnits in doors, low panes and panels beside doors must be toughened or laminated. We check this on every measuring visit.
• Georgian bars or leadsInternal bars or applied leads can be copied so a new unit matches the rest of the house.

Misted units in Liverpool homes

Across the city the pattern is fairly consistent. Inter-war and post-war semis in places like West Derby and Childwall often have uPVC that has been in for decades, with several units failing within a few years of each other. Victorian and Edwardian terraces in areas such as Wavertree have uPVC in older openings, sometimes with tall bay units. Homes near the coast, including Crosby, take a lot of wind-driven rain, which tests drainage and seals. Landlords and sellers often ask us to replace misted units before an inspection or a sale, because a fogged window is one of the first things people notice.

What affects the cost of replacing a misted unit?

We do not publish set prices because no two windows are the same, but these are the factors that matter most:

• The size of the unit, and whether it is a standard rectangle or a shaped unit such as an arch or a triangle.
• The glass type: toughened, laminated, obscure, low-E or triple glazing.
• Extras inside the unit, such as Georgian bars.
• Access – ground-floor windows are simpler than first-floor ones or units over a conservatory roof.
• How many units are being replaced at the same time.

Our double glazing repair cost guide covers this in more depth, with national guide figures from published sources.

When should you get a misted unit replaced?

A failed unit is not dangerous, but it will not get better. The insulating value drops once the seal has gone, the view gets worse, and the white staining inside can become permanent. If a window is also letting in draughts or water, the problem may be the frame seals rather than the glass – our double glazing problems guide helps you tell the difference. If you are not sure, a photo is often enough for us to give you a first opinion on the phone.