Misted Double Glazing Units
Causes, signs, and solutions for blown or failed sealed units – you almost certainly don't need new windows.
What Causes Misting in Double Glazing?
A double glazed sealed unit consists of two panes of glass separated by a spacer bar, with the cavity between them filled with dry air or an insulating gas such as argon. The entire unit is sealed around its perimeter with a primary butyl seal and a secondary polysulphide or silicone seal to keep moisture out and the gas in.
Inside the spacer bar is a desiccant – typically silica gel – that absorbs any trace amounts of moisture that make their way into the cavity over time. This system works well for years, but it doesn't last forever. Here's what causes it to fail:
- Seal degradation over time – the perimeter seal gradually breaks down through exposure to UV light, heat, and moisture. No seal lasts indefinitely, and even the best-quality units will eventually fail.
- Desiccant saturation – the silica gel inside the spacer bar can only absorb a finite amount of moisture. Once it's fully saturated, any additional moisture entering the cavity condenses on the inner surfaces of the glass.
- Age – a typical sealed unit manufactured to BS EN 1279 durability testing standards lasts between 15 and 25 years. Units approaching or exceeding this range are far more likely to develop misting.
- Thermal pumping – every day, as the sun heats the unit and the temperature drops again at night, the gas or air inside the cavity expands and contracts. This daily cycle of expansion and contraction places repeated stress on the perimeter seal, gradually weakening it over thousands of cycles.
- Poor original installation – if the unit was not glazed correctly (inadequate drainage, incorrect packers, or beads that trap water against the seal), the seal can fail prematurely. Poorly drained frames are a common cause of early seal failure.
- Physical damage – impact damage, building movement, or even aggressive window cleaning with pressure washers can compromise the seal, allowing moisture ingress well before the unit's expected lifespan.
Signs of a Failed Sealed Unit
Not sure whether your double glazing has actually failed? Look for these telltale signs:
- Condensation between the panes – this is the definitive sign. If the moisture is between the two sheets of glass (not on the inside room surface or the outside street surface), the seal has failed. You can confirm by wiping both surfaces – if the condensation doesn't clear, it's trapped inside the unit.
- Milky or hazy appearance – even when there are no visible water droplets, a failed unit often looks cloudy, milky, or slightly hazy. This is caused by mineral deposits left behind as moisture repeatedly condenses and evaporates inside the cavity.
- Water droplets trapped inside – in more advanced cases, you may see actual water droplets or even a small pool of water sitting at the bottom of the unit between the panes.
- Patches that come and go with temperature – misting often appears in the morning when the temperature is cool and disappears during the warmest part of the day. This intermittent pattern is a classic early sign of seal failure.
- Visible seal degradation around the edges – look at the perimeter of the unit where the glass meets the spacer bar. If you can see gaps, cracks, or crumbling sealant, the seal has broken down and moisture is entering freely.
If you recognise any of these signs, your sealed unit has failed and will need to be replaced to restore thermal performance and clear the misting permanently.
Can Misted Units Be Repaired?
You may have seen advertisements for "defogging" or "mist removal" services. Here's how the process works and why we recommend full replacement instead.
Defogging (Temporary Fix)
A technician drills a small hole through the glass or spacer bar, injects a drying agent or flushes the cavity with dry air to remove the trapped moisture, and then reseals the hole with a one-way valve or plug. The process typically costs £60–£120 per unit.
While defogging can clear the misting in the short term, there are significant limitations:
- The original gas fill (argon, krypton, or xenon) is lost and replaced with ordinary air, reducing the thermal performance of the unit
- The underlying seal failure is not repaired – moisture will re-enter the cavity over time
- Defogging typically lasts 1–3 years before the misting returns
- Drilling the unit voids any remaining manufacturer's warranty
- The mineral deposits (haze) already formed on the inner glass surfaces are not removed
Full Replacement (Permanent Solution)
Replacing the sealed unit is the only permanent solution. A new unit is manufactured to your exact measurements with fresh glass, a new spacer bar with dry desiccant, your choice of gas fill, and a factory-sealed perimeter. The new unit is fitted into your existing frame – there's no need to replace the window frame itself, provided it's in good condition.
A new replacement sealed unit restores full thermal performance, eliminates the misting permanently, and comes with a fresh manufacturer's warranty. It's the option we recommend for every customer. Read our complete replacement guide for more detail on the process.
The Replacement Process
Replacing a misted sealed unit is straightforward and far less disruptive than you might expect. Here's how it works, step by step:
Contact Us
Get in touch by phone, email, or our online quote form. Tell us how many units need replacing and we'll guide you through the next steps.
Measure
We can arrange a free measuring survey, or you can measure the units yourself using our step-by-step guide and send us the dimensions.
Manufacture
Your new sealed units are manufactured to your exact specifications – size, glass type, gas fill, and spacer bar – in our UK facility to BS EN 1279 standards.
Remove Old Unit
Our installer carefully removes the internal beading and lifts the failed sealed unit out of the frame. The frame rebate is cleaned and checked.
Install New Unit
The new sealed unit is positioned into the existing frame with correct packers to ensure proper drainage and load distribution.
Refit Beading
The beading is replaced to hold the new unit securely in the frame. Job done – most replacements take just 30–60 minutes per window.
The entire process is clean, quick, and can usually be completed in a single visit. There's no scaffolding, no mess, and no disruption to your daily routine.
Cost of Replacing Misted Units
Replacing a misted sealed unit is a fraction of the cost of replacing the entire window. Prices depend on the size of the unit, the type of glass, and the gas fill you choose. Here's a guide to typical costs:
| Unit Size (W × H) | Standard Float + Air | Low-E + Argon | Toughened Low-E + Argon |
|---|---|---|---|
| 500 × 500mm | From £70 | From £100 | From £140 |
| 600 × 900mm | From £90 | From £130 | From £170 |
| 800 × 1200mm | From £120 | From £170 | From £220 |
| 1000 × 1000mm | From £130 | From £180 | From £240 |
| 1200 × 1200mm | From £160 | From £220 | From £290 |
Prices shown are supply only. Installation is typically £80–£160 per unit depending on access, location, and frame type.
Factors That Affect the Cost
- Size – larger units require more glass and materials
- Glass type – upgrading from standard float to Low-E glass improves thermal performance but increases the cost
- Gas fill – argon gas adds a small premium over air-filled units but significantly improves insulation. Krypton is more expensive but ideal for slimline units
- Toughened glass – required by Building Regulations in certain locations (doors, low-level windows, near stairs) and costs more than standard glass
- Spacer bar type – warm-edge spacer bars reduce heat loss at the edge of the unit and cost slightly more than standard aluminium spacers
- Number of units – replacing multiple units in one visit is more cost-effective per unit
For exact pricing on your requirements, visit our full price breakdown or get a free quote.
Frequently Asked Questions
Condensation between the two panes of a double glazed unit means the perimeter seal has failed. Every sealed unit contains a desiccant (silica gel) inside the spacer bar that absorbs small amounts of moisture entering the cavity. Over time – typically 15 to 25 years – the seal degrades through thermal pumping (daily heating and cooling cycles that expand and contract the unit), UV exposure, and general age. Once enough moisture gets past the seal and saturates the desiccant, water vapour condenses on the inner glass surfaces, causing the familiar misting or fogging effect. The only permanent fix is to replace the sealed unit.
There is a process called defogging where a technician drills a small hole in the unit, injects a drying agent to remove the moisture, and reseals the hole. While this can temporarily clear the misting, it does not restore the original gas fill or thermal performance of the sealed unit. Defogging typically lasts only 1 to 3 years before the misting returns, and it voids any remaining warranty on the unit. The only permanent solution is to replace the failed sealed unit with a new one – which can be fitted into your existing frames without needing new windows. Contact us for a free replacement quote.
Replacement costs depend on the size of the unit, the type of glass, and the gas fill. A standard 600×900mm sealed unit with float glass and argon fill starts from around £90 for supply only. A larger 1200×1200mm Low-E argon-filled unit typically costs £160–£240. Installation labour is usually £80–£160 per unit depending on access and location. Replacing just the sealed unit is significantly cheaper than replacing the entire window and frame. Visit our cost guide for a full UK price breakdown.
It depends on where the condensation appears. Condensation on the inside surface of the glass (room side) is caused by high indoor humidity and is normal – it does not indicate unit failure. Condensation on the outside surface (street side) can actually be a sign that your sealed unit is working well, as it means the outer pane is cold because the unit is effectively insulating. However, condensation between the two panes – inside the sealed unit – is a definite sign of seal failure. The unit will need to be replaced to restore thermal performance and clear the misting permanently.