Blown double glazed windows occur when the airtight seal between two panes of glass fails, allowing moisture to enter. This leads to window condensation between panes, creating a cloudy or misted appearance.
While some DIY methods claim to fix misted double glazing, the most reliable and lasting solution is to replace the failed sealed unit. Professional double glazing replacement restores insulation, clarity, and energy efficiency — saving money in the long run.
What causes double glazing to blow?
The sealed unit in a double-glazed window is engineered to keep the insulating gas (often argon) trapped between two panes, while blocking moisture and external air. When the seal breaks (i.e., you get failed window seals), air and moisture enter, and you end up with misted double glazing or full blown units.
This seal failure might be due to age, manufacturing defects, poor installation, high-indoor humidity or use of harsh chemicals around the seals.
Signs you have a blown double-glazed window
Condensation or fogging between the panes
A sure sign of a compromised seal is visible window condensation between panes—cloudiness, mist or fog inside the unit.
Rising energy bills
When the insulating layer is lost, your heating system has to work harder—so you’ll notice higher costs.
Cold draughts and increased noise
A window with broken seals may let cold air, draughts or more outdoor noise in—so if you experience that, it’s worth checking.
Mould, damp or white dust inside the unit
Left unaddressed, moisture in the sealed unit can lead to damp patches or mould. If you spot a fine white dust inside the glass, that’s likely the desiccant (moisture-absorbing material) breaking down.
How to fix blown double glazing
Replace the sealed glass unit
Often the most practical and cost-effective option is to replace only the sealed unit (rather than the whole frame). A glazier measures up and installs a new unit into your existing frame—restoring clarity and insulation.
DIY repair kits – worth it?
DIY kits promise to “defog” the unit by drilling holes, injecting drying agents and resealing—but they don’t restore the original insulating gas, results are inconsistent, and they may void warranties.
Replace the entire window
If the frame is warped, rotting or suffering major issues, then full window replacement is better. This also allows upgrading to high-performance glazing (for instance via our uPVC Windows With Double Glazing & Installation service page).
Costs of Repairing or Replacing Blown Double Glazed Windows
When it comes to blown double glazed windows, the cost depends on the extent of the problem and the condition of your existing frames. Sometimes, a simple seal repair can fix the issue; other times, replacing the glass or even the entire window is the smarter long-term option. Here’s what UK homeowners can expect to pay.
Repairing the Window Seals
If your window is only showing small signs of failure — perhaps a faint draught or minor condensation at the edges — then repairing the seals could be enough.
Professional resealing typically costs between £50 and £100 per window, depending on size and accessibility. This involves removing and replacing the failed seal to restore the airtight barrier.
It’s a good short-term fix if the frame and glass unit are still in good condition, but it won’t always solve deeper problems such as window condensation between panes. If the misting keeps coming back, the seal inside the glass unit itself has likely failed and needs replacing.
Replacing Just the Glass Unit
When you can see visible mist or fog trapped between the panes — a sure sign of failed window seals — the most effective fix is to replace the sealed glass unit while keeping your existing frame.
For standard-sized double glazing, costs typically range from £140 to £400 per window, depending on the size, location, and type of glass. Smaller panes may be as low as £50 to £190.
This option restores insulation, clears the fog, and brings the window’s energy efficiency back to normal — without the cost or disruption of a full window replacement. It’s ideal when your frames are still structurally sound and visually in good shape.
Replacing the Full Window
If your frames are old, warped, or showing signs of rot, then it’s worth replacing the entire window rather than just the glass. This is a bigger investment, but it gives you a brand-new, fully sealed and energy-efficient system that should last for decades.
For a standard uPVC double-glazed window, the cost generally falls between £200 and £700 per window, including installation. For premium materials like timber or aluminium, expect anywhere from £600 to £1,200+ per window.
For full-house replacements — typically eight to ten windows in a three-bed home — total costs range between £3,000 and £10,000+, depending on style, size, and specification.
While this is the most expensive route, it’s also the most effective long-term solution if several windows are failing or you want to upgrade to higher-performance or triple-glazed units.
Repair vs Replacement: Which Is Better Value?
| Option | Typical Cost | What You Get | Best For |
|---|---|---|---|
| Repair the window seals | £50 – £100 per seal | Quick fix to stop small draughts; re-establishes airtight barrier | Minor leaks or early-stage condensation, good frames |
| Replace the glass unit | £50 – £400 per window | Restores clarity, insulation and energy efficiency | Misted or fogged units with solid frames |
| Replace the full window | £200 – £1,200+ per window | Complete upgrade with new frame and sealed unit | Old, warped or multiple failing windows |
In many cases, replacing just the glass unit gives you the best balance of cost and performance. However, if your windows are over 15 years old, or you’re seeing widespread misting across the house, full replacement may save more in the long run through better insulation and lower energy bills.
If your problem is isolated and your frames are in good shape, double glazing replacement of just the sealed unit is the most economical fix. But if your windows are reaching the end of their lifespan, investing in full replacement can increase comfort, improve efficiency, and enhance your home’s overall value.
Preventing future double-glazing failure
Keep your windows and seals clean
Regular cleaning with mild soapy water helps avoid grime build-up that can damage rubber seals. Avoid oil-based or harsh solvents.
Manage indoor humidity
Cooking, showering and drying laundry indoors all boost indoor moisture—use extractor fans and ventilate properly to protect the seal.
Inspect sealant and caulking
Look for cracked or failing silicone around the frame; resealing helps prevent infiltration that can lead to deterioration of the sealed unit.
Handle windows gently
Avoid slamming or forcing windows open or shut. Smooth operation ensures the seals and hardware don’t suffer undue stress.
Use professional installation
Quality really matters—choosing an installer from Home Windows ensures your windows receive the correct fit, specification and support (see our New Windows Cost guide).
Expert help for blown double glazing
Whether you’re dealing with misted double glazing, worrying about double glazing replacement or simply want to know when it’s time to upgrade, Home Windows offers trusted advice and reliable installation across the UK. Our page on Bay Window Cost can help if you have non-standard shapes or large units.
FAQS
Can you replace just the glass in double glazing?
Yes – in many cases you don’t need to replace the full window frame. Swapping the sealed glass unit within the existing frame is often sufficient to restore performance. This makes it a smart route for homeowners dealing with blown double glazed windows who want to avoid the cost and disruption of full replacement.
How long should double glazed windows last before they blow?
Most quality double glazed windows last between 15 and 25 years, depending on the quality of the seals, installation, and environmental exposure. Windows in south-facing or coastal properties may degrade faster due to UV and salt exposure.
Is condensation inside double glazing covered by a warranty?
Yes, in most cases. Many manufacturers offer 10- to 20-year warranties against seal failure or misting inside the glass. However, if the issue is due to accidental damage, poor maintenance, or expired warranty, you’ll likely need to pay for a replacement yourself.
Can you fix blown double glazing without replacing the glass?
There are DIY defogging or resealing kits that claim to fix misted windows, but these only provide temporary results. They don’t restore the insulating gas between the panes or fix the underlying seal failure. For a lasting solution, it’s best to have the sealed unit professionally replaced.
Why do my windows keep misting up even after replacing the glass?
If misting returns after a new glass unit is installed, it could be due to poor ventilation inside the property, excess humidity, or a small leak around the frame that wasn’t properly sealed during installation. Ensuring good airflow and professional fitting usually resolves the issue permanently.
Will a blown window make my home less energy efficient?
Yes — once a window seal fails, it loses its insulating gas layer, allowing heat to escape and cold air to enter. Even a single blown double glazed window can noticeably affect your energy efficiency and increase heating costs.
Can you just replace one blown window, or do you need to do them all?
You can absolutely replace just one blown unit if the rest of your windows are performing well. However, if several are misting or the frames are showing their age, replacing multiple at once can be more cost-effective and consistent in appearance.
Do blown windows cause damp or mould in the home?
They can contribute to dampness over time. Moisture trapped between panes doesn’t directly enter your home, but a failed seal can reduce the window’s insulation, leading to condensation on interior surfaces and a higher risk of mould growth around the frame.
Are triple glazed windows less likely to blow than double glazed ones?
Yes — triple glazing uses improved spacer bars and seal technology, making it more resistant to failure. However, no sealed unit lasts forever. Regular cleaning, ventilation, and professional installation are key to long-term durability.
How can I tell if my window’s misting is internal or external condensation?
If the moisture is inside the glass unit, you won’t be able to wipe it away — that’s a sign of a blown or misted unit. If it’s on the inner or outer surface, it’s usually due to temperature differences and normal household humidity, not a blown seal.
Who should I contact to repair or replace blown double glazing?
You should contact a trusted FENSA-registered installer or glazing specialist, such as the team at Home Windows, who can inspect the window, confirm the cause of the failure, and advise whether repair or replacement is the best option.
