Key summary: There are two types of window seal failure. If you have condensation between the panes that won’t wipe away, the glass unit (IGU) has failed and needs replacing; you can usually keep the frame. If you have draughts around the frame edge, the rubber gasket has worn out and can often be replaced cheaply, sometimes as a DIY job. Most seals last 10-20 years. Home insurance won’t cover either. Defogging services are a temporary fix, not a solution.
You’ve probably stood at a closed window and felt cold air coming through anyway. Or wiped at foggy glass and realised the condensation is trapped between the panes, not on them. Both problems get described as a “broken window seal,” but they’re caused by different things and need different fixes.
The cost gap between the two is significant. Replacing a worn rubber gasket around the frame typically costs very little and is sometimes something you can do yourself. Replacing the glass unit inside the frame costs more and needs a professional. Replacing the whole window is only necessary in specific circumstances.
If your seals have failed and a repair isn’t the answer, Apex Windows have been manufacturing and installing uPVC windows across Bedfordshire and the Home Counties since 1989. Get a free, no-obligation quote and they’ll tell you on the day whether you’re looking at a simple reseal or a full replacement.
What does a broken window seal actually mean?
A broken window seal can refer to two separate things: a failed seal inside the double-glazed unit itself (which causes condensation between the panes) or a worn rubber gasket around the frame edge (which causes draughts). Knowing which type has failed determines whether you need a new pane of glass, a new rubber seal, or a full window replacement.
The glass unit in a double-glazed window is a sealed assembly, known as an IGU (insulated glass unit), that contains two panes of glass with argon gas trapped between them. That gas is what gives double glazing its insulating properties. The perimeter seal holds it in. When that seal fails, the argon escapes, moist air enters the cavity, and you get the fogging or condensation that won’t wipe away.
The gasket is a different component entirely. It’s the flexible rubber strip fitted into a groove around the opening edge of the frame, and its job is to create a weatherproof barrier when the window closes. Over time it hardens, cracks, or pulls away from the groove. The result is a draught you can feel even with the window shut.
How to tell if your window seal has failed
The clearest sign of a failed IGU seal is condensation or fogging between the panes that you cannot wipe away from either side. For a failed gasket or weather seal, the giveaway is a draught you can feel around the frame edge when the window is fully closed, or visible cracking and hardening of the rubber strip itself.
Signs your IGU seal has failed
- Condensation, fogging, or a milky haze between the panes that won’t clear
- Water droplets or mineral deposits forming inside the glass unit
- A misted appearance that changes with temperature or time of day
- Increased outside noise coming through a window that previously blocked it well
A failed IGU loses some of its acoustic insulation alongside its thermal performance, so a window that suddenly seems noisier may have a seal problem worth investigating.
Signs your rubber gasket needs replacing
- A draught around the frame edge when the window is fully closed
- Visible cracking, flattening, or hardening of the rubber strip
- The gasket has pulled away from its groove in places
- Difficulty getting the window to close flush against the frame
One thing worth knowing: condensation on the outside surface of the glass is not a sign of failure. It means the outer pane is cold relative to the air outside. If the fogging is on the exterior surface and clears as the day warms up, your windows are fine.
What causes a window seal to fail?
Window seals fail for four main reasons: solar pumping, UV degradation, poor installation, and age. South and west-facing windows tend to fail sooner due to greater sun exposure and more extreme temperature swings.
- Solar pumping is the most common cause. Every time the sun heats the air between the panes, the glass expands slightly. Overnight, it cools and contracts. That cycle repeats thousands of times, putting daily stress on the perimeter seal until it fails.
- UV exposure hardens and degrades the rubber gasket over time, making it brittle and prone to cracking. Gaskets on south-facing windows often need replacing before the IGU seal shows any problems.
- Poor installation accelerates failure regardless of product quality. Frames that aren’t square, glass units fitted under tension, or seals applied without adequate preparation all create stress points that wouldn’t exist in a correctly fitted window.
- Most uPVC window seals last between 10 and 20 years, and if yours are approaching that range and showing symptoms, the seal hasn’t failed prematurely.
uPVC window seal types: what’s in your window frame
A standard uPVC window contains two distinct types of seal: the IGU perimeter seal bonded inside the glass unit, and the rubber gasket fitted into a groove around the frame. The gasket comes in several profiles depending on the window type and age. The four most common in UK uPVC windows are:
- Wedge gasket: fitted between the glass unit and the frame, holding the glass in place and acting as a weather seal. More common in older-style frames.
- Bubble (or bulb) gasket: runs around the opening sash edge and compresses when the window closes to block draughts. The most widely used type.
- Flipper gasket: similar to the bubble gasket but with a fin that deflects water away from the frame. Often used alongside wedge gaskets.
- E-gasket: fits into a channel and grips the frame bead. Common in modern uPVC profiles.
Unlike the gasket, the IGU perimeter seal is factory-bonded inside the glass unit and can’t be replaced on its own. When it fails, the whole glass unit needs replacing, not just the seal around it.
Can a broken window seal be fixed?
Whether a broken window seal can be fixed depends entirely on which type has failed. A worn rubber gasket can usually be replaced without touching the glass, at low cost and sometimes as a DIY job. A failed IGU seal cannot be repaired: once the argon gas has escaped and moisture has entered the unit, the glass unit must be replaced. Defogging services exist but address the appearance only, not the underlying failure.
Replacing a worn gasket
Push-fit gaskets can be pulled out of their groove and replaced without specialist tools or adhesive. For a ground-floor, accessible uPVC window, it’s a realistic DIY job: the replacement seal presses back into place and the whole job takes one to two hours per window.
Upper floors, heritage frames, or windows still under warranty are worth handing to a professional. If you’re unsure of the gasket type, a glazier can identify and source the correct one quickly.
When the IGU has failed
Once moisture has entered between the panes, the glass unit needs replacing. If the frame is in good condition, you can keep it and replace only the glass unit, which costs significantly less than a full window replacement. According to Which?, window seals can sometimes be replaced without the entire window or door being replaced, particularly where the seals are separate components rather than built into the frame. If you want a fuller breakdown of your options at this stage, our guide to condensation inside double glazing covers this in more detail.
Defogging services, which involve drilling a small hole in the unit, extracting moisture, and pumping in an anti-fog solution, treat the appearance only. They don’t restore the argon gas or reseal the unit. Defogging is a short-term cosmetic fix.
When to replace the whole window
Full window replacement makes sense in four situations: multiple units have failed across the same frame, the frame itself has warped or deteriorated, the windows are 20 or more years old, or the energy performance gains from upgrading outweigh the cost of repeated repairs.
If you’re weighing up repair against replacement, our uPVC windows come with a 10-year insurance-backed guarantee, which is worth factoring into the comparison.
What does it cost to fix a broken window seal in the UK?
Costs vary significantly depending on which type of seal has failed. Replacing a rubber gasket yourself costs very little: replacement gasket rolls are available for £15-22 from hardware retailers. Having a professional replace a blown IGU typically costs £55-145 per unit. A full professional resealing job including materials, labour, and call-out runs £125-180 on average.
| Job | Typical cost |
| DIY gasket replacement (materials only) | £15-22 per roll |
| IGU unit replacement (glass only, keeping frames) | £55-145 per unit |
| Professional resealing (labour, materials, call-out) | £125-180 per window |
Checkatrade’s 2026 cost data puts the average professional resealing job at £125-180 including call-out. For IGU replacement, MyJobQuote puts the average at £55-145 per unit, with a full set of units in a three-bedroom semi typically running £1,400-2,500.
Standard home insurance does not cover broken window seals. Seal failure is considered wear and tear and is consistently excluded from standard policies. Some accidental damage add-ons may cover sudden breakage, but gradual seal failure is not the same thing. Check your policy wording, but in most cases the cost will be out of pocket.
Conclusion
A failed window seal is not always the disaster it looks. Rubber gasket failure is often a quick, affordable fix. A failed glass unit means replacement, but in most cases you can keep the frame and replace just the glass.
The longer you leave a failed IGU, the more likely moisture ingress is to affect the surrounding frame. A repair job left too long becomes a full replacement.
If you’re in Bedfordshire or the surrounding area and your windows need attention, Apex can help. Use the instant online calculator to get a price for new windows in under 10 seconds, or speak to our team directly. You can also find more guides in our guides if you’re still weighing up your options.
Frequently asked questions
What does a window with a broken seal look like?
Condensation or fogging between the two panes that you cannot wipe away from either side is the clearest sign. You may also notice water droplets, a milky haze, or mineral deposits forming inside the unit over time. For a failed gasket, look for cracked, hardened, or flattened rubber around the frame edge, or visible gaps where the seal has pulled away from the groove.
Can a broken seal in a window be fixed?
It depends on which seal has failed. A worn rubber gasket around the frame can be replaced, sometimes as a DIY job. A failed IGU seal cannot be repaired: once moisture has entered between the panes, the glass unit must be replaced. Defogging services treat the appearance only and are not a lasting fix.
Do windows with broken seals need to be replaced?
Not always. If only the rubber gasket has failed, you can replace the seal without touching the glass. If the IGU seal has failed, the glass unit needs replacing, but in most cases you can keep the existing frame. Full window replacement is usually only necessary if the frame itself is damaged or the windows are very old.
What does it cost to replace a seal on a window?
For a rubber gasket, DIY materials cost £15-22 for a roll of replacement seal. Professional resealing including labour, materials, and call-out typically runs £125-180 for a single window. Replacing a blown glass unit costs £55-145 per unit depending on size. For larger jobs or full window replacement, use Apex’s online calculator for a fast, accurate estimate.
Can I replace a window seal myself?
Replacing a rubber gasket on a ground-floor, accessible uPVC window is a realistic DIY task. The old seal pulls out of its groove and a new push-fit seal presses in without tools or adhesive. Replacing a blown glass unit is not a DIY job: it requires accurate measurement, custom manufacture, and professional fitting to ensure the new unit seals correctly.
What causes a window seal to fail?
The most common cause is solar pumping: repeated heating and cooling of the air between the panes puts daily pressure on the perimeter seal, weakening it over years. UV exposure degrades the rubber components. Poor installation creates stress points that accelerate failure. Most uPVC window seals last 10-20 years; south and west-facing windows tend to fail sooner.
How long do window seals last?
The IGU perimeter seal in a well-installed double-glazed unit typically lasts 10-20 years, with quality units sometimes lasting longer. Rubber gaskets around the frame last a similar period but are more susceptible to UV degradation and may need replacing sooner on south or west-facing windows.
What is the difference between a window seal and a window gasket?
The terms are often used interchangeably, but they refer to different components. The window seal is the bonded perimeter seal inside the glass unit, which holds the argon gas between the panes. A gasket is the rubber strip fitted into a groove around the frame, creating a weatherproof barrier when the window closes. The gasket can be replaced; the IGU seal cannot.
Will home insurance cover a broken window seal?
Standard home insurance does not cover broken window seals, as seal failure is considered normal wear and tear. Some accidental damage add-ons may cover sudden breakage, but gradual seal failure is consistently excluded. Check your policy wording, but in most cases repair or replacement costs will be out of pocket.