Why Has Your Window Seal Blown?

If you’ve noticed mist, condensation, or fog trapped between your window panes, you’re likely dealing with a blown window seal. It’s a common issue across UK homes—especially with ageing double glazing—and while it might seem cosmetic at first, it can lead to reduced energy efficiency and higher heating bills.

In this guide, we’ll explain exactly what a blown window seal is, why it happens, and what you can do about it.

What Is a Blown Window Seal?

Modern double-glazed windows are designed with two panes of glass separated by a sealed gap filled with insulating gas (usually argon). This seal keeps moisture out and helps retain heat inside your home.

A blown window occurs when that airtight seal fails, allowing moisture to enter the gap between the panes. This results in condensation or fogging that you can’t wipe away from either side.

Common Signs Your Window Seal Has Blown

You might have a blown unit if you notice:

• Condensation trapped between the glass panes

• A cloudy or misty appearance

• Water droplets inside the unit

• Cold spots or draughts near the window

• Reduced clarity or visibility

These symptoms indicate the insulating barrier has been compromised, meaning your window is no longer performing as it should.

Why Has Your Window Seal Blown?

There are several reasons why window seals fail in UK homes. Understanding these can help you prevent future issues.

1. Age and Natural Wear

Double glazing isn’t designed to last forever. Most units have a lifespan of around 15–20 years. Over time, exposure to weather, UV rays, and general wear causes seals to deteriorate.

2. Temperature Changes (Thermal Stress)

The UK climate is known for its fluctuations—warm days followed by cold nights. These changes cause materials like glass and uPVC to expand and contract, putting stress on the seals until they weaken or crack.#

3. Poor Installation

If your windows weren’t fitted correctly, the seals may have been under strain from day one. Misalignment or uneven pressure can lead to early failure and moisture ingress.

4. Moisture Ingress

If water finds its way into the window unit—through damaged seals or blocked drainage—it can accelerate deterioration. Once moisture gets inside, condensation forms and the unit becomes compromised.

5. Frame Movement or Property Settlement

Over time, buildings naturally shift slightly. This movement can affect window alignment, putting additional pressure on the sealed unit and causing it to fail.

6. Manufacturing Defects

Occasionally, faults during production mean the seal wasn’t fully effective from the start. While less common, this can cause premature failure even in relatively new windows.

What Happens If You Ignore a Blown Window?

It might be tempting to leave it—especially if it’s “just a bit misty”—but a blown window can lead to:

• Increased heat loss and higher energy bills

• Reduced comfort due to draughts

• Potential damp or mould around the frame

• Decreased property value

• Further damage to surrounding materials

  
  

Once the seal has failed, the insulating gas has usually escaped, meaning the window is no longer energy efficient.

Can a Blown Window Be Repaired?

In most cases, the best solution is to replace the sealed glass unit rather than the entire window frame.

This is good news for homeowners, as it’s often more cost-effective than full window replacement and restores both clarity and thermal performance.

How to Prevent Window Seals from Blowing

While some causes are unavoidable, you can extend the life of your windows by:

• Ensuring professional installation

• Keeping drainage channels clear

• Avoiding harsh chemicals on seals

• Maintaining good ventilation in your home

• Addressing minor issues early

Why Choose a Professional?

Diagnosing and replacing a blown unit requires the right tools and expertise. A professional installer will ensure:

• Correct measurements

• Proper sealing

• Long-lasting performance

If you’ve noticed misted or blown windows, don’t wait for the problem to worsen.

👉 Book a free quote today

👉 Get your windows inspected by experts

👉 Restore clarity, warmth, and efficiency in your home

Contact St Austell Bay Plastics today for reliable, local service across Cornwall.

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FAQs

How long do double-glazed window seals last?

Typically 15–20 years, depending on quality, installation, and exposure to weather.

Can you fix condensation inside double glazing?

No—once moisture is inside the sealed unit, it usually requires replacement of the glass unit.

Do I need to replace the whole window?

Not usually. In most cases, only the glass unit needs replacing, not the entire frame.

Will a blown window increase my energy bills?

Yes. A failed seal reduces insulation, allowing heat to escape more easily.