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Blown Double Glazing – Why It Happens, and How to Fix It

Failed double glazed window

Double glazed windows can become blown when its seal is compromised or damaged. Double-glazing has many advantages, such as, lowering heating bills, improving home security, and adding value to a property. However, double glazing can sometimes become blown. 

In this post, we’ll explore the issue of blown double glazing, including how it occurs and the problems it causes. You’ll also find out how to tell if your windows have blown and what you can do about it.


What Is Blown Double Glazing?

To understand why double glazing becomes blown, we need first to look at the components of the double-glazed unit that fits within the window frame.

The two sheets of glass of a double-glazed window are separated by a spacer bar. This creates a cavity, which is filled with an insulating layer of gas – argon, krypton, xenon, or dehydrated air.

A seal prevents air or moisture getting into the double-glazed unit. Additionally, the spacer bar contains a silicon material – a desiccant – to dry up any moisture that does get trapped within the gap. 

A blown window occurs if the seal breaks or becomes damaged, or the desiccant in the spacer bar becomes saturated. Double glazing can also become blown if the insulating gas escapes or the window panes are damaged.

If any of this happens, moist air will quickly build up between the two panes of glass, which can eventually damage the inside of the glazing permanently.


What Causes Blown Double Glazing?

Several factors can result in the damage to seals or window panes that causes double glazing to become blown.

Seals can break or develop faults because of wear and tear over time. As temperatures fluctuate outside and inside your home, the gas within the double-glazed unit expands and contracts. This puts pressure on the seal that can eventually weaken it.

Cleaning products containing harsh chemical solvents can also damage window seals over time, increasing the risk of water seeping into the unit. The glass in double glazing and the window frames can also be damaged by bad weather such as storms or strong winds.

A further reason for blown double glazing is poor installation. This can lead to water seeping inside the unit rather than running off the outside of the panes of glass.

Whatever the reason for blown glazing, the result will be same: condensation. 


Double Glazing Condensation

Condensation occurs when warm air hits a cold surface and quickly cools to form liquid droplets. This is a common problem with single-pane windows. Double glazing, though, is designed to reduce condensation, by up to 85%.


Exterior Condensation

Condensation may occasionally appear on the exterior of double-glazed windows.  This happens when the air outside is warmer than the glass.

It’s nothing to worry about. In fact, it shows your double glazing is working properly by keeping heat inside your home. The condensation usually disappears when the sun raises the temperature of the window surface.


Interior Condensation

If condensation forms on the interior of a double-glazed window, it’s usually due to steam building up in the bathroom when you shower or take a bath, or in the kitchen during cooking. 

It occurs when water vapour turns back into liquid as it comes into contact with the cooler temperature of the window, and may mean the room doesn’t have adequate ventilation. 


Condensation Inside Double Glazing

Condensation happens inside double glazing when fresh air and water vapour get into the space between the two panes of glass, causing them to mist up.

It means the unit has broken down and is no longer functioning properly, allowing heat to escape from your home.

The solution to the problem, and the cost involved, will depend on what’s caused the double glazing to become blown. The question facing homeowners is whether they should have the window repaired or replaced.


Can Blown Double Glazing Be Repaired?

Repairing a blown double-glazed window is technically possible and would be cheaper than replacing it, but it’s unlikely to be the best long-term solution.

The design of double glazing – with an inert gas within hermetically-sealed glass – makes it more difficult to repair a blown window than replace it. This is why a blown window is invariably replaced.

If your window is less than 10 years old, check whether it’s still under warranty. If so, the installers may fix the problem at their own cost, depending on the terms of the warranty. With older windows, consider their expected life cycle and whether it’s worth repairing instead of replacing.


Replacing Blown Double-Glazed Windows

The best way to fix blown double glazing and ensure the problem doesn’t recur is to replace the entire window unit with a new one. In the unlikely event the window becomes blown again in the future, your investment would be protected by a warranty.

In most cases, fixing blown double glazing doesn’t require replacing the window frames, just the double-glazed unit. This makes it far less expensive than replacing the whole window or installing brand-new double-glazing.

It could also be an opportunity to upgrade to A-rated energy-efficient double glazing to get better insulation and reduce your heating bills.

Replacing a blown double-glazed unit will generally take a professional window installer less than an hour. It will cost from around £50 to £150, depending on the size of the window.


How Do I Know if My Double Glazing Has Failed?

The most obvious sign of blown double glazing is misting between the glass panes, which makes your windows look cloudy, foggy, or hazy.

Other indications your double glazing may have blown or otherwise failed include:

  • Water leakage. If water is leaking through the frame, the window’s weather seals may have failed or the drainage system become blocked.
  • Cracked or chipped glass. This damage can make your windows less efficient, allowing draughts into your home. 
  • Draughts. Draughts can come in through your windows if sealing in or around them has broken down. 
  • Noise. Failure to absorb external sounds is another sign your double glazing has failed.

You may also have difficulty opening and closing your windows. This could be down to faulty hardware but it could be a bigger problem, with the window’s structural integrity.


How to Avoid Blown Double Glazing

You can take a few simple measures to minimise the risk of blown double glazing. This will help avoid the cost of any repairs or replacement windows.

Clean your windows regularly, avoiding use of harsh chemicals. Fit exterior window shutters if you live in an area prone to storms or strong winds. Fill any gaps with silicone sealant to prevent moisture getting into the double-glazed unit.


Financial Implications of Blown Double Glazing

Quality double-glazed windows should last 15 to 20 years without becoming blown or developing any other major problems. But if you do get blown double glazing, it’s important to fix the issue, ideally by replacing the double-glazed unit.

The longer your windows are failing, the more money it will cost you in heating bills. You’ll find yourself cranking up the heating in winter as warm air continues to escape from your home and cold air gets in.

Furthermore, the unsightly, fogged-up appearance of a blown double-glazed window will negatively impact the aesthetic appeal of your home.

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