What is Blown Double Glazing?
Blown double glazing occurs when the airtight seal surrounding a double-glazed unit (IGU) fails. Once this seal breaks, the insulating gas (usually Argon) escapes, and moist air enters the gap.
The result is the tell-tale sign of “blown” windows: misting, fogging, or visible water droplets trapped between the two panes of glass. Once moisture is inside, the unit is permanently compromised and cannot be cleared.
Quick Verdict
The Cause: A failed perimeter seal
The Sign: Permanent condensation between the panes that cannot be wiped away
The Fix: The glass unit must be replaced (not repaired)
The Savings: You can usually keep your original frames, saving up to 70% compared to a full window replacement (read guide here).
How Can You Tell if Double Glazing Has Blown?
Before calling a professional, check where the moisture is located. Common signs of a blown unit include:
Internal Condensation: Fog or droplets between the panes that cannot be wiped from either side
Cloudy Appearance: The glass looks milky or “dirty” even after a thorough clean
Visible Water Pooling: Water collecting at the bottom of the spacer bar inside the glass
Note:
Condensation on the room side usually means the room needs better ventilation
Condensation on the outside is normal and indicates the glass is well-insulated
Why Does Double Glazing Fail?
Seal failure is usually gradual and can be caused by:
Age: Most units last 15–25 years
Solar Pumping: Daily expansion and contraction from sunlight stresses the seal, eventually causing tiny cracks
Poor Drainage: Blocked internal drainage holes allow water to sit in the frame, rotting the edge sealant
Failed Desiccant: The spacer bar contains desiccant beads that absorb moisture. Once saturated, fogging appears
Does Blown Double Glazing Affect Insulation?
Yes. A blown window no longer functions as proper double glazing and effectively becomes two sheets of glass with damp air in between. This leads to:
Increased Heat Loss: Noticeable cold spots near the window
Higher Energy Bills: Your heating system works harder to compensate for the thermal bridge
Mould Risk: Cold internal glass encourages condensation, which can lead to black mould on frames
Can Blown Double Glazing Be Repaired?
No. Cosmetic “defogging” services that drill holes to vent moisture are temporary. They do not restore the vacuum seal, replace lost Argon gas, or prevent fogging from returning.
The only long-term solution is to replace the sealed glass unit professionally.
Do You Need to Replace the Whole Window?
Usually, no. In most cases, the failed glass unit can be removed and replaced within the existing frames.
A full window replacement is only necessary if:
The uPVC or timber frame is warped, rotten, or physically broken
You want to change the style or opening mechanism of the window
How Much Does It Cost to Replace Blown Double Glazing?
Replacing just the glass is significantly cheaper than a full window replacement. Typical UK fitted costs for 2026 are:
| Window Size / Type | Estimated Cost (Fitted, UK) |
|---|---|
| Small (e.g., Bathroom / Top Fanlight) | £120 – £180 |
| Medium (e.g., Standard Bedroom Window) | £150 – £250 |
| Large (e.g., Living Room Picture Window) | £250 – £400 |
| Specialist (e.g., Patio Door / Toughened Glass) | £350 – £550+ |
Note: Prices include glass, labour, and VAT. Costs may vary based on glass specifications such as Low-E coatings or acoustic glass. You can calculate it from this page (link).
How to Prevent It From Happening Again?
When replacing your glass, ask for Warm Edge Spacer Bars. Unlike old aluminium spacers, these plastic composite bars do not conduct cold and are more flexible, reducing the risk of blown units in the future.
Get a Professional Assessment
Not sure if your windows are blown or just need a good clean? Contact us today for free advice. We’ll give you an honest assessment and a quote for a high-performance glass replacement, saving your frames and your money.
