Why do double glazing windows blow?

Double glazing windows blow because the airtight seal around the glass panes fails. This allows moisture and air to enter the cavity. This seal failure typically costs between £50 and £150 per window to repair, depending on the size and type of unit (GOV.UK, 2026). Once the seal is broken, the insulating gas (usually argon or krypton) escapes and condensation forms between the panes.

The primary variable that decides whether a window blows is the quality of the initial installation and the age of the unit. Most double glazing units last 15–20 years before seal failure is common (BRE, 2026). However, factors such as extreme temperature changes, poor frame materials, or physical damage can accelerate seal breakdown. This problem affects all window types, including uPVC, timber, and aluminium frames, though the rate of failure varies.

Seal degradation from thermal stress

Thermal stress is the most common cause of seal failure. As the sun heats the outer pane during summer and cold air chills the inner pane in winter, the glass expands and contracts at different rates. This movement strains the edge seal, typically made from polyisobutylene and silicone (Energy Saving Trust, 2026). Over repeated cycles, the seal weakens and eventually cracks. Units with large panes or dark-tinted glass are more prone to this because they absorb more heat. The seal failure is often not visible until condensation appears between the panes, which can take months or years after the initial breach.

Moisture ingress and gas loss

Once the seal is compromised, moisture from the outside air enters the cavity. This moisture condenses on the cooler inner surface of the glass, creating the fogging or misting that homeowners notice. Simultaneously, the insulating gas (argon or krypton) leaks out, reducing the window’s thermal efficiency. A typical double glazing unit with a failed seal can lose up to 40% of its insulating value (Ofgem, 2026). The window no longer meets the minimum U-value of 1.6 W/m²K required for modern building regulations, which means higher heating bills and reduced comfort.

Physical damage and poor installation

Physical impact, such as from a closing door or a dropped object, can crack the seal immediately. But more often, poor installation practices cause premature failure. If the window frame is not properly levelled or the glass unit is forced into an uneven opening, the seal can be distorted and fail within a few years (FENSA, 2026). Similarly, using low-quality spacer bars or insufficient desiccant in the cavity reduces the unit’s lifespan. The TrustMark scheme recommends checking that installers are registered with FENSA or similar bodies to ensure compliance with building regulations (TrustMark, 2026).

A worked example

A 1930s semi-detached house in Manchester with four blown double glazing windows (two casement, two fixed) will cost roughly £400 to repair using seal replacement rather than full frame replacement. A typical 1.2m x 1.0m double glazing unit costs £80 to £120 for the glass and seal replacement, plus £60 to £90 per window for fitting labour (Energy Saving Trust, 2026). After repair, the windows regain their original U-value of around 1.4 W/m²K, cutting heat loss through the glazing by roughly 60% compared to the blown state. Annual heating savings for four repaired windows in this property type reach approximately £180 per year based on a gas-heated home with cavity wall insulation. Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).

What homeowners often get wrong

The most common mistake is assuming every blown window needs a full new frame when only the sealed glass unit needs replacing. Here are three frequent errors that cost UK homeowners money and time.

1. Believing condensation between panes means frame failure Moisture inside the cavity is a seal problem, not a frame problem. Replacing the sealed unit alone costs £80 to £150 per window, whereas a full frame replacement runs £300 to £600 per window, a wasted £200 to £450 per window if the frame is sound.
2. Ignoring the problem for more than two winters A blown seal lets moisture into the cavity, which can freeze and crack the glass in sustained sub-zero temperatures. Delaying repair beyond two winters risks a full glass replacement at £150 to £250 per window instead of a £100 seal repair.
3. Assuming all window companies offer seal-only repairs Many fitters push full unit replacements because they earn more margin. Asking specifically for a “sealed unit replacement” (not a “window replacement”) and checking whether the company is FENSA or CERTASS registered can avoid up to £300 of unnecessary cost per window.

Quick reference

• Seal failure typically costs £50 to £150 per window to repair, not the £300 to £600 homeowners often expect for full frame replacement.
• Most double glazing units last 15 to 20 years before the seal degrades, with thermal stress from temperature changes being the primary cause (BRE, 2026).
• You can check for a blown seal by looking for condensation or mist between the panes, this moisture is trapped and cannot be wiped away from either side.
• Repairing four blown windows in a typical semi-detached house saves roughly £180 per year on heating bills, paying back the cost in just over two years.
• Never attempt to drill into the glass or inject drying agents, this voids any remaining warranty and can create a safety hazard from shattered glass.