Failed double glazing: what buyers need to know

Double glazing works as a sealed unit. The space between the two panes is filled with an insulating gas that reduces heat transfer and limits condensation. When that seal fails, the gas escapes, moisture enters and the unit loses its effectiveness. The signs are usually visible, but the implications for a property can vary considerably depending on the extent of the problem.

Condensation between the panes is the most obvious sign of a blown unit. When the seal fails, moisture enters the space between the glass and cannot escape. The result is a persistent milky or cloudy appearance that cannot be cleared from the inside or outside.

Black mould around frames can develop where condensation is allowed to settle repeatedly. Mould is a more serious concern where there are children or vulnerable adults in the property, as the spores can affect air quality and health.

Draughts around windows or doors suggest either a failed sealed unit, a cracked pane or a deteriorated rubber seal around the frame. The source matters: a failed unit needs replacement, while a seal issue is usually cheaper to address.

Increased noise from outside can indicate a defective sealed unit. One of the primary benefits of double glazing is noise reduction, so a noticeable change in noise levels is worth flagging.

Difficulty opening or closing windows or doors is sometimes a sign of a more serious underlying issue. Where this has developed suddenly rather than always being the case, it may indicate subsidence or movement in the structure of the building. A surveyor will look for supporting evidence such as diagonal cracks in surrounding walls.

Failed glazing is rarely a structural issue in itself, but the implications can be wider than they first appear. Blown units increase heat loss, raise energy bills and can contribute to damp if condensation is left to accumulate. Where multiple units have failed or frames are deteriorating, the cost of replacement across a property can be significant.

The age of the glazing matters too. Double glazing typically has a lifespan of 20 to 25 years. On older properties where the original installation is approaching or past that point, widespread failure may be imminent even if only some units are currently showing signs.

A surveyor will assess the extent of any failed glazing, note the likely cause and advise on whether individual units or full replacements are needed. Where failure is extensive, the findings can be used to renegotiate the agreed price or request that the seller addresses specific units before exchange.

As part of a RICS Level 2 HomeBuyer Report or RICS Level 3 building survey, the surveyor will inspect all accessible windows and doors for signs of failed seals, condensation, frame deterioration and any evidence of associated damp or mould.

Where difficulty opening or closing is noted, the surveyor will look for any indication of structural movement that might explain it, and flag this accordingly. Failed glazing will be rated by severity and any recommendations for remedial work included in the report.

If you are buying an older property and can already see signs of blown units at the viewing, it is worth factoring potential replacement costs into your budget. Our house survey cost guide sets out what each survey level covers and what to expect to pay.

A property survey will assess the condition of all windows and doors and give you a clear picture of any glazing issues before you commit to a purchase.

At Surveyor Local, we work exclusively with RICS-accredited chartered surveyors across England and Wales. Our friendly team can arrange a survey quickly and at a competitive price.

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