Double glazed unit
Plain English. Practical detail. UK context.
A double glazed unit is a sealed assembly of two panes separated by a spacer and an enclosed cavity. It is also called an insulating glass unit. The unit sits within a window or door frame; replacing the glass unit is not necessarily the same job as replacing the complete window.
Last reviewed: (editorial review against the public sources listed below).
What affects its performance
Glass coatings, cavity width, gas fill, spacer design and pane type all influence performance. The complete window’s heat loss also includes the frame and edge details, so a centre-pane figure should not be presented as the performance of the whole installed window. Safety, solar control, acoustics and appearance may require different glass specifications. The location matters: glass near doors, at low level or in other critical positions may need appropriate safety performance.
Understanding condensation
Moisture on the room-side surface can relate to indoor humidity, surface temperature and ventilation. Moisture trapped between panes commonly indicates a failed edge seal or loss of the unit’s intended sealed condition. External condensation can occur under particular weather conditions and does not necessarily mean failure. Establish which surface is affected before recommending replacement. A photograph taken from one angle can make these cases look similar, so a clear inspection and description are more useful than a generic misted-window diagnosis.
Common mistakes
Do not assume all replacement units have the same thickness, coating or safety glass arrangement. Incorrect packers, drainage or edge clearance can damage a unit or affect operation. Drilling a misted unit is not equivalent to replacing it with a properly specified sealed assembly. A replacement glass unit will not necessarily fix failed frame seals, poor installation or blocked drainage. Nor should it be expected to solve every condensation problem elsewhere in the room.
Alternatives and relevant guidance
A replacement sealed unit may be sufficient where the frame and hardware remain serviceable. Secondary glazing or complete window replacement may suit different constraints, including historic buildings. BS EN 1279 concerns insulating glass units; BS 6262 and relevant safety-glass standards inform glazing selection and installation. Replacement windows can involve building regulation notification and competent person schemes, but the route depends on the actual work and nation. Agree what is being replaced and which performance and compliance documents will be provided.
Frequently asked questions
Does misting mean I need a whole new window?
Not always. The sealed glass unit may be replaceable if the frame and other components are sound.
Is condensation outside the glass a failed seal?
Not necessarily. Identify the surface and conditions before diagnosing a fault.
Is every pane in a double glazed unit toughened?
No. Safety glass specification depends on location and requirements, not merely the number of panes.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.