Double glazing
Plain English. Practical detail. UK context.
Double glazing uses two panes of glass separated by a sealed cavity, commonly supplied as an insulating glass unit. The frame, spacer and installation also affect the performance of the complete window.
Last reviewed: (editorial review against the public sources listed below).
What it means in practice
Distinguish the glass unit specification from the whole-window specification. Glass coatings, cavity fill, edge details, frame design and installation junctions all influence the result. Replacement work should also address weather sealing, supports and the surrounding construction.
Check safety, ventilation and other applicable requirements for the location rather than choosing solely on pane count. A good sealed unit installed in a defective frame or junction will not correct every problem.
For homeowners
Ask whether the quote replaces only the glass unit or the complete frame. Condensation between the panes suggests a different issue from condensation on the room-side surface. Explain exactly where moisture appears, when it occurs and whether the frame also leaks or lets draughts through.
Practical checks
Keep the window and glass specifications, installation records and relevant compliance documents. When comparing offers, compare complete window arrangements rather than a single isolated glass figure.
The specification needs to match the location
An insulating glass unit is one part of an installed window. The complete arrangement may need to satisfy thermal, safety, ventilation, accessibility or escape requirements depending on the location and work. Safety glass selection is not determined merely by the number of panes. Opening lights, hardware and perimeter weathering also affect everyday performance. Ask for the whole-window information and relevant installation detail when comparing complete replacements, rather than compare one centre-pane figure with another supplier's whole-window rating. For repair, identify whether the defect is the unit seal, the frame drainage, the opening seals or the installation junction; replacing glass cannot correct every frame or wall defect.
Compliance and handover
In England, Approved Documents L, F and K provide relevant guidance on energy, ventilation and safety matters; the work and national jurisdiction determine the applicable route. Scheme certification and building control evidence concern the installation process, not a manufacturer's glass label alone. The Glass and Glazing Federation explains why misting between panes can be addressed by replacing a sealed unit where the frame is serviceable. Retain the specification and care instructions, and learn how background ventilation and opening hardware work. No universal savings or installed price is stated because size, access, frame condition and specification change the scope substantially.
Frequently asked questions
Does misting always mean the whole window needs replacement?
No. Misting between panes may involve the sealed glass unit while the frame remains serviceable. A competent assessment should establish what actually needs replacing.
Is triple glazing automatically the right upgrade?
Not for every property or budget. Compare the complete window, location, ventilation, installation and intended benefit rather than pane count alone.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.