Double Glazing, Explained

Double glazing uses two panes of glass separated by a sealed gap, usually filled with air or an inert gas such as argon, to slow heat loss compared with a single pane. The gap reduces conduction and convection, a low-emissivity coating on one pane reflects room heat back inside while letting daylight through, and a warm-edge spacer at the unit edge reduces the cold bridge around the perimeter. Together those parts make a window that loses far less heat, feels warmer to sit beside and usually reduces outside noise as well.

The sealed unit sits in a frame, and the whole window performance, not just the glass, is what matters. A good sealed unit in a poor frame, or a good window badly installed with gaps at the wall junction, will not deliver its rated performance in a real home.

Choosing double glazing means choosing the unit, the frame and the installation together. Unit thickness, gas fill, coating type and spacer all affect the U-value, while frame material, whether timber, uPVC, aluminium or composite, affects durability, maintenance, appearance and the frame U-value. Opening style matters too: casement, tilt-and-turn, sash and fixed lights have different seals, ventilation options and cleaning access, and a style that suits a modern flat may be wrong for a period house.

Secondary benefits are real but should not be oversold. Double glazing usually improves comfort near windows and reduces condensation on the inner pane, because the inner glass stays warmer, but it does not cure condensation caused by high indoor humidity and poor ventilation, which may simply move to a colder wall or a different window.

Check each existing window for misting between panes, which signals a failed seal, for draughts, for rotten or distorted frames, and for whether the window opens, locks and vents properly. Note the frame material, the reveal depth and any heritage or planning constraints, especially for period properties where original windows may be protected.

Ask for the whole-window U-value, not just the centre-pane figure, and for the window energy rating where available, plus the frame material, gas fill, coating and spacer specification in writing. Compare quotes on those documented specifications, not on the word double glazing alone, which covers a wide range.

In the EU, double glazing products must meet product standards and labelling rules, while national building regulations set minimum U-values and installation requirements that differ by member state and climate zone. Some countries also run window energy labelling schemes that go beyond the basic U-value, and those schemes should be read in their national context.

In the UK, window energy ratings from BFRC, administered with BRE involvement, give a familiar A to G style rating, while building regulations set U-value limits and competent-person schemes cover installation notification. Devolved standards add local differences, and listed buildings or conservation areas may require special consent or alternative approaches.

A common mistake is replacing sound single-glazed sash windows in a period home with standard double glazing that harms character and may breach planning rules, when secondary glazing or careful repair plus draught-proofing would have served better. Another is buying the cheapest sealed unit available, with air fill and a cold spacer, and then wondering why performance and comfort gains are modest.

A third mistake is ignoring ventilation: new, well-sealed double glazing can reduce background air leakage so much that a home that previously ventilated through gaps now needs trickle vents or fans that were never fitted. Ventilation should be part of the window quote, not a later complaint.

Keep the whole-window U-values and ratings, the sealed unit specification, the frame details, the installation guarantee and photographs of the finished junctions. Those papers prove what was fitted and help any later repair, since sealed units are made to size and specification.

Ask a qualified installer: what is the whole-window U-value for each window, what gas, coating and spacer are used, how will ventilation be provided, and what guarantee covers the sealed unit, the frame and the installation separately. Specification answers show a real comparison.