Double Glazing

A window or door construction featuring two panes of glass separated by a sealed air or gas-filled gap, providing significantly better thermal and acoustic insulation than single glazing.

Double glazing refers to windows and doors that use two panes of glass separated by a sealed gap (typically 12-20mm) filled with air or an inert gas like argon. This sealed unit — often called an insulated glass unit (IGU) — dramatically reduces heat loss compared to single glazing, making your home warmer, quieter, and cheaper to heat.

How double glazing works

The sealed gap between the two glass panes acts as an insulating barrier. Heat transfers much more slowly through a trapped layer of air or gas than through a single pane of glass. Argon gas (which is denser than air) in the cavity slows heat transfer even further. Most modern double-glazed units also have a Low-E (low emissivity) coating — an invisible metallic layer on the glass that reflects heat back into the room while still allowing light through.

Benefits of double glazing

  • Thermal insulation — reduces heat loss through windows by up to 50-65% compared to single glazing, lowering heating bills
  • Sound insulation — noticeably reduces outside noise, particularly beneficial on busy roads or under flight paths
  • Reduced condensation — the inner pane stays warmer, so moisture from the air is less likely to condense on it
  • Security — two panes of glass are harder to break than one. Many modern units use toughened or laminated glass.
  • Property value — double glazing is expected in modern homes and adds value when selling

Frame materials

  • uPVC — the most common and affordable option. Low maintenance, good insulation, wide colour range. Less favoured on period properties for aesthetic reasons.
  • Aluminium — slim frames that maximise the glass area. Very durable. Modern thermal-break technology makes them well-insulated. Premium pricing.
  • Timber — the traditional choice, and often required for conservation areas and listed buildings. Warm aesthetic, excellent insulation, but requires periodic painting or staining.
  • Composite — an aluminium exterior with a timber interior. Combines low external maintenance with a warm timber look inside. Premium pricing.

Double vs. triple glazing

Triple glazing adds a third pane and second gas-filled cavity for even better insulation and noise reduction. It is standard in Scandinavian countries and is becoming more common in the UK and US, particularly in new builds and high-performance renovations. The additional cost (typically 20-30% more than double glazing) may be justified for north-facing windows, properties in cold climates, or rooms where noise is a significant issue.

Renovation considerations

When planning window replacement as part of your renovation, factor in:

  • Building regulations — replacement windows must meet current thermal performance standards. Your installer should provide a FENSA certificate (UK) or equivalent compliance documentation.
  • Planning restrictions — in conservation areas or historic districts, there may be rules about window styles, materials, and glazing bar patterns. Check before ordering. See sash windows for more on period properties.
  • Lead time — custom windows can take 4-8 weeks from order to delivery. Factor this into your renovation timeline.
  • Making good — budget for replastering and redecorating around new window openings, including new architraves if the frame size changes.