Double glazed sash windows

Upgrading from single glazing to double glazed sash windows transforms how a room feels. The most immediate gains are thermal and acoustic insulation together with improved security, and these are the reasons most of our clients begin the conversation. A sealed double glazed unit dramatically reduces heat loss through the glass, so rooms hold their warmth for longer and feel less draughty in winter. Lower heat loss also means lower energy bills and a smaller carbon footprint over the life of the window.

Acoustic comfort is often the benefit that surprises people most. In busy parts of London, a well‑made double glazed sash window noticeably softens traffic noise and street sound, making bedrooms and living rooms markedly more peaceful. Because our units are set into hand‑made timber frames rather than plastic ones, you gain this performance without the hard, characterless look of modern replacements.

Security improves too. Double glazed units are considerably harder to break through than a single pane, and when combined with quality ironmongery and modern sash locks, the window becomes far more resistant to forced entry. The result is a sash window that looks entirely period‑correct from the street yet performs to a contemporary standard for warmth, quiet and safety.

Traditional sash windows were never designed to carry the thickness of a standard modern double glazed unit, which is why slimline glazing matters so much in period and listed properties. For most traditional windows the maximum unit thickness we would recommend is around 18mm, allowing a sealed double glazed unit to sit within the sash without distorting the delicate proportions of the glazing bars.

In listed buildings and conservation areas a slimline double glazing unit is usually required. Its thickness is typically around 12mm, in a 4/4/4 build‑up, argon filled with a low‑E glass unit. This slim construction preserves the fine sightlines that conservation officers expect while still delivering a genuine improvement in thermal and acoustic performance over single glazing. We can also convert suitable existing single‑glazed sashes to accept slimline double glazing, retaining the original timber wherever the condition allows.

Where extra safety is needed, we can supply toughened glass for the lower sash at no additional cost, which is particularly useful in homes with children or in low‑level windows. Whether the project calls for a standard sealed unit or a conservation‑grade slimline unit, we specify the glass to suit the building, the location and, where relevant, the requirements of the local conservation authority.