What are secondary glazed windows?

Secondary glazed windows are two separate window sashes set a short distance apart, each with its own glass and frame, used to improve thermal and acoustic insulation without replacing the original window. Also called double windows or secondary glazing, they are popular in listed and heritage buildings.

Difference from double glazing

Double glazing consists of two panes of glass joined into a single unit with a sealed gap of a few millimetres. Secondary glazing uses two separate windows with a much larger air gap between them (often 5-15 cm). This gives excellent acoustic insulation and preserves the existing outer window.

Advantages:

• Original frames preserved — important for heritage buildings
• High sound reduction — thanks to the large air space
• Good thermal insulation — extra glazed layer with still air
• Minimal intervention — no change to the external facade
• Reversible — can be removed without damage

Application

• Listed buildings — as an alternative to replacing single glazing
• Conservation areas — where exterior changes are not permitted
• Noise-affected facades — alongside railways, motorways or industry
• Energy upgrading — in older homes without major works
• Interim solutions — before a full refurbishment

Points of attention

• Seal the inner side of the cavity well
• Stop warm indoor air from entering the cavity — otherwise condensation forms
• Maintain proper ventilation in the building, as a tighter facade increases moisture risk
• Combine with good draught-proofing of the outer window
• Choose suitable frame material — timber, aluminium or slim steel

Related terms

• Glazing
• Insulating glazing
• Internal glazing
• Window
• Window frame

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