Secondary Glazing vs Double Glazing
Both add a second pane of glass. Everything else is different — cost, thermal performance, acoustic performance, disruption and consent.
Where secondary glazing wins
It's around 40–60% cheaper per opening, needs no exterior alteration, is fully reversible, keeps the original window intact and is the only option on most listed and conservation-area properties.
- 40–60% cheaper per opening
- No exterior alteration
- Fully reversible
- Listed/conservation friendly
- Better acoustic performance
Where double glazing wins
On a modern property with no historic frontage, a full replacement gives you tighter frames from day one and no cavity to look through — but at 2–3x the cost.
- Slightly cleaner sightlines
- Single sealed unit
- Right for post-1990s homes with no character
The thermal reality
A modern secondary glazing unit with Low-E soft-coat glass and a 100 mm cavity delivers U-values as low as 1.868 W/m²K — comfortably outperforming most 1990s and early 2000s sealed double-glazed units still in service.
The acoustic reality
Secondary glazing wins outright on noise. Double glazing's narrow cavity (12–20 mm) can't decouple the acoustic energy the way a 100 mm secondary cavity can. Typical Rw improvement is 15–20 dB better with secondary.
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