Secondary Glazing vs Double Glazing — Which is Better?

The key difference is the air gap. Standard double glazing has 12–24mm. Secondary glazing reveal-fixed creates 150–200mm — up to 15x larger. This is why secondary glazing consistently outperforms double glazing for noise.

Noise Reduction

Secondary glazing: up to 54dB (80% perceived reduction). Double glazing: 30–35dB (40% perceived). Secondary glazing wins clearly.

Thermal Performance

Modern double glazing achieves U-values of 1.2–1.4 W/m²K. Secondary glazing with Low-E glass achieves 1.5–1.8 W/m²K. Comparable, with double glazing marginally ahead for thermal performance.

Listed Buildings

Secondary glazing wins outright. Double glazing replaces the original window — typically refused for listed buildings. Secondary glazing is endorsed by English Heritage.

Cost

Secondary glazing: £300–£500 installed per window. Double glazing: £500–£1,500 per window. Secondary glazing is considerably more affordable.

Which to Choose?

Choose secondary glazing if: noise reduction is a priority; listed building or conservation area; preserving original windows; budget matters. Consider double glazing if: modern property where original windows have no historic value and maximum thermal performance is the priority.