The short answer
Secondary glazing is one of the most effective retrofit options for noise. A standard system typically cuts external noise by around 30 dB, while an acoustic specification — thicker or laminated glass with a wide air gap — can reach roughly 45–54 dB, often quoted as up to about an 80% reduction in perceived noise. The two things that matter most are the air gap (a deep cavity of roughly 100–150 mm is the acoustic sweet spot) and the glass (acoustic laminate outperforms plain float). This wide cavity is why secondary glazing often beats a like-for-like double-glazed unit on sound, where the panes sit only millimetres apart.
Noise is the reason many people fit secondary glazing, and it is where the system genuinely shines. The result depends on getting the air gap and glass right, so here is what each does and what to ask for.
Typical noise reduction
- Standard system~30 dB
- Acoustic system~45–54 dB
- Perceived reductionup to ~80%
- Optimal air gap~100–150 mm
- Best glassacoustic laminate
What controls the noise reduction
- Air gap: the distance between the existing window and the secondary pane. A deep cavity of roughly 100–150 mm is the acoustic sweet spot — far wider than the few millimetres inside a double-glazed unit.
- Glass type: acoustic laminated glass (with a sound-damping interlayer) cuts more than plain float, and thicker glass helps with low-frequency traffic rumble.
- Sealing: good perimeter seals stop noise leaking around the frame; a great pane in a leaky frame underperforms.
- Mismatched glass: using a different glass thickness from the original window helps, because the two panes then resonate at different frequencies.
| Specification | Typical reduction | Best for |
|---|---|---|
| Standard, modest gap | ~30 dB | general traffic, draughts |
| Thicker / toughened glass | ~40–49 dB | busier roads |
| Acoustic laminate, deep gap | ~50–54 dB | main roads, rail, flight paths |
Indicative figures; real-world results depend on the window and fit. Sources: acoustic and trade guides.
How to specify it for serious noise
If a main road, railway or flight path is the problem, the brief to give an installer is a wide air gap (toward 100–150 mm where the reveal allows), acoustic laminated glass, and tight perimeter seals. Using a glass thickness different from the original pane helps the two work across different frequencies. Decibels are logarithmic, so a move from 30 dB to 45 dB of reduction is a large change in perceived quiet, not a small one — which is why the acoustic specification is usually worth the extra cost when noise is the reason you are fitting it.
Want secondary glazing specified for noise?
We'll match you with a vetted secondary-glazing installer who surveys your windows and quotes an acoustic specification — glass, air gap and seals — suited to your noise problem.
Frequently asked questions
How much noise does secondary glazing block?
A standard system typically cuts external noise by around 30 dB, while an acoustic specification with thicker or laminated glass and a wide air gap can reach roughly 45–54 dB — often described as up to about an 80% reduction in perceived noise.
What air gap is best for noise reduction?
A deep cavity of roughly 100–150 mm between the existing window and the secondary pane is the acoustic sweet spot. That wide gap is why secondary glazing often outperforms a double-glazed unit, where the panes sit only millimetres apart.
Does secondary glazing stop all noise?
No glazing makes a room silent, and very low-frequency noise is the hardest to reduce. But a well-specified acoustic secondary system makes a clear, measurable difference to typical traffic, rail and aircraft noise.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific windows. They are guidance, not a quotation.