Secondary glazing for noise reduction works by adding an independent inner window behind the existing one. The new frame, glass, seals and the space between the two layers interrupt the path from the street to the room. It can be a strong option for traffic, aircraft, rail and neighbourhood noise, especially where the original window needs to stay in place.
The figures in this guide are indicative, specification-dependent references. A stated dB result may relate to a glass sample, a complete tested window or a particular project rather than your room. Noise can also travel through walls, doors, floors, roofs, vents and adjoining windows. No glazing system can guarantee silence.
How much noise reduction is realistic?
The answer depends on the noise source and the whole window assembly. A sound primary window, a suitable air gap, acoustic glass, accurate measurement and continuous seals can all help. An open vent, a failed sash joint or a flanking path can limit the lived result even when the inner pane is well specified.
As a broad orientation, simpler secondary-glazing arrangements are sometimes described around 30–45 dB, while a well-specified acoustic arrangement may be described around 45–54 dB. Some marketing material also translates a suitable result into 70–80% less perceived noise. These are indicative descriptions, not guaranteed measurements or a direct percentage reduction in sound pressure.
| Arrangement or description | Indicative reference | Important qualification |
|---|---|---|
| Original single-glazed window | About 20–30 dB reduction in some references | Window condition, seals and the frequency of the noise vary |
| Simpler secondary-glazing arrangement | About 30–45 dB | Product, glass, cavity, seals and test method must be stated |
| Well-specified acoustic secondary glazing | About 45–54 dB | A tested or modelled result for a defined build-up, not every room |
| 70–80% perceived reduction | Indicative description used for suitable projects | Not the same as a guaranteed measured dB or percentage result |
For a deeper look at acoustic glass, read the acoustic-glass comparison. For the wider principles of separating the panes, see the soundproof secondary glazing overview.
Why the air gap matters
The gap between the original and secondary panes helps decouple the two surfaces. A 100mm air gap is a useful starting point for discussing demanding traffic noise, but it is not a universal rule or a guarantee of a particular dB result. The available reveal, shutters, skirting, radiators, curtains, ventilation and opening method determine what is practical.
A larger cavity is not the only consideration. Glass mass, acoustic laminate, asymmetric pane thicknesses, frame rigidity and seal continuity also matter. A narrow unit with a perfect seal may outperform a badly fitted wide unit, while low-frequency rumble may need a different balance from high-frequency voices or sirens. The Red Route soundproofing guide explains why the noise spectrum matters.
Glass, seals and the original window
Acoustic laminated glass can add mass and damping through its interlayer. It is often considered where traffic or aircraft noise is the main concern, but “acoustic” on its own does not select a complete system. Ask which glass thickness, interlayer, frame, cavity and seals are included and whether the claimed result is for the glass or the entire window.
The original window remains part of the acoustic barrier. Repair loose putty, failed joints, damaged meeting rails and gaps around the frame where appropriate. The inner unit should be sealed carefully, but it should not conceal rot, water ingress or a ventilation problem. A survey can identify whether the sound is entering through the opening or taking another route through the building.
Survey stages before a specification
1. Describe the disturbance
Note whether the problem is buses, tyres, aircraft, trains, voices, music or intermittent sirens. Low-frequency rumble and high-frequency voices behave differently. Record the room, time of day and whether the noise is strongest at the window, wall, roof or door.
2. Inspect the existing opening
The survey should record the window type, condition, reveal depth, meeting rails, handles, shutters, vents, curtains and radiators. It should also consider whether the sash or casement can continue to open for ventilation and maintenance.
3. Compare a complete build-up
Ask for the proposed glass, frame, seals, air gap, fixing method and opening style in writing. A dB claim without its test basis is difficult to compare. Ask whether it is measured, modelled or a project observation, and which frequencies and locations were used.
4. Set expectations for the room
A quieter window may leave sound through a party wall, ceiling, floor, door, vent or another elevation. The installer should explain what the proposal addresses and what will remain outside scope. A realistic improvement is more useful than a promise of complete silence.
Cost and practical trade-offs
Acoustic glass, wider cavities, bespoke frames, repairs, access and installation can all increase the cost. A broad planning range for professionally fitted standard work may be around £300–£800 per window, with acoustic specifications sometimes around £600–£1,200. These are indicative ranges only; size, finish, location, VAT, access and making good can change the quote. Use the 2026 UK cost guide to compare inclusions.
Secondary glazing also creates a second layer to open and clean. Hinged, sliding, fixed and lift-out systems each make different compromises. The best acoustic specification is not useful if it blocks ventilation, prevents safe escape or makes the original window impossible to maintain.
Planning and period properties
Internal secondary glazing may leave the external elevation unchanged, which is why it is often explored for listed buildings and conservation areas. That does not create a blanket planning or Listed Building Consent exemption. Fixings, shutters, architraves, historic fabric, Article 4 Directions, local policy and lease terms can all affect the answer. Check before commissioning work in the listed-building guide.
FAQs
Can secondary glazing reduce traffic noise by 70–80%?
It may produce a substantial perceived improvement in a suitable installation, but 70–80% is an indicative description rather than a guarantee. The dB result depends on the original window, glass, air gap, seals, installation, noise frequency, room and flanking paths.
Is 100mm the best air gap?
It is a useful starting point for some acoustic designs, not a universal requirement. The reveal, glass, frame, seals, ventilation, shutters, curtains and access determine what is suitable.
Do I need acoustic glass?
Not always. Acoustic laminated glass may help with a demanding noise source, but the complete window build-up matters more than a label or thickness in isolation. Ask for the evidence behind the proposed glass and dB result.
Will secondary glazing make my room silent?
No. It may reduce noise entering through a suitable window, while sound through walls, doors, floors, roofs, vents and other windows can remain audible.
What should I do first?
Write down the noise pattern, photograph the window and arrange a measured survey. Contact SecondaryDoubleGlazing to discuss an acoustic specification that fits the actual opening.
SecondaryDoubleGlazing helps homeowners make measured decisions about warmer, quieter period homes. The right acoustic result starts with the window and the building, not a headline number.