How to Sound Insulate a Window: Methods, Decibels & Costs (2026)
Traffic on a red route, the Northern Line two floors down, a pub kicking out at 11:30 — most "window soundproofing" advice ignores the physics and leaves you doing all the same things twice. This guide gives you the realistic decibel reduction, cost, and lifespan of every method, so you can pick the right one first time.
Quick reference: a 10 dB reduction halves perceived loudness. So a window that drops noise from 75 dB to 45 dB is roughly 8× quieter, not 40% quieter.
Why Most "Soundproofing" Fails
Sound is fluid. It finds the weakest link and pours through it. A 2mm gap around a window frame transmits as much noise as if the rest of the wall didn't exist. That's why heavy curtains and 4mm double glazing barely help with traffic noise — they don't seal the leak, and they don't have enough mass or air gap to block low frequencies.
The two things that genuinely reduce noise through a window: mass (heavier, laminated glass) and a large dead-air gap (100–150mm between the two panes). See the physics: The 100–150mm Air Gap Rule.
Sound Insulation Methods, Ranked by Real-World dB Reduction
The gap from "DIY acrylic" to "professional 10.8mm acoustic secondary glazing" is roughly 40 dB — that's the difference between "I can still hear the bus" and "I forgot the road was there".
Step 1: Measure What You're Up Against
Use a free smartphone SPL meter app at the window, with traffic at peak. Typical readings:
- Quiet suburban street: 45–55 dB
- Busy London side road: 60–70 dB
- Red route / A-road: 70–80 dB
- Heathrow flight path: 75–95 dB peaks
WHO recommends bedroom noise stays under 30 dB at night. Aim for that.
Step 2: Seal the Leaks (Do This First, Always)
Before spending £600 on glass, spend 20 minutes with a tube of acoustic sealant (Acoustiseal or similar). Run a bead round the outside of the frame where it meets the brick or render, and round the inside where it meets the plaster. Add brush or compression seals to any opening sash. This alone delivers 3–8 dB on most older windows and stops you wasting money on glass that's defeated by a 2mm gap.
Step 3: Match the Glass to the Noise
- Under 60 dB outside (light traffic): Standard 4mm secondary glazing is enough.
- 60–75 dB (busy road, occasional Tube): 6.4mm laminated glass with a 100mm gap.
- 75 dB+ (red route, flight path, nightlife): 10.8mm acoustic laminated glass with a 100–150mm gap — the only spec that brings indoor levels under 35 dB.
Why acoustic laminate beats plain thick glass: the PVB interlayer damps the resonance frequency where ordinary glass "rings" and lets sound through. Full explanation: The Science of Silence.
Step 4: Get the Air Gap Right
A 20mm DIY gap = ~15 dB reduction. A 100mm professional gap with the same glass = ~35 dB. A 150mm gap = ~45 dB+. The gap is doing more of the work than the glass. This is why a £200 magnetic panel mounted directly on the existing frame can't get anywhere near the performance of a £550 properly-recessed installation, even with identical glass.
Common Window Types — Specifics
Sash windows
Sashes have built-in leak paths: the meeting rail, the parting bead, the cord-pulley voids. Draught-strip first (£180–£300), then fit vertical-sliding secondary glazing so you keep the operation. Full pricing: Sash Windows Cost 2026.
Single-glazed
Single 4mm glass alone offers ~25 dB reduction — useless against modern traffic. Adding 10.8mm acoustic secondary glazing pushes the combined system to 50 dB+, comfortably better than triple glazing.
Bay windows
Bays act like a horn — they collect and amplify street noise into the room. Treat every pane individually, plus the cill and the bay roof above (usually missed, often the loudest leak). Detailed approach: Edwardian Bay Windows.
What to Skip
- "Acoustic" curtains marketed at £200+: 3–5 dB at best. Buy normal lined curtains and put the money into glass.
- Replacing with modern double glazing for noise: The 16–20mm gap is too small. You'll spend £800/window and gain only 25–30 dB.
- Foam-filling the frame: Damages the timber and locks in moisture. Use acoustic sealant only.
The Bottom Line
For under 60 dB outside, draught-proof and add basic secondary glazing. For anything louder — and that's most of central London — only professionally-installed 10.8mm acoustic laminate with a 100mm+ air gap will get bedroom levels under the WHO 30 dB threshold.