How to Soundproof Sash Windows: 6 Ways to Cut Noise

Old sash windows can let in a surprising amount of street noise, but most of it gets in through gaps around the sashes rather than through the glass. That means you can usually make a room much quieter without replacing the windows. Draught-proofing makes a noticeable difference on its own, and well-fitted secondary glazing with a gap of 100mm or more can cut noise by up to 45 decibels, according to Historic England. This guide compares every option, with costs.

Why sash windows let in so much noise

• Gaps: sound travels wherever air can. Worn beads, loose sashes and gaps at the meeting rails let noise straight in.
• Thin glass: most original sashes have a single pane of thin glass.
• Rattles: loose sashes vibrate in wind and when lorries or buses pass.

Historic England puts the noise reduction of a single-glazed window without seals at about 18 to 25 decibels. Every 10 decibels you take off sounds roughly half as loud, so even modest improvements are easy to notice.

The options compared

Option	How much quieter	Typical cost per window	Keeps original windows
Draught-proofing or full overhaul	Noticeable, by sealing the gaps	£150 to £350 (overhaul £350 to £650)	Yes
Secondary glazing, 100mm+ gap	Best: up to about 45dB with the original window	£180 to £400 (acoustic versions more)	Yes
New standard double glazed sashes	About 30dB	£1,200 to £3,500	No
New double glazing with acoustic laminated glass	Up to about 40dB	Higher than standard	No
Internal shutters or heavy curtains	Some, especially shutters when closed	Varies	Yes

Prices are typical 2026 UK ranges including VAT. London and the South East are usually 20 to 30% higher. Use our sash window cost calculator to work out a total for your house.

1. Start by sealing the gaps

Draught-proofing is the cheapest place to start, and it helps with noise as well as heat. A specialist removes the sashes, fits brush seals into the beads and meeting rails, and adjusts the sashes so they sit tight in the frame. Historic England’s advice is that, for noise, the most important thing is that windows are well fitted and draught-proofed. A full overhaul also stops rattles. See how to stop draughts from sash windows.

2. Add secondary glazing for the biggest difference

Secondary glazing is a slim second window fitted inside the existing one. The air gap between the two is what stops the sound, so the gap matters more than anything else:

• Aim for a gap of 100mm or more. Historic England notes that secondary glazing with a 100mm+ gap typically gives up to 45dB of noise reduction, and that there is little extra gain beyond about 200mm.
• Ask for thicker or acoustic laminated glass in the secondary panel.
• Line the reveals between the two windows with sound-absorbing material for extra performance on very noisy roads.
• Choose vertical sliders to match the sash behind, so they can still be opened for air.

Secondary glazing also keeps heat in and adds security. Read our full secondary glazing guide.

3. Choose acoustic glass if you’re replacing

If your sashes are beyond repair and you’re buying new windows, ask for acoustic glass. Standard double glazing reduces noise by about 30dB, while units with acoustic laminated glass can reach about 40dB. Glass of different thicknesses on each side of the unit also helps. Ask each firm for the tested sound reduction figure (Rw) of the glass they propose. See double glazed sash windows: costs and options.

4. Slim or vacuum glazing in your existing sashes

Fitting slim double glazing or vacuum glass into your original sashes is mainly an energy upgrade, but it does make rooms quieter than single glazing, especially when combined with draught-proofing. If noise is your main concern, ask for the acoustic figure before you choose this over secondary glazing. See can you double glaze existing sash windows?

5. Use shutters and heavy curtains

Many period houses were built with internal shutters, and Historic England notes that shutters and heavy curtains can significantly improve noise insulation. If your shutters are still there, getting them working again is a cheap win for bedrooms facing the street.

6. Check the other ways noise gets in

Once the windows are sealed, noise can still come through trickle vents, air bricks and a letterbox. Acoustic trickle vents and a draught-excluding letterbox are inexpensive. Keep enough ventilation to avoid condensation; see condensation inside double glazing.

Do you need permission?

• Draught-proofing and secondary glazing don’t usually need planning permission, even in a conservation area, because they don’t change the outside of the house.
• Listed buildings: internal changes can still need listed building consent. Secondary glazing is often accepted, but check with your council first. See our listed building windows guide.
• New windows in a conservation area may need permission. See replacing sash windows in a conservation area.

Which should you choose?

• Busy road, original windows in decent condition: draught-proofing plus secondary glazing with a 100mm+ gap.
• Moderate noise, tight budget: a full overhaul with draught-proofing, plus shutters or heavy curtains.
• Windows rotten beyond repair: new timber sashes with acoustic laminated double glazing.
• Listed building: repair and draught-proof, then secondary glazing, with consent if needed.

Getting quotes

Tell each firm that noise is the main problem, which road or source it comes from, and which rooms matter most. Ask for the gap size and glass for secondary glazing, or the tested Rw figure for new glass, so you can compare like with like. Find sash window specialists near you.

Frequently asked questions

What is the best way to soundproof sash windows?

For most period homes, draught-proofing the existing sashes and adding secondary glazing with a gap of at least 100mm gives the biggest reduction in noise for the money, and keeps the original windows.

Is secondary glazing better than double glazing for noise?

Usually, yes. The wide air gap in secondary glazing blocks sound better than the narrow gap inside a double glazed unit. Historic England puts well-fitted secondary glazing at up to 45dB, compared with about 30dB for standard double glazing.

Does draught-proofing reduce noise?

Yes. Much of the noise through old sash windows comes through gaps, so sealing them and stopping rattles makes a noticeable difference, though less than secondary glazing.

How much does it cost to soundproof a sash window?

Draught-proofing typically costs £150 to £350 per window and secondary glazing £180 to £400, so doing both is often £350 to £750 per window. New acoustic double glazed timber sashes cost £1,200 to £3,500 or more.

Sources: Historic England, Energy Efficiency and Historic Buildings: Secondary Glazing for Windows (2016); Crittall Windows, acoustic glazing figures. Prices from our cost guides. A guide only; always get a written quote.