Soundproof your original sash windows.

A traditional timber sash window was designed for a quieter age. As traffic, deliveries and busier streetscapes have grown around heritage properties, the very details that make these windows beautiful — slender glazing bars, a single pane, a sash that slides freely in its boxes — are also the details that let outside noise in.

The reassuring news is that the original window does not need to be replaced to be quieted. Two complementary upgrades, carried out within the existing frame, do the great majority of the work: reglazing with vacuum-insulated glazing, and discreet, high-quality draught-proofing.

Where the noise actually comes through

Outside sound enters a period sash window along two paths. The first is straight through the glass itself: a single 3–4 mm pane offers a weighted sound-reduction index (R_w) of roughly 29 dB — adequate against an 18th-century street, less so against modern traffic, voices and aircraft. Standard sealed double glazing is, perhaps surprisingly, only marginally better at R_w 29–31 dB: its air-filled cavity sets up a resonance that hands sound back through at certain frequencies. The second path is air itself — the perimeter gap around each sash, the meeting rail where the two sashes overlap, and any clearance in the staff and parting beads. Sound travels through air, so even a small open route between sash and frame leaks a disproportionate amount of noise into the room.

A useful way to think about it: the glazing controls how much sound transmits through the surface of the window, and the seals control how much sound flanks around it. As a rule of thumb, the ear perceives every 10 dB reduction in R_w as roughly halving the loudness — and to reach that perceived halving, both paths have to be addressed. Treat only one, and the other quickly becomes the limiting factor.

Vacuum-insulated glazing, retrofitted into original frames

The first intervention is to replace the original single pane with a vacuum-sealed double-glazing unit. Two sheets of glass are bonded with a 0.1–0.3 mm evacuated cavity between them; with no air in the gap, there is no medium for sound waves to cross efficiently. On the glass alone, this delivers an R_w of 36–39 dB — a 7–10 dB step over the original single pane, which takes outside noise toward what the ear perceives as roughly half its previous loudness. Acoustically, the unit performs in the same band as a well-specified acoustic-laminated double-glazed window many times its thickness.

That slim profile is what makes the upgrade possible on heritage windows. The whole vacuum unit is only 6–10 mm thick — against the 20–28 mm of a standard sealed double-glazed unit — so it fits within the existing rebate of the original wooden sash. The frame, glazing bars and external sight lines are preserved; the window still reads as a period sash from the street, and we routinely complete these upgrades on listed buildings for that reason. As a useful additional benefit, the units we fit use toughened, laminated glass, which both improves their security and adds further mass to dampen sound.

Draught-proofing — closing the airborne paths

Reglazing alone will not soundproof a sash if the perimeter still whistles. The second intervention, draught-proofing, addresses the gaps around the moving sashes. Historic England's retrofit research finds that a properly draught-proofed sash leaks around 80% less air than an untreated one — and because sound travels through air, closing those routes typically adds a further 3–5 dB on top of the glazing upgrade, and removes the discrete leaks (voices outside, doors closing in the street) that the ear locks onto. Each sash is taken out, the parting and staff beads are routed to accept a fine brush or compression pile, and the meeting rail is sealed with a discreet weather strip.

The seals we use are slim, neatly hidden within the beads, and chosen so the sash still slides smoothly and locks in the usual positions. Internally, the window looks unchanged. Externally, there is nothing visible at all. What the occupant notices is that conversation outside no longer drifts in word-for-word, and that the constant low background of a nearby road drops away.

Repairs that complete the result

Two further details, addressed during the same renovation, help the upgrade achieve its full benefit:

• Structural repairs to the boxes, beads and sashes — softened or shrunken timber leaves cavities that resonate and rattle. Replacing degraded sections with sound, dense timber restores the mass the window relies on.
• Functional repairs to the cords, weights and pulleys — a sash that no longer rattles in its box, and that closes properly against its seals, is a sash that performs as designed.

Reglazed with a vacuum-insulated unit, sealed around its perimeter, and operating cleanly within sound timber, the original wooden window typically gains around 10–12 dB of real-world acoustic improvement — an outcome the ear registers as a near-halving of outside noise — while keeping every aesthetic detail that made the window worth preserving in the first place.