Cutting Traffic Noise Through Sash Windows in London

The 6.20am bus is the problem. Not the traffic generally, the specific one that pulls away from the stop outside your bedroom. You have had two quotes: one to replace all the sashes with double-glazed timber units, one to fit secondary glazing, and they are thousands apart. The replacement quote promises "up to 70% noise reduction", which is not a unit of anything.

Traffic noise through a sash window is a solvable problem, but it is solved in a specific order, and the order is not the one most quotes assume. Almost everything useful happens before you get to the glass.

Noise gets in through gaps first, and glass second

Sound is air pressure. It travels through openings far more readily than through solid material, and the arithmetic is brutally unforgiving: a very small open area in an otherwise good barrier dominates the result. On a typical unsealed box sash you have a continuous gap down both sides of both sashes, a gap at the meeting rails, a gap under the bottom rail and a gap over the top rail. Add them up and it is not a crack, it is a slot several centimetres square.

Historic England's technical guidance puts a number on the consequence: a single-glazed window without seals may only achieve a noise reduction of 18 to 25 dBA. That is the whole window performing worse than the glass alone would, because the gaps are short-circuiting it.

Two things follow. First, sealing the window properly is the highest-value intervention per pound on almost every job. Second, any acoustic measure fitted over an unsealed window is being wasted, because the leak sets the ceiling.

For scale on the numbers: decibels are logarithmic, roughly 3 dB is the smallest change most people notice in steady noise, and a 10 dB reduction is generally perceived as about half as loud. So the difference between 25 dBA and 40 dBA of sound insulation is not a modest improvement. It is the difference between hearing individual vehicles and hearing a distant hum.

Why new double glazing so often disappoints on a main road

This is the part that surprises people who have just spent five figures.

In a sealed double-glazed unit the two panes are held rigidly apart by a spacer, typically 12mm to 20mm, and they are joined at the edges. Acoustically that makes them behave partly as one panel: the trapped air acts like a spring, the two panes resonate together, and at some frequencies the assembly performs no better than a single sheet of the same total thickness. Historic England is blunt about it, noting that sealed units are often no better than single glazing for noise and "can be slightly worse for traffic noise". The reason is that traffic noise is weighted heavily towards low frequencies, engine and tyre roar rather than the treble, and low frequency is exactly where a narrow-cavity sealed unit is weakest.

That does not make double glazing useless. Where new units genuinely help is that they arrive in a new, well-fitted, properly draught-proofed frame, and as noted above, most of the noise was coming through the gaps. The comfort improvement is real, it just is not coming from the second pane in the way the brochure implies. Our comparison of single glazing and double glazing sets out where the sealed unit does earn its keep, which is heat rather than noise.

If you do go down the glazing route on a period sash, asymmetric glass matters. Two panes of the same thickness share the same weak frequency; different thicknesses do not, and laminated glass with an acoustic interlayer damps the panel rather than just adding mass. Ask for the tested Rw figure for the exact build-up rather than a marketing percentage.

The 100mm rule, and why secondary glazing wins on noise

Secondary glazing is a separate glazed frame fitted inside the reveal, in front of the original sashes. It is the single most effective thing you can do to a period window for traffic noise, and the reason is purely geometric.

With an air space of 100mm or more between the primary and secondary glass, the two panes are genuinely decoupled: they no longer resonate together, and the air gap stops behaving like a stiff spring. Historic England's guidance gives the achievable figure as sound insulation of up to 45 dBA, against the 18 to 25 dBA of an unsealed single-glazed window. That is an enormous margin, and it is why acoustic consultants specify secondary glazing on main-road sites rather than replacement windows.

Three practical details decide whether you get near that figure.

Depth. Deeper is better, but with diminishing returns: improvements are small beyond about 200mm of cavity. In most London reveals you can get 100mm to 150mm without difficulty, which is comfortably into the useful range. If the reveal is shallow, the frame can sometimes be packed forward from the face of the wall to buy depth.

The seal on the secondary unit, not the primary. This is the detail installers get wrong. The secondary unit should be very well sealed, typically with compression gaskets rather than brush, because it is not sliding under load in the way a sash does. The original window, though, should not be sealed airtight, so that a little ventilation reaches the air gap and limits condensation on the inner face of the primary glass. Draught-proofing the sashes and sealing the secondary are different jobs with different products, which is worth understanding before you buy either. There is a rundown of the types in our article on sash window seals.

Glass and reveals. Thicker or acoustic laminated glass in the secondary unit measurably improves performance, and lining the reveals with an absorbent material improves it further, particularly at deeper cavities. On a bad road, 6.4mm or 6.8mm acoustic laminate in the secondary panel is money well spent.

Hinged casement secondary systems with high-compression seals generally outperform sliders acoustically, because a slider has to leave running clearance somewhere. The trade-off is that a casement swings into the room, so it needs space and a restrictor stay.

What each measure is actually worth

Prices swing on access, window size, condition and borough. Take these as the shape of the market.

Cost per decibel, draught-proofing wins outright, and it is the only measure on that list that also pays back in heating. The detailed method is in our guide to draught proofing a sash window, and per-window pricing for the secondary route is broken down in our article on what secondary glazing costs.

The ventilation trap

Sealing a bedroom against a main road creates an obvious problem: you still have to breathe, and you still have to get moisture out of the room.

Two things in the Building Regulations bear on this. Approved Document F says that account should be taken of outside noise when considering whether openable windows are appropriate for purge ventilation, which is the polite way of saying that a window you cannot open at 3am because of the traffic is not doing its ventilation job. It also says that where an exposed façade is close to an area of sustained and loud noise, such as a main road, a noise-attenuating background ventilator should be fitted. Those are real products: an acoustic trickle vent with a lined, baffled path, considerably deeper and more expensive than a plain slot vent.

The trap is in the other direction too. If you replace windows rather than repair them, ventilation obligations attach to the new units: where the old windows had background ventilators, the new ones must have at least the same, and where they had none, the standard provision is 8,000mm² equivalent area in habitable rooms and kitchens and 4,000mm² in bathrooms. A trickle vent is a hole in your acoustic barrier. Fitting a plain one in a bedroom on a bus route undoes a good part of what you have just paid for, which is a strong practical argument for repairing and sealing the existing sashes instead, since repair carries no such obligation.

Traditional buildings also want more air movement than modern ones, roughly twice as much as a rule of thumb, because the fabric relies on being able to release moisture. Sealing a solid-walled Victorian house tight and then complaining about condensation is a common sequence. There is more on that failure mode in our piece on condensation on the inside of windows.

The cheap wins, and the flanking paths nobody quotes for

Before spending on glass, work through the free and nearly-free measures.

1. Close the meeting rail properly. A sash fastener that no longer pulls the rails together leaves a horizontal slot right at ear height. A screw-down fitch fastener or a new keep, correctly set, closes it.
2. Put the shutters back into use. Original panelled shutters are dense timber across the whole opening, and Historic England notes that shutters and heavy curtains make significant improvements to noise insulation. Most London shutters have been painted shut rather than removed. Our guide to shutters for sash windows covers freeing them.
3. Hang heavy interlined curtains, full width, floor length, on a return-ended track. The return is the point. A curtain that stops at the reveal leaves the sound a path around the edge.
4. Block the disused chimney, with a vented cap or a chimney balloon that still allows a trickle of air. An open flue is a direct duct from the roof to the room.
5. Check the sub-floor. In a ground-floor front room, noise comes up through floorboards over an air-brick-ventilated void as well as through the window. Sealing the boards makes an audible difference and costs very little.

Be realistic about what remains. A window is one element in a wall, and once it outperforms the wall, the letterbox, the front door and the party structure, further spending on glazing buys nothing. On a Georgian house with a thin front door and a large stairwell, the door is often the weak point long before the windows are.

What to do next

Stand in the room with the traffic noise and put your hand round the perimeter of each sash on a windy day. If you can feel air moving at the meeting rails or under the bottom rail, seal the window before you consider anything else, because until that is done every other measure is capped.

If it is already sealed and still too loud, secondary glazing at 100mm or more with laminated glass is the honest answer, not replacement windows, and it will normally cost less. Where the room is habitable but the windows are also cold, tired and stiff, it is usually worth doing the repair and the acoustic work as one visit. Our sash window repair service covers London, and if you are weighing sealed units against a secondary system, our comparison of secondary glazing and double glazing sets out both, while our double glazing work explains what can and cannot be fitted into an original sash.