Shea CullenNSW Registered Architect 9748 · Updated 15 August 2026You inspected on a Saturday morning, you fell for the house, and you moved in to discover the street is busier than you thought. I get called about this more often than you would think.
Before anyone lists the house, it is worth knowing that road noise is one of the more fixable problems in a building, and that most of the money people spend on it goes to the wrong places. So here is what actually works, in the order I would invest in each.
The three things that stop sound
There are three ways to block sound.
Mass. Heavy things are hard to vibrate. This is the mass law: double the weight per square metre of a barrier and you gain roughly 5 to 6 dB.
Really solid walls stop sound. You can add extra insulation to an external wall and adobe to internal walls. A really solid masonry fence that is high will help a lot as well. It is also worth considering external 'rooms', outdoor partitions and screens designed to block sound.
Sealing. Sound is air movement, and air finds gaps. This one matters more than mass, which surprises people, and I will come back to it because it is where the cheap wins live.
Separation. Two layers with a gap between them beat one layer of the same total weight, because the vibration has to cross the gap. This is why double glazing exists, why a cavity wall outperforms a solid one of similar mass, and why resilient mounting is the best way to stop sound travelling through different floors.
There is a fourth thing, absorption, which is what soft furnishings, carpet, curtains and the batts in your walls do. Absorption is genuinely useful, but it does a different job. It soaks up sound that is already bouncing around inside a room, making the space feel calmer and less echoey. It does very little to stop sound getting in from outside.
Confusing absorption with blocking is the most common and most expensive mistake people make here, and plenty of products marketed for noise trade on the confusion.
The weakest link decides everything
The overall performance of a wall is not the average of its parts, it is dominated by the worst part. A wall built to Rw 55 with an unsealed gap around a door performs like Rw 25.
It is the difference between a quiet room and a loud one, caused by a small gap.
Run the same logic across your whole street facing wall. A typical brick veneer wall with insulation sits somewhere around Rw 50 or better. A standard single glazed aluminium window sits around Rw 25. Even though the window is a small fraction of the wall area, essentially all of the noise you can hear is coming through it, and through the gaps around the frame. Which means:
- Adding mass or batts to a wall while the original windows stay in place doesn't really help.
- Sealing costs almost nothing and often delivers the largest single improvement of anything on this list.
- Once the windows are dealt with, the next weakest link takes over, and it is usually somewhere you have not thought about.
When people do the windows and are still disappointed, the noise is almost always arriving somewhere else: wall vents and weepholes, the gap under an external door, an unsealed exhaust fan, a range hood duct, downlight penetrations into an uninsulated ceiling, an old chimney, or the roof space itself.
Sound coming through the roof and down through a lightweight ceiling is the one people miss most often, particularly in a single storey house with a large roof plane facing the road.
Traffic noise is low, and that changes the answer
Above about 30 km/h the dominant sound from a car is not the engine, it is the tyres on the road surface. That produces a broad spectrum with a lot of energy down low, and low frequencies are the hardest thing in acoustics to stop. They have long wavelengths, they go straight through fabric, they set light building elements vibrating, and they are exactly what light, thin, soft solutions fail against. It is why you can hear a truck through a closed window while a conversation at the same distance is inaudible.
This has one very practical consequence when you try to stop it.
Rw and Ctr, and why the number on the brochure misleads you
Rw is a single figure for how much sound a window stops. It comes from testing the window across a range of pitches and boiling the results down to one number. Higher is better, so Rw 35 stops more than Rw 30.
The catch is in what that average leans on. Rw weights the result towards the middle and upper pitches, roughly where human speech lives. That is genuinely useful if your problem is the neighbour's television. It flatters a window badly if your problem is a road, because tyre noise sits well below the range Rw cares most about.
Ctr is the correction for exactly this. It is a second number, always zero or negative, that you add to the Rw to get the performance against traffic. You will see it written in brackets after the Rw, so Rw 35 (Ctr -5) means about 30 against road noise. Suppliers quote the Rw because it is the bigger number.
Two examples of why the difference matters.
Same headline, different window. Say you are quoted two products, both Rw 35. The first is Rw 35 (Ctr -2), the second is Rw 35 (Ctr -6). On the brochure they look identical. Against traffic the first delivers about 33 and the second about 29. Four decibels is easily heard, and you would have had no way of knowing from the headline figure.
The lower number that wins. Now say the choice is between Rw 38 (Ctr -7) and Rw 34 (Ctr -1). The first looks 4 dB better and will usually be sold that way. Add the corrections and they come out at 31 and 33, so the one with the worse headline is the quieter window on a busy road. Buy on Rw alone and you buy the wrong one.
The reason the two numbers drift apart is the low pitches. A double glazed unit with a narrow cavity and two panes of the same thickness has a resonance down low where it stops very little, and that dip is what drags Ctr down. Laminated glass, a wider gap and mismatched pane thicknesses all lift the low end, which shows up as a smaller Ctr penalty.
So when the quotes come in, ask for both numbers and add them together before you compare anything. If a supplier cannot tell you the Ctr, that in itself tells you something. This single question will save you from the most common bad glazing purchase I see.
Where to fix things
1. Seal everything
Cheapest, fastest, and routinely the biggest single gain. Draught seals to external doors including a proper bottom seal, new perimeter seals to every openable window, sealing around the window and door frames where they meet the wall lining, closing off redundant wall vents and old chimneys, gasketed covers on exhaust fans, and sealing the ceiling penetrations that open into the roof space.
A home that has been properly draught sealed is usually noticeably quieter before you have bought a single window, and you get lower energy bills and fewer draughts along with it. If you do nothing else on this list, do this one.
The warning that goes with it is in the ventilation section below. Do not seal a house without thinking about air.
2. Fix the glass
There are two routes.
Secondary glazing adds a second pane on the inside of the existing window, on its own frame, with a sealed air gap between. Magnetite is a window system that goes over your existing windows. The reason it works is the air gap. A replacement double glazed unit has a cavity of around 12mm, sized for thermal performance, which is acoustically quite narrow. A secondary system can run a gap of 50 to 100mm, and that wide gap is worth more against low frequency traffic noise than an extra pane. Tested configurations with a wide cavity get up into the high 30s for Rw, which is better than a lot of replacement double glazing, at lower cost, without touching the existing windows. On a character home where the original windows are part of why you bought it, that last point matters.
Replacement double glazing is the right call if the windows are due for replacement anyway, or if you want the thermal and condensation benefits at the same time. If you go this way, specify for acoustics deliberately: unequal pane thicknesses so the two sheets do not resonate together, at least one laminated pane because the interlayer damps vibration, the widest cavity the frame will take, and good compression seals. If you have two panes of the same thickness, the identical sheets share a resonance and the pair performs worse than the numbers suggest, so always use a different thickness of glass to your existing ones. Older homes are usually 3 to 4mm.
Worth knowing: a single sheet of acoustic laminated glass around 10mm can outperform standard double glazing for noise, because laminate damping and mass beat a narrow air gap. If the window has to stay a single pane for heritage or budget reasons, laminated is the upgrade.
My separate piece on whether double glazing is worth it here covers the thermal side of the same decision, and the window specification guide has the frame detail.
3. Deal with the roof and ceiling
If the house is single storey and the road faces a big roof plane, the roof is often the second biggest path once the windows are sorted. Insulation batts in the ceiling help a little, but the real gains come from mass and separation: a second layer of plasterboard on the ceiling, sealing every downlight and duct penetration, or replacing open downlights with sealed fittings. Batts alone are absorption, which is comfort inside the room rather than a barrier to what is coming in.
4. Add mass to the wall, if there is still a problem
You can fill the wall with adobe. In practice, adding mass to the street facing wall usually means an extra layer of plasterboard, ideally a high density acoustic board, on resilient mounts so it is not rigidly connected to the frame. The resilient mounting is doing as much work as the mass. Screwing a second sheet directly to the studs bridges the vibration straight across and wastes most of the benefit.
On a new build or a substantial addition, this is where the answer gets easy and cheap: build the street facing wall heavy in the first place. Cavity brick, a concrete or block wall, or an insulated cavity system costs very little extra when it is in the original drawings and it removes the problem permanently. If you are planning an addition or renovation on a noisy street, tell your architect about the noise at the briefing stage, not at the end.
5. Barriers: the berm and the solid fence
A berm and a solid fence are the two measures people ask me about most, and both come with conditions attached.
An earth berm or a solid masonry fence works by blocking the line of sight between the tyres and your ears. Sound then has to bend over the top, and the amount you gain depends almost entirely on how far the barrier pushes the sound out of its direct path. Break the line of sight properly and you get somewhere in the range of 5 to 10 dB, which is a real and worthwhile improvement. Fail to break it and you get close to nothing, no matter what the barrier is made of.
Three things follow from that.
Mass barely matters, past a point. A barrier needs about 10 kg per square metre to stop sound passing straight through it. Above that, performance is capped by the sound bending over the top, not by the material. Which means a well built, properly sealed solid timber fence performs much like a masonry wall of the same height, it is just that it is a lot harder to seal properly. If you are choosing a masonry fence for looks, longevity or privacy, that is a perfectly good reason.
Gaps destroy it. Slatted, hit and miss, or lapped fencing with daylight through it is not an acoustic barrier at all. Everything needs to be closed up, ideally with no gap above 5mm anywhere, including at the base where the fence meets the ground.
Barriers only work downstairs. This is the big one. A barrier helps a receiver that sits in its shadow. Upper floor rooms usually look straight over the top and get almost no benefit. If the bedrooms that are keeping you awake are upstairs, a berm or a fence is not your solution and the money belongs in the glazing.
Between the two, a berm and a wall of the same height perform within a decibel or two of each other. A berm needs a lot more land to reach a given height, so on a normal suburban block a fence usually wins on practicality. A berm's advantages are that it is quiet to look at, absorbs a little rather than reflecting sound back across the road, and can be planted. If you have the room for it and you like the look, it is a good move. If it means a 3 metre wide mound eating your front yard to reach 1.8 metres, a fence does the same job on a 200mm footprint.
6. Planting
Honest answer: the acoustic effect of a garden screen is close to zero. The research is consistent that you need roughly 30 metres depth of dense planting to achieve about 5 to 6 dB, and that in most real situations anything above 3 to 5 dB is unlikely. A row of lilly pillies along the front fence is not a noise barrier.
I still recommend planting there. Two reasons. Not being able to see the road measurably changes how intrusive people report the noise to be, and that perceptual effect is real even though an instrument would not register it. And planting in front of a solid fence softens what would otherwise be a hard wall along your boundary. Green views are also really good for your mental and physical health. Plant for the look and the psychology, not for the decibels, and do not let it substitute for the fence.
7. Soft furnishings
Last on the list, and worth doing anyway. Heavy curtains, rugs on hard floors, upholstered furniture and a bookcase on the street wall all reduce the reverberation inside the room. A room with hard surfaces everywhere lets whatever noise does get in rattle around and sound louder than it is, so damping that down genuinely makes the space more pleasant.
What soft furnishings do not do is block traffic noise from outside. Curtains marketed as soundproof typically manage a couple of decibels at low frequencies, which is not perceptible. The exception is a heavy curtain in a sealed pelmet, which starts to act a little like a second layer over the glass, and even then it is a supporting act to the glazing rather than a replacement for it.
The move that costs nothing: change what faces the road
The most effective thing an architect can do about noise is not a product, it is a plan.
Put the rooms that do not care about noise on the road side, and the rooms that do on the quiet side. A garage, laundry, bathroom, hallway, robe or stair against the street facade is a solid buffer that you were building anyway. Bedrooms and living areas go to the back. On a renovation this is often achievable by swapping which rooms do what, sometimes for the cost of moving a door.
If you are relocating a main bedroom, that alone can solve the complaint that is driving the conversation, for a fraction of the cost of treating the whole street facade. It is the first thing I look at on a noisy site, and it is why I would want to see the floor plan before recommending you spend anything.
Do not seal a house without thinking about air
If you tighten a house up, you have to give it a way to breathe.
The National Construction Code requires habitable rooms to have openable windows, doors or vents totalling at least 5% of the room's floor area. The acoustic criteria that apply near busy roads, on the other hand, are assessed with the windows shut, which tells you something important: the design assumption is that on a noisy street you will keep the windows closed, and the ventilation has to come from somewhere else.
So a genuinely quiet house on a busy road usually needs mechanical ventilation, whether that is a heat recovery system, acoustically treated vents, or at minimum well chosen exhaust and supply. Skip this and you trade traffic noise for condensation, mould and stuffy bedrooms, which is not a good trade in our humid coastal air. My piece on natural ventilation here sets out how I normally approach it, and a noisy street is the case where the usual advice needs adjusting.
If you are building new near a busy road
The planning system has views on this. Under the State Environmental Planning Policy (Transport and Infrastructure) 2021, residential development in or next to identified road and rail corridors has to demonstrate internal noise levels of 35 dB(A) in bedrooms and 40 dB(A) in other habitable rooms, assessed with windows and doors closed. Transport for NSW publishes the traffic volume maps that determine which roads trigger it.
If you are near the M1, the Central Coast Highway, the Pacific Highway or the rail line, this is likely to apply to you and it means an acoustic report with the DA. The upside is that designing to the criteria from the start costs far less than retrofitting later. It forces the useful decisions at the point where they are cheap: heavier street facing walls, properly specified glazing, and ventilation that was planned rather than added.
So what would I actually do
For the house you have already bought, in order:
- Get the plan right. Move the bedrooms and living spaces away from the street if the layout allows it. Costs the least, does the most.
- Seal everything. Doors, windows, frames, vents, chimneys, ceiling penetrations. Cheap, quick, and usually the biggest measurable jump.
- Fix the glass on the rooms that matter. Secondary glazing if the existing windows are worth keeping, acoustic double glazing if they are due for replacement. Buy on Rw+Ctr.
- Sort ventilation at the same time as steps 2 and 3, not afterwards.
- Chase the next weakest link. Usually the roof and ceiling, sometimes a door or a duct.
- Barriers, if your problem rooms are at ground level. Sealed and tall enough to break the line of sight. Berm or fence, whichever suits the site.
- Mass on the street wall, resiliently mounted, if a problem survives all of the above.
- Soft furnishings and planting throughout, for comfort and for the look, understanding what they are and are not doing.
Ten decibels of reduction is heard as roughly halving the loudness, so the improvements here are real, not marginal. You will never get to silence though. The goal is a house where the road is background rather than the thing you notice, and that is achievable on almost any site.
Weighing up whether to fix it or sell? Send me the address and the floor plan through the enquiry form, and I will tell you which of these applies to your house and roughly what the useful version costs, before you spend twice the stamp duty finding out.