How the two differ in kind
A facade is a composite: wall, window, frame, and any vent or trickle opening in it. Sound transmission through a composite adds on ENERGY, weighted by area, which has one dominant consequence — the assembly performs like its WEAKEST element, and improving anything else changes very little until that element has been dealt with.
In almost every ordinary building, the weakest element is the glazing. A masonry or well-built framed wall substantially outperforms a window of any normal specification, so the facade's effective performance is close to the window's, degraded further by any gap or vent. Upgrading the wall in that situation is spending on the element that was not setting the level, and the measured improvement is disappointing in a way that feels inexplicable until the arithmetic is written down.
The order therefore starts with the glazing and the openings around it, and only reaches the wall once the glazing is good enough that the wall has become the limit — which for a serious upgrade against a busy road genuinely happens.
Two properties of glazing follow from the same physics and are worth knowing before buying anything. Thicker glass transmits less, because mass is what resists being moved by sound — but two panes of the SAME thickness resonate at the same frequency and let that frequency through, so an ASYMMETRIC pair outperforms a symmetric one of the same total glass. And laminated glass outperforms plain glass of equal thickness, because the interlayer damps the panel's vibration; acoustic laminates use an interlayer chosen specifically for that.
The third property surprises people: the air gap in a sealed double-glazed unit is sized for THERMAL performance, at a spacing that suppresses convection. Acoustic performance wants a much wider gap than that. Which is why secondary glazing — a second window with a large air space behind the original — routinely outperforms a replacement sealed unit for noise, while doing less for heat.
The factors that actually differ
Show
| Acoustic glazing | Treating the wall | |
|---|---|---|
| Which element usually governs | This one. The window is normally the weakest part of a facade by a wide margin. | Rarely, until the glazing has been upgraded substantially. |
| Return on spending | High while the glazing is the limit — which is most of the time. | Low until the glazing is no longer the limit, then significant. |
| What improves it | More mass, ASYMMETRIC pane thicknesses, laminated glass with an acoustic interlayer, and a wider air gap. | Mass, decoupling, cavity absorption — and sealing, which is usually the cheapest gain. |
| The gap | A sealed unit's gap is optimised for heat. Acoustics wants it much wider, which is what secondary glazing provides. | A decoupled cavity, where width and absorption both matter. |
| Low frequency | Hard. Traffic rumble and bass need mass and width, and thin units do little. | Also hard, and mass is the only reliable lever. |
| Disruption | Replacement windows, or secondary glazing fitted inside the existing frames with far less disturbance. | Substantial — linings, finishes, sockets, skirtings and the room out of use. |
| Room size lost | Almost none for a replacement; a secondary unit takes a little from the reveal. | Real — an independent or resilient lining takes depth from the room on every treated wall. |
| Ventilation | The catch. A trickle vent or an openable window undoes the upgrade whenever it is open, so ventilation has to be solved separately. | Unaffected. |
| Flanking | Not addressed. Sound entering through the roof, the floor, or around the facade is a separate path. | Also not addressed, and it is what limits results when everything else has been done. |
| Cheapest first move | Sealing — the perimeter, the opening light's seals, and any redundant vent. Frequently the largest single gain available. | The same principle: seal penetrations and perimeters before adding mass. |
Which one, and when
Choose acoustic glazing when…
- There is ordinary glazing in a solid wall — the usual case, and the glazing is the limit.
- The noise is external and arrives through the facade, particularly road or rail.
- Disruption matters, and secondary glazing can be fitted with the room largely intact.
- Sealing and glazing have not yet been addressed, which is where the available improvement is.
Choose treating the wall when…
- The glazing has already been upgraded substantially and the wall is now the weakest element.
- The wall is genuinely lightweight — a thin partition, a timber-framed wall with little mass, a converted space.
- The noise is coming through a party wall or a partition rather than through the facade.
- There is no window in the wall concerned, which settles it.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does improving the wall do so little?
- Because transmission adds on energy weighted by area, so the assembly's performance is dominated by whatever is weakest. Think of it as leakage: if the window is passing far more sound energy than the wall, then halving what the wall passes barely changes the total, in the same way that fixing a small leak does not help when a large one is open beside it. In most buildings the wall is substantially better than the window, so the facade's effective performance sits close to the window's — and improving the wall moves a number that was not setting the total. The corollary is that this reverses once the glazing is good: a seriously upgraded window in an ordinary wall can leave the wall as the limit, at which point treating it is exactly right. The order matters more than the products.
- Why do the two panes need to be different thicknesses?
- Because identical panels resonate at the same frequency and let that frequency through together. Every panel has a coincidence frequency at which it transmits sound far more readily than its mass would suggest, and that frequency depends on the panel's thickness and stiffness — so two panes of the same glass share the same weak point, and the assembly has a pronounced dip in performance there. Making the panes different thicknesses moves their weak points apart, so each pane is still working where the other is weak, and the combined performance across the spectrum improves markedly. This is why acoustic glazing is specified as a pair of dissimilar thicknesses rather than as a total, and why a unit made of two equal panes underperforms an asymmetric one containing the same weight of glass.
- Is secondary glazing better than new double glazing for noise?
- For noise specifically, usually yes, and the reason is the gap. A sealed double-glazed unit has a cavity sized for thermal performance — narrow enough to suppress convection between the panes — and that spacing is far below what acoustics wants. Secondary glazing puts an independent window inside the existing one with a much larger air space between them, and that width, combined with the two frames not being rigidly coupled, produces a substantially better acoustic result. It also retains the existing window, which matters in a listed or conservation context. Its costs are practical rather than acoustic: two windows to open and clean, a loss of a little reveal depth, and less thermal improvement than a modern sealed unit. Where both heat and noise matter, an upgraded unit plus secondary glazing is the strongest combination.
- What about trickle vents and openable windows?
- They are the hole in the plan, and they undo the upgrade whenever they are open. Composite transmission is unforgiving of a path with no resistance, so an open trickle vent or a window on the latch can dominate the facade's performance regardless of the glass specification — which is the mechanism behind the familiar complaint that expensive new windows made no difference, discovered later to have their vents open. The resolution is to solve ventilation separately rather than to seal the room: acoustic trickle vents with a lined labyrinth give a worthwhile attenuation while still ventilating, and mechanical ventilation removes the need to open windows for air at all. That is not a detail to leave to the window supplier, since a facade upgraded acoustically and ventilated by opening the window has bought very little.
- Does laminated glass help?
- Yes, more than its thickness alone would suggest, because the interlayer damps the panel. A plain sheet of glass vibrates readily at its coincidence frequency and transmits sound efficiently there; a laminate of two sheets bonded by a polymer interlayer has that vibration damped by shear within the interlayer, which flattens the dip and improves performance across the range. Acoustic laminates go further by using an interlayer formulated specifically for damping rather than for the safety and security properties standard laminates are made for, and the difference between a standard laminate and an acoustic one is measurable. Laminated glass also brings safety and security benefits and blocks ultraviolet, which frequently makes it the right choice for reasons beyond acoustics — and it is the single most cost-effective upgrade within a glazing specification.
- What is the cheapest thing to do first?
- Seal, before buying anything. A gap passes sound with no resistance at all, so the perimeter of the window frame where it meets the wall, the seals on the opening light, redundant vents, and any service penetration through the reveal are all potential paths that can dominate an otherwise decent facade. Replacing perished seals on an opening casement and running a bead of sealant round a frame perimeter costs very little and can produce an improvement that a glazing upgrade would have been credited with. It is also diagnostic: sealing first establishes what the facade is actually capable of, which makes the subsequent decision between glazing and wall a measured one rather than a guess. The same principle applies to the wall, where penetrations and perimeters matter more than mass until they are closed.
- What if I do everything and the noise remains?
- Then it is arriving by another path, and the usual candidates are the roof, the floor, and flanking through the structure. A room in a roof space has a large area of lightweight construction overhead that may be far weaker than the facade. A room above a shared entrance or a plant space may be receiving noise through the floor. And structure-borne transmission from road traffic or a railway travels through the ground and into the building frame, arriving as a low-frequency rumble that no facade treatment touches. The diagnostic is to identify which surface the sound is loudest against, which usually points at the path directly. It is also why a serious noise problem is worth measuring before spending: an acoustic survey identifies the dominant path and frequency range, and it is cheap against the cost of treating the wrong element.
- How much improvement can I expect?
- Enough to be worth doing and less than the product ratings imply, for two reasons that are worth understanding before setting expectations. Laboratory ratings are measured on an isolated element with every other path suppressed; a real facade includes a frame, a perimeter, a vent and a wall, and the composite performs below its best component. And perception is logarithmic: a reduction that looks modest in decibels is quite noticeable, but halving the perceived loudness takes a substantial change rather than a few points. The realistic framing is that a good glazing upgrade on a noisy facade takes a room from intrusive to acceptable rather than to silent, that the improvement is largest at mid and high frequencies where speech and tyre noise sit, and that low-frequency rumble is the part most likely to remain.