Acoustic windows: what actually reduces noise at home

Acoustic windows reduce noise thanks to three things: glass made of panes of different thicknesses with an acoustic interlayer, a closure with no leaks, and an installation that leaves no gaps in the joint with the wall. A special gas in the cavity or a third pane of glass counts for much less than is commonly believed.

The assembly performs only as well as its weakest path. With a gap of 0.1% of its surface, acoustic windows rated at 40 dB drop to about 30 dB, and the German information centre ifz Rosenheim cites losses of around 10 dB from small gaps in the installation joint.

This guide brings together what laboratory tests published by glass and system manufacturers say about the sound insulation of windows. All figures are laboratory values, with a measurement tolerance of ±2 dB according to Saint-Gobain. No window guarantees silence, however well it is specified.

Seven decisions determine how well acoustic windows perform:

1. Glass: two panes of different thicknesses and, in one of them, an acoustic interlayer.
2. Cavity: anything between 6 and 16 mm makes little difference, and argon adds nothing.
3. Profile: one that accepts that glass and closes with continuous seals.
4. Opening type: one that does not leak, and never with the sash left ajar.
5. Installation: a joint filled and sealed without interruptions.
6. Roller shutter box: treated as part of the window.
7. Ventilation: acoustic trickle vents or another route, without opening the sashes.

How noise gets in through a window

Noise passes through a window by vibration and by leakage: it makes the glass and frame vibrate, and it slips through any air gap, whether between sash and frame, between frame and wall, in the roller shutter box or in a trickle vent. The insulation of the assembly ends up close to that of its weakest element, not its best.

Well-designed acoustic windows close off all five entry paths, and each one has its own remedy.

Why the assembly performs worse than its best component

The sound energy crossing each part of a building envelope adds up, and because decibels are a logarithmic scale, an area with little insulation lets through far more energy than one with a lot.

The CTE application guide states that the assembly never exceeds its weakest part by more than 10 dB. And with windows taking up between 30 and 40% of a façade, the overall insulation ends up at most 4 to 5 dB above that of the window.

An example shows the effect of a leak. For a 1.8 m² window with an Rw of 40 dB, a gap equivalent to 0.1% of its surface (about 18 cm²) brings the assembly down to 29.6 dB. With 1% of the surface open, the insulation cannot exceed 20 dB; with 10%, 10 dB.

This is our own indicative calculation: it sets the order of magnitude, it does not predict a specific window. It explains why excellent glass in a poorly closing frame disappoints, and why seals, installation, roller shutter box and ventilation deserve as much attention as the glass.

Rw, C and Ctr: how to read acoustic data

Rw is the weighted sound reduction index for airborne noise, in decibels and measured in a laboratory; C and Ctr adjust that value for the type of noise, and for traffic and aircraft the most informative figure is Rw + Ctr. The number comes from the method in UNE-EN ISO 717-1 and is the value, at 500 Hz, of a reference curve shifted until it fits the measurements.

Data sheets give the figure as Rw (C; Ctr). A real example: Saint-Gobain's Stadip Silence 44.1 laminated glass has Rw 37 (−1; −3). Against traffic, the effective insulation is 37 − 3 = 34 dB, which is the RA,tr index the manufacturer publishes for this glass. Against voices or television it would be 37 − 1 = 36 dB.

Which term applies to which noise

Pilkington assigns noise sources to the two terms in its technical documentation.

The CTE application guide puts it more briefly: C for pink noise and railway noise, Ctr for cars and aircraft. And it points to the underlying reason: all materials insulate low frequencies (125 and 250 Hz) less well than medium and high ones, and traffic noise is concentrated mainly in the low frequencies.

When Rw gives the wrong ranking

Comparing glass on Rw alone can give the wrong order. In Pilkington's table for its Optiphon glass, a 6 mm / 16 mm / 8.8 mm unit scores Rw 41 (−2; −6), i.e. 35 dB against traffic. Another, with 8 mm in the outer pane, rises to Rw 42 (−3; −8) and drops to 34 dB. The second wins on Rw and loses exactly where it matters for a noisy street.

How many decibels you can notice

According to Pilkington, a 3 dB difference is barely perceptible, 5 dB is clearly noticeable and 10 dB is equivalent to doubling or halving the perceived noise. Kömmerling gives the same reference: 10 dBA less sounds like half, and 20 dBA less like a quarter.

A point or two of difference between two catalogue glass types should not decide a purchase, because it falls within the scatter of the test itself. An improvement of 8 to 10 dB, on the other hand, really does change the room.

The glass in acoustic windows

The glass is the component with the most headroom: with different thicknesses and an acoustic interlayer, acoustic windows gain several decibels over ordinary double glazing without changing the profile. Mass helps, but only a little; it pays more to distribute the thicknesses well and to laminate with an acoustic butyral interlayer.

Mass and thickness: thicker helps, within limits

In Pilkington's generic values (those of EN 12758), 4 mm float glass has an Rw of 29 dB, 8 mm 32 dB and 12 mm 34 dB. Doubling the mass, from 4 to 8 mm, adds 3 dB. Mass is the starting point, not the solution.

The theoretical mass law promises 6 dB for each doubling, according to the CTE guide. The guide itself warns that it only holds between the resonance frequency and the critical or coincidence frequency, and that around the latter the insulation drops. Near that frequency, a pane lets through more sound than its mass would suggest.

Hence a counter-intuitive result: 4/12/4 double glazing has the same Rw as 4 mm single glazing (29 dB) and, against traffic, one point less (25 versus 26 dB of Rw + Ctr). Kömmerling sums it up in one sentence: the air cavity provides thermal properties, not acoustic ones.

Different thicknesses: asymmetric glazing

If both panes are the same, the coincidence effect occurs in the same frequency band in both. A training course on noise protection, published by the Swiss canton of Lucerne and written by an acoustician (W. Stalder, 2004), therefore calls for different thicknesses in the inner and outer panes to reduce coincidence and resonance effects.

Pilkington IGP test reports show the effect in argon-filled double glazing.

Pilkington IGP test reports, double glazing with an argon-filled cavity (source 4). The first row has a 20 mm cavity; the others, 16 mm.

With a 16 mm cavity and one 4 mm pane, going from 6 to 8 mm in the other adds 3 dB, and with a large difference (4 and 10 mm) it reaches 38 dB. When the panes are more alike, 8 and 6 mm, the value drops one point compared with 8 and 4.

Rw does not tell the whole story. In Pilkington's generic values, the 10/4 and 10/6 combinations tie on Rw (35 dB), but 10/6 reaches 32 dB of Rw + Ctr while 10/4 stays at 30. For traffic, that second figure is the one that counts.

The usual practice is to put the thicker pane on the outside, although according to the Swiss source this is not acoustically necessary. The reason is practical: the low-emissivity coating usually goes on a standard 4 mm pane, and it is simpler to make the special, uncoated glass for the outer side.

Laminated glass with an acoustic interlayer

Laminated glass bonds two or more panes with a polyvinyl butyral (PVB) interlayer. If the PVB is acoustic, it damps vibration in the coincidence zone where float glass loses insulation. Saint-Gobain states that its acoustic interlayer almost eliminates that loss around the critical frequency.

Saint-Gobain's data for single glazing illustrate this with three panes of very similar thickness.

Saint-Gobain Glass, Stadip Silence data sheet (April 2023; source 5). The designation 44.1 means two 4 mm panes bonded by a 0.38 mm interlayer.

The manufacturer puts the gain from the acoustic interlayer at around 2 dB over a laminate of the same make-up and around 3 dB over monolithic glass of the same thickness. At 8.38 mm, Stadip Silence 44.1 reaches the same Rw of 37 dB as 15 mm monolithic glass.

The same happens in double glazing. According to Saint-Gobain's data sheet, Climalit Plus 6/12/8 scores Rw 35; 6/12/44.2 with standard laminate, 37; and the same 6/12/44.2 with an acoustic interlayer, 39. Pilkington gives similar figures for its Optiphon: 36 dB at 6.8 mm and 37 dB at 8.8 mm in single glazing, compared with the 32 and 33 dB its table of generic values gives for 6 and 8 mm laminates with ordinary PVB.

Safety laminates with ordinary PVB, whether impact-resistant or burglar-resistant, add only around 1 dB over the equivalent float glass according to those same generic values. The improvement you are looking for in acoustic windows comes from the acoustic interlayer.

Cavity and gas: what counts and what does not

Pilkington notes that, in insulating glass units, the difference in insulation between 6 and 16 mm cavities is small, and that its data apply equally to air- or argon-filled cavities. That is two fewer decisions to worry about.

The manufacturer's tables confirm it: in the unit with 8.8 mm Optiphon and a 10 mm outer pane, widening the cavity from 16 to 20 mm raises Rw from 44 to 46 dB. In the IGP reports, 8-16-4 and 8-20-4 both give 37 dB, and 4-16-10 and 4-20-10 give 38 and 39.

Widening the cavity from 16 to 20 mm therefore yields between 0 and 2 dB. It is not much, but it is one of the few improvements the cavity offers when the profile accepts the thickness.

Heavy gases such as krypton add between 0 and 3 dB depending on the case in the IGP reports, and in practice they are hardly used. Argon, which is the norm, has no effect on acoustics.

Double or triple glazing

Triple glazing insulates better against the cold; against noise, it only gains if its panes are designed for it. Pilkington IGP test reports for triple and double glazing allow them to be compared.

Pilkington IGP test reports (sources 3 and 4). These are different test series, so the comparison is indicative.

A symmetric triple unit of three 4 mm panes (32 dB) falls below a double unit of 8 and 4 mm (37 dB) and one of 4 and 10 mm (38 dB). The triple unit pulls ahead when its panes have different thicknesses, as in the 36 and 37 dB rows, or when it is laminated with an acoustic interlayer, as in the 40 and 42 dB rows.

In Pilkington's table for Optiphon, a double unit with a 10 mm outer pane, a 16 mm cavity and 8.8 mm of Optiphon scores 44 dB, and the brand itself states that Optiphon can also be used to improve a triple unit. If triple glazing is chosen for thermal reasons, as we explain in the article on why a coastal house feels cold in winter, the acoustic glass is designed afterwards, pane by pane, without assuming that the third pane helps.

Acoustics, thermal insulation and security in the same glass

All three properties fit in a single unit. Pilkington states that Optiphon accepts a low-emissivity coating on face 3 of the unit, a solar control coating on face 2 and a self-cleaning coating on face 1. All its Optiphon products achieve impact class 1(B)1 under EN 12600, and versions are available for the EN 356 classes.

Saint-Gobain states that Stadip Silence has the same mechanical strength and the same safety performance as a Stadip or Stadip Protect of identical make-up, with the acoustic improvement on top. And it points out a site detail: the tightness of the glass-to-frame seal and its drainage must be checked.

The RC burglar-resistance class, by contrast, is awarded to the complete window, with its profile, hardware and glass, not to the glass on its own. The details are in the article on RC2 and RC3 classes in burglar-resistant windows.

Profile, seals and opening type

The profile does not determine the insulation of acoustic windows on its own: what counts is that it accepts thick glass and that it closes airtight. Kömmerling puts it this way: sound insulation depends not only on the profile, but also on the thickness and type of glazing and on the air permeability of the window.

Glass capacity and Rw declared by manufacturers

The glass in acoustic windows is thick. An Optiphon 6 mm / 16 mm / 8.8 mm unit adds up to 30.8 mm; a 10 / 16 / 8.8 unit, 34.8 mm; and the 10.8 / 24 / 16.8 unit, 51.6 mm. Such glass only fits a profile that accepts that thickness, and it is also heavy: float glass has a density of 2,500 kg/m³ according to the CTE Catalogue of Construction Elements, so each millimetre weighs 2.5 kg per square metre.

The table brings together the maximum Rw values published by two system manufacturers, along with the maximum glass thickness of each.

Cortizo technical data sheets and Reynaers technical pages (sources 10 and 11). These are maximum laboratory values, with a specific glass and size that the manufacturer does not always state.

Aluprof states in its documentation that the MB-86 system accepts glass up to 67.5 mm, including acoustic and safety glass, but it does not publish Rw values. Nor do they appear in Cortizo's data sheet for the 4600 Plus RPT lift-and-slide door.

The table can be read in three ways. The published maximums for windows range from 42 to 46 dB. Glass capacity does not predict the result: ConceptSystem 59Pa reaches 44 dB with 35 mm, while ConceptSystem 77 stops at 42 with 63 mm. And Reynaers warns in its data sheets that performance depends on size, profile combinations, hardware and infill. A published Rw is the ceiling of a test, not a promise for a specific window.

Seals, hardware and closing pressure

Air slipping between sash and frame carries sound with it, and the insulation depends on that closure. The Swiss course already cited lists what is advisable: windows with rebate seals, because without them the insulation is insufficient; two perimeter rubber seals instead of one, which is generally better; and good closing pressure from the hardware.

The same source adds two useful observations. Merely adjusting the hardware and seals of an old window rarely achieves the desired values. And with periodic inspections, replacement of damaged seals and readjustment of the sashes, a window keeps its insulation over time. A technician with a trained ear can even detect leaks by comparing the sound with the sash open and closed, with an ear close to the closure, although this only gives a rough estimate.

Casement, tilt-and-turn and sliding windows

For acoustic windows, Kömmerling recommends casement or tilt-and-turn systems over the traditional sliding window wherever possible, with an improvement that can reach 10 dB. Between the first two, the Swiss course ranks side-hung casements above tilt-and-turn windows acoustically. And in either, a sash in the tilt position or left ajar stops insulating: with an opening equivalent to 10% of the surface, the theoretical limit is 10 dB.

In large-format acoustic windows, the modern lift-and-slide and minimalist systems from Cortizo and Reynaers publish between 42 and 46 dB in the table above, the same range as those manufacturers' windows, unlike the traditional sliding window. Even so, you need to look at the specific system and its test protocol. For its MB-77HS lift-and-slide door, Aluprof mentions that the shape of the seals and the hardware ensure high air and water tightness, although it gives no acoustic value.

Between lift-and-slide and minimalist, the acoustic criterion is added to the usual ones: sash size, integration into the building, threshold and cost, which we cover in the article on large-format minimalist sliding doors and lift-and-slide doors. The comparison of profile materials is in aluminium or PVC on the coast.

Installation: where acoustic glass pays off or is wasted

In acoustic windows, an installation joint that insulates only as well as the window takes 4 to 6 dB off the assembly, and a small gap or a hairline crack in the junction costs in the order of 10 dB. These are the figures given by ifz Rosenheim, which brings together work by the ift institute.

The same document proposes a rule to avoid this: the insulation of the joint (Rst,w) should exceed the Rw of the window by at least 10 dB, preferably 15. With that margin, the window loses no more than 1 dB because of the joint. For windows with an Rw between 40 and 50 dB, ift Rosenheim has measured joints of over 60 dB, although they require very careful workmanship.

What the joint needs

The installation guide from ASEFAVE, the Spanish association of curtain wall and window manufacturers, states that any air leakage affects sound insulation. Filling materials must have the highest possible sound insulation and retain their cellular structure through cycles of expansion and contraction. Polyurethane foam and self-expanding tapes must match or exceed the attenuation of the window itself.

ifz Rosenheim divides the joint into three levels. The inner level separates the indoor climate from the outdoor one; the middle level is the functional zone for thermal and sound insulation; the outer level protects against rain. A 10 to 20 mm joint filled with foam gives surprisingly high laboratory values, above 50 dB, but the institute refuses to transfer them to the building site without correction: the seals at the inner and outer levels are needed. At corners and junctions, the joint's insulation drops by around 3 dB due to diffraction and resonance effects.

The most overlooked detail: the roller shutter box

If the window has a roller shutter box, the seal between window and box and between window and wall must be continuous. The ASEFAVE guide warns that any discontinuity in that seal can result in a lack of sound insulation. And in renovations with an existing box, the new compact box must not be placed inside the old one, because the old one can act as a sound box and amplify the noise of small air leaks.

What to ask the installer, and how installation fits into the sequence of works, is covered in the 20 questions to ask before signing a contract.

Weak points: roller shutter box and ventilation

The two most common weak points in a home with acoustic windows are the roller shutter box and ventilation with the sashes closed, because in both cases there is a lightweight element or an opening that connects directly to the outside. If they are not dealt with, they limit the return on what has been invested in the glass.

Roller shutter box

The Swiss course already cited states that roller shutter boxes often act as acoustic weak points, that they must be identified as such and that they should be dealt with in a refurbishment. Manufacturers offer dedicated boxes: the data sheet for the Cortizo Isolation PVC box declares up to Rw 44 dB, measured with a 200 × 230 mm box 1,230 mm long.

The box as a thermal weak point is covered in the article on why a coastal house feels cold in winter; here the concern is acoustics: choose a box with a declared Rw, seal it to the window and the wall without discontinuities, and do not fit a new box inside the old one.

Trickle vents and ventilation

With acoustic windows that close well, fresh air has to come in some other way. One solution is frame-mounted trickle vents, which have their own sound attenuation. Siegenia publishes values for its range of window ventilators of up to 41 dB (AEROMAT midi), up to 47 dB (AEROMAT 80), up to 49 dB (AEROMAT 100) and up to 51 dB (AEROMAT 150), expressed as Dn,e,w.

Each trickle vent declares its own index, Dn,e,w, which is not directly comparable with the window's Rw, so it cannot simply be subtracted. An indicative calculation shows the effect: a 1.8 m² window with Rw 40 dB and a trickle vent of Dn,e,w 40 dB ends up at about 32 dB; with one of 50 dB, at about 38 dB.

A mediocre trickle vent can take off as much as a leak. That is why, in a noisy area, the trickle vent is specified by its Dn,e,w just as the glass is by its Rw + Ctr. And with mechanical ventilation or air conditioning, the sashes can stay closed at all times.

Realistic expectations: from the laboratory to the room

The Rw on a data sheet is the result of a laboratory test on a specific window; in the room it usually comes out lower, and the gap depends on the installation, leaks and the rest of the façade. Four things separate it from reality:

• The test itself. Saint-Gobain gives a measurement tolerance of ±2 dB for its values, and Reynaers warns that its performance depends on size, profile combinations, hardware and infill. Change the dimensions or the glass and it is no longer the tested window.
• The joint with the wall. According to ifz Rosenheim, a joint that merely matches the window's Rw reduces the assembly by 4 to 6 dB, and small defects by more. In addition, laboratory values for the joint cannot be transferred to a real building site without correction.
• The room. The ASEFAVE guide states that on-site measurements are carried out in accordance with UNE-EN ISO 16283-3, with the trickle vents closed. They express the difference between the outdoor and indoor levels, corrected for the reverberation time of the receiving room. It is not the same as Rw: it is a different quantity, with a different correction. And they must be carried out by specialist technicians.
• The rest of the façade. With windows taking up between 30 and 40% of the façade, the assembly ends up at most 4 to 5 dB above the window, according to the CTE guide. Replacing only the windows with acoustic windows has a ceiling set by the other elements.

When replacing the windows does not solve the problem

Acoustic windows do not cure noise that does not pass through them. If the sound comes through a lightweight roof, low-mass walls or the structure itself (machinery, vibration, impact noise), upgrading the glass barely touches the path that matters. And if the box, the trickle vents or the joint are already limiting the result, changing the glass without dealing with them means improving the wrong link.

Before deciding, it is worth finding out where the noise is getting in: listen with the sashes closed, locate leaks in the seals and the box and, if you want an objective figure, commission an on-site measurement from an acoustic technician. The reasonable approach is to talk about decibels, with their source, and about noticeable reductions. Promising silence is not.

How to choose acoustic windows for the type of noise

To choose acoustic windows, first identify the noise (traffic, aircraft, voices or music), then look at the corresponding value (Rw + C or Rw + Ctr) and, finally, check that the whole window, not just the glass, has been tested in that configuration. The order matters because each type of noise puts the emphasis in a different place.

A six-step procedure

1. Identify the noise. What the source is, at what times and from which side. This determines the correction term.
2. Choose the reference value. Rw + Ctr for urban traffic, propeller aircraft, distant jets and bass-heavy music; Rw + C for voices and television. If the source varies, Rw + Ctr is the safer figure: in the Pilkington, Saint-Gobain and Reynaers tables, Ctr is always more negative than C.
3. Set an improvement you will notice. According to Pilkington, 3 dB is barely perceptible and 5 dB is clearly noticeable; 10 dB is equivalent to halving the noise. An improvement of less than 3 dB over the current window will hardly be noticed.
4. Choose the glass for the acoustic windows. Two panes of different thicknesses and an acoustic interlayer in at least one. Ask for the test report with Rw (C; Ctr), not just Rw.
5. Check the system. It must accept the total thickness of the glass, have been tested in that configuration and close with continuous seals. If possible, a casement rather than a traditional sliding window.
6. Close off the weak points of the acoustic windows. A roller shutter box with a declared Rw and properly sealed, acoustic trickle vents with their Dn,e,w, and an installation joint with continuous filling and inner and outer seals.

What to prioritise for each type of noise

A scale for reference

Pilkington's documentation ranks configurations from least to most insulating and shows how much is gained against traffic at each step. The first three rows are generic values from EN 12758; the rest are measurements of Optiphon glass, so the comparison between the two groups is indicative.

Pilkington, Optiphon technical documentation (January 2020; source 1). The total thickness is the sum of the panes and the cavity; for laminates it includes the interlayer.

From the first to the fifth row there are 13 dB against traffic: on the scale above, more than halving the noise. But each row calls for a deeper profile and more weight, and the last one (51.6 mm) only fits systems that accept more than 50 mm of glass. Choosing acoustic windows is, to a large extent, a balance between three things: decibels, thickness and weight.

Acoustic windows on the Costa del Sol: which source, which solution

On the Costa del Sol, four sources of outdoor noise often coincide (motorways, aircraft, nightlife and construction work), and each one changes the value you need to look at and the weak points you need to close off. The rest of the article applies anywhere; here it is applied to a coastline with fast roads running parallel to the sea, an international airport and a lively street life.

Motorways

The A-7 and AP-7 run along the Málaga coastline. Pilkington classifies urban traffic under Ctr and motorway traffic above 50 mph (about 80 km/h) under C, so a fast road and a street in a residential development are not read the same way. If a home is exposed to both, acoustic windows should be compared on both Rw + C and Rw + Ctr, taking the lower of the two.

Aircraft

Málaga-Costa del Sol airport predominantly uses the South configuration, according to the noise action plan published by Spain's Ministry of Transport and Aena with 2022 data. In that configuration, take-offs head out to sea and landings fly over various built-up areas, depending on the runway end in use.

Aircraft noise is intermittent and, according to Pilkington, depends on distance: Ctr for propeller aircraft and distant jets, C for nearby jets. For acoustic windows facing aircraft noise, the sensible course is to ask for both values for the glass and to keep the sashes closed when there are flights.

Nightlife

Nightlife brings mainly music, and Pilkington places bass-heavy and disco music on the Ctr side. Bass is the hardest to stop, because all materials insulate low frequencies less well, according to the CTE guide. In acoustic windows facing music, what counts is the mass of the glass, the asymmetry, the acoustic interlayer and, above all, that the sashes stay closed and there are no leaks.

Construction work

Construction noise varies with the machinery, and it is often unclear which will dominate. In such cases, Rw + Ctr is the safer figure. There is no special solution for construction work: the same decisions on glass, closure, joint, box and trickle vents apply.

Windows open in summer

In summer, with the window open, no acoustic glass works: with an opening equivalent to 10% of the surface, the maximum theoretical insulation is 10 dB. If you want to sleep with the sashes closed, air conditioning, mechanical ventilation or acoustic trickle vents need to be planned from the outset.

For villas with large sliding doors opening onto the terrace, the choice of system, the threshold and the installation are covered in the article on minimalist sliding doors and lift-and-slide doors.

How we work at Safe Smart Energy

Safe Smart Energy (SSE) has 22 years of experience, works across the Costa del Sol and also carries out projects throughout Europe, on complex jobs of any scale. We install aluminium systems from Cortizo, Schüco, Aluprof and Reynaers, and we give a 5-year guarantee on installation work.

We often replace windows in homes where the family continues to live. The crew tidies and cleans up after itself, and the house is left in the same condition it was in before work began. We do our best to keep any delays that depend on us to a minimum.

If you would like to see what we do, you can browse our solutions or a selection of published projects.