Acoustic laminated glass for a rooflight, and the inner pane that does the work

Laminated glass is two or more panes bonded permanently to a flexible interlayer. Acoustic laminated glass uses a particular kind of interlayer, tuned to behave like a damper rather than a stiff glue.

Take a standard laminated pane first. Two sheets of glass are laid either side of a clear plastic film, usually polyvinyl butyral, known as PVB. The stack is heated and pressed in an autoclave until the film bonds to both faces and turns optically clear. The result is one pane that cannot be pulled apart by hand. If it breaks, the film holds the fragments in place, which is why laminated glass is the normal choice when glass sits above people's heads.

Now the acoustic version. The interlayer is built differently, usually as a soft core between two firmer skins, so that it stays flexible across the range of temperatures a roof sees in a year. A soft interlayer does something a stiff one cannot. It absorbs some of the vibration passing from one sheet of glass to the other, and turns that movement into a tiny amount of heat.

That is the whole trick. No foam, no mesh, nothing you could see or feel. The pane is thicker than a single sheet of the same glass, and the film is the quiet part.

• Two sheets of glass, each cut and toughened or annealed to suit the job.
• One interlayer, specially formulated so it damps vibration instead of passing it on.
• One finished pane, bonded and clear, handled on site like any other glass of its weight.

A sound wave hitting glass sets the glass shaking. The pane then pushes the air on the far side, and that is the noise you hear in the room. The job of a quiet build is to make the pane a poor loudspeaker.

Every sheet of glass has a natural way of flexing, like a drum skin that wants to ring at certain pitches. When the pitch of the incoming sound lines up with the way the sheet bends, the glass gives in more readily and passes more sound through. Acousticians call this the coincidence effect. It is the reason a plain sheet of glass, however thick, leaks noise in a narrow band of mid and high pitches, which happens to include the hiss of jet engines and the whine of tyres on a wet road.

A stiff, single sheet has nothing to stop that ringing. A laminate with a soft interlayer does. The two sheets cannot move in perfect step, because the film between them flexes and drags, and that shearing action steals energy from the vibration. The pane rings less, and less of the sound reaches the room.

This is also why an ordinary laminate helps a little, and an acoustic laminate helps more. The standard film is fairly rigid, so the two sheets behave as one thicker pane. The acoustic film lets them work against each other. If you want the comparison between plain laminated and toughened glass in a shorter form, we answer it separately in is laminated glass quieter than toughened glass.

In a sealed double-glazed rooflight, the inner pane faces the room. Put the acoustic laminate there and three things go right at once: sound is damped last, safety is handled where it matters, and the film stays out of the weather.

Start with safety, because it is the reason laminated glass is on most overhead units anyway. If the inner pane ever breaks, it has to stay in its frame rather than drop onto a bed or a kitchen worktop. A laminated inner pane does that. The outer pane is then free to be toughened glass, chosen for strength against hail, a stray branch or a dropped tool.

Next, the sound. The two panes of a double-glazed unit are separated by a sealed cavity, and each pane rings in its own way. If both are the same thickness and the same kind of glass, they tend to ring at the same pitch, and the pair can pass a noticeable amount of sound through the same narrow band. Making the two panes different, with the inner one laminated, breaks that shared resonance. The outer toughened sheet takes the first hit, the cavity interrupts the path, and the soft film in the inner laminate damps whatever gets through.

Last, the film itself. PVB does not love the combination of heat, damp and ultraviolet light over many years. Inside the sealed unit, behind the outer pane, it is shielded from most of that. On the inside face of the unit it also stays away from the edge seal, which is the part of any rooflight that ages first.

That is why the quiet spec that comes with every quote names the inner pane specifically. The instruction is never "acoustic glass" in general. It is a description of the inner sheet, its thickness and its interlayer, written for that room. You can read how the written note is laid out on the quiet spec page.

Glass is described in a shorthand that looks like a code, and the code tells you a great deal once you know the order. It is read from the outside in.

A typical build for a quiet rooflight might be written as a toughened outer pane, a sealed cavity, and a laminated inner pane made of two thinner sheets with one interlayer. The thicknesses are given in millimetres, the cavity width is given on its own, and the interlayer is named. A line on a quote might read "six millimetre toughened, sixteen millimetre cavity, 6.8 millimetre acoustic laminate", with the coatings listed after it.

The table below turns the usual items into plain words, and says which ones have an effect on sound.

Two rows deserve a second look. The coating and the spacer belong to the thermal side of a rooflight, which we cover in energy efficient rooflights. They keep heat in and condensation off, and they are chosen alongside the acoustic build rather than in competition with it. The interlayer row is the one people miss. A quote that says only "laminated" has not told you which film is inside.

A thicker pane is heavier, and heavy things are harder to shake. That much is true. The catch is that thickness alone does not solve the ringing problem, and past a point it only adds weight.

Think of a church bell and a biscuit tin. Both are metal, both are rigid, and both ring when struck. Thickening the tin would not stop it ringing, it would only change the pitch. Glass behaves the same way. What stops the ringing is damping, and the soft interlayer supplies that.

So a well-judged acoustic build spends its thickness where it helps. A modest inner laminate with the right film often does more than a much thicker single sheet. It is also lighter, which matters on a roof. Every extra millimetre adds load to the frame, to the kerb and to the structure holding it all up, and on a large lantern that load is real. We weigh it on the survey, along with the span and the roof type. If the structure needs work to carry a heavier unit, that belongs in the quote, not as a surprise on the day.

The other limit is light. Additional layers of glass and film absorb a little more of the daylight passing through. The loss is small with clear acoustic laminate, and far smaller than the loss from a tinted or heavily solar-controlled coating. Where a bedroom skylight needs to be kept cool and quiet both, we handle those two jobs with separate parts of the build, rather than stacking glass until something gives.

A quiet pane set into a leaky frame is a quiet pane that still lets noise in. Sound finds the weakest route, and on a rooflight that route is often the seal, the upstand or the frame rather than the glass.

Think of the glass as one part of a chain. The others are:

1. Seals. Perimeter seals that have hardened, shrunk or never quite closed leave a hairline gap, and a hairline gap passes sound very well. New compression seals, fitted to a frame that closes evenly, close the route.
2. The upstand. On a flat roof the kerb is a box of timber or insulated upstand. If it is hollow and lightly built, it can ring like a drum. An insulated, filled upstand with a properly dressed membrane is much less lively.
3. The frame. Aluminium and timber frames both can be made quiet. The test is how tightly the sash closes and how the glass is seated against its gaskets.
4. The ceiling reveal. Inside the room, the lining around the opening should be solid and sealed at the plasterboard, so there is no route into the loft void.

When we replace a tired unit, it is the combination that makes the difference. A sound acoustic inner pane in a new frame, on a sealed upstand, will behave very differently from the same glass dropped into the old kerb with the old gaps. The detail of how those parts are chosen is covered on the page about what goes into a quiet rooflight specification.

Different sounds need different answers. An acoustic laminate is strongest against the noises that travel through the air at mid and high pitches, and more modest against low rumble.

Rain is a good first example. Drops landing on a single thin sheet of glass make it ring, and on a flat rooflight or a lantern over a kitchen, that can sound like a tin roof. A laminated inner pane with a soft film takes the edge off the drumming because the film damps the ring. The outer pane, the cavity and the way the glass is bedded matter as well, and we describe those in more detail in the guide to reducing rain noise under a flat rooflight.

Traffic and trains sit in the same band as tyre hiss and wheel noise, and respond well to a damped inner pane. Crawley has the A23, the M23 and the Brighton main line running through and around it, so this is often the noise a survey finds. The wider picture is in keeping road and rail noise out through a rooflight.

Aircraft are more complicated. Gatwick is inside the borough, and a loft bedroom under a skylight can hear a departure clearly. Jet noise contains both mid-range hiss and lower rumble. A laminated inner pane helps with the first and does less with the second, because low pitch moves whole structures rather than just shaking a pane. For the loft itself, the answer is the whole build: glass, seals, upstand and the roof structure around it. We set that out in rooflights under the Gatwick flight paths.

Whatever we hear at the survey, the quiet spec records it. It says what the room sounded like, which noise we are addressing, and which parts of the build are there for that reason.

Some rooflights gain little from it, and it is better to know which before you pay. Honest advice on glass sometimes means leaving the laminate out.

A small sun tunnel or a rooflight over a landing, stairwell or utility room has no one sitting under it for hours. A standard build with a laminated inner pane for safety may be all the room asks for. A walk-on rooflight is a different case again, where the glass is a multi-layer structural laminate chosen for load first. You can see how that build is approached on the walk-on rooflight installation page.

Some rooms hear almost nothing overhead: a utility room on a quiet cul-de-sac, say. Others are sensitive, such as a nursery, a bedroom used by a shift worker, or an office where calls are taken. The value of the upgrade rises with how much the room is used at quiet times and how loud the surroundings are. The fuller argument for and against is in is acoustic glass worth it for a rooflight, and it is worth reading before you ask anyone to specify anything.

Finally, a laminate cannot fix a problem of position. A skylight directly under a flight path or beside a road will always see more of the noise than one on the far slope. Sometimes the right advice is to move the opening, or to use two smaller units rather than one large one, and we say so at the survey.

Choosing acoustic glass exempts a rooflight from no rule at all, and the usual rules still apply. The glass is one line in a larger specification.

On a replacement or a new unit, the thermal performance of the whole rooflight has to meet current Building Regulations, in particular Part L. Overhead glazing should be safe if it breaks, and good practice meets that with a laminated inner pane, since Part K itself is written around doors and low glazing. If the room has no other means of ventilation, Part F may come into play, and on some roofs Part B covers the fire performance of the roof covering near a boundary. We install to current Building Regulations and handle the Building Control notification where the work is notifiable. The guide to Building Regulations for rooflights goes through each part in order.

Acoustic build and thermal build are separate choices on the same pane. A laminated inner sheet can sit beside a low-emissivity coating, so a quiet unit can also be a warm one. The two are specified together, because the position of the coating and the order of the panes matter.

The installation carries our 10-year workmanship guarantee, and the glass itself comes with the manufacturer's own warranty on the sealed unit.

A survey for a quiet rooflight starts with listening, not measuring. We stand in the room, beside the existing unit, and listen for the noise you find hard to bear.

That might mean a loft bedroom at the time the first aircraft come in, or a kitchen during a shower. If the weather does not help, we ask you what you hear and when, and we look at the roof, the glass and the seals for the physical explanation. A misted or failing sealed unit is often the first clue, because the original seals have lost their grip. You can read the signs on the page about signs a rooflight has failed.

From there, we write the quiet spec. It names the noise, the glazing build including the inner pane, the seals and the upstand. You get it with the quote, and you can show it to anyone. When you are ready to ask for one, use the quote form on this page, or send a message on WhatsApp. The overview of how we approach all of this is on the home page.

These are the points that come up when someone is choosing between a standard unit and an acoustic build.

Can I tell acoustic laminate from ordinary laminate by looking?

Not reliably. Both are clear and both look like normal glass. Some panes carry a small etched mark in one corner naming the product, and the quote or the delivery note should say which interlayer is inside. If it does not, ask.

Does the laminate make the room darker?

Very slightly. Clear laminate takes a little more light than a single sheet, but far less than a tint or a strong solar coating would. For most rooms the difference is hard to see. If glare or heat is the issue, that is solved separately with the coating or a blind.

Will a blind help instead of new glass?

A blind softens echo inside the room, and it can make a bedroom feel calmer, but it does little to stop sound entering through the glass. We look at that question in whether a blind reduces rain noise.

Is triple glazing the same thing?

No. A third pane adds weight and thermal performance, and it does not automatically damp sound, because three panes of the same kind can still ring together. The answer depends on how the panes are chosen, which is covered in does triple glazing make a rooflight quieter.