Guide · Energy Efficient Rooflights
Acoustic glazing and rain noise
Rain on a rooflight is louder than rain on a tiled roof by a wide margin, and the reason is not the water. It is that glass is a thin, stiff, lightweight membrane stretched over a room with nothing between it and your ears, while a tiled slope has a covering, a ventilated void, insulation and a plasterboard ceiling doing the same job in four stages. The glazing specification can close a good deal of that gap, but only if the decision is made at order.
This page covers what actually reduces rain noise on a rooflight, which ratings on a glazing schedule mean anything for this particular problem, and where the money stops buying quiet. Acoustic specification is one strand of the wider glazing decision set out on the energy efficient rooflights hub.
How loud rain on glass actually gets
Steady moderate rain on an unmodified double glazed flat rooflight puts something in the region of 45 to 55 dB into the room below. Heavy rain and hail push it higher, and hail is a different event entirely, brief and startling rather than sustained.
For scale, a quiet bedroom at night sits near 30 dB and normal conversation is around 60. So moderate rain overhead lands squarely in the range that interrupts sleep, competes with a television and makes a video call awkward. It is generally described as pleasant during the first autumn after the extension is finished and considerably less so by the third.
Why glass drums and a tile does not
A raindrop hitting a surface delivers an impulse. What happens next depends on the mass, the stiffness and the damping of what it hits. Glass is stiff and lightly damped, so the impulse sets the pane ringing and the pane radiates that vibration into the air below as sound. It behaves like a drum skin because acoustically it is one.
A clay or concrete tile is heavier per unit area, sits loose on a batten so energy is lost in the joint, and has a ventilated cavity and an insulated ceiling underneath absorbing what does get through. Each of those is a stage of damping. A rooflight has one stage: the glass, and then the room.
Decibels, and what a difference of ten actually sounds like
Sound is logarithmic and human hearing more so, which makes small looking numbers meaningful. A reduction of 3 dB halves the acoustic energy but is only just detectable. A reduction of 6 dB is clearly noticeable. A reduction of 10 dB is heard as roughly half as loud.
This matters because glazing upgrades trade in increments of two and three. Moving from a standard unit to a well chosen acoustic one might buy 8 to 10 dB against rain, which genuinely changes the room. Moving from a good acoustic unit to an exceptional one might buy another 2, which almost nobody can hear over the sound of the rest of the house.
Rw, Ctr and what a glazing schedule is telling you
Sealed units are rated for airborne sound with a single figure weighted index, Rw, in decibels, usually followed by two correction terms in brackets: C and Ctr. C corrects towards mid and high frequency sources such as speech. Ctr corrects towards low frequency sources such as road traffic.
Rain impact is not the same thing as airborne sound, and there is a separate test method for rain noise on glazing, which is why a high Rw is a useful indicator rather than a guarantee. As a rule the specification choices that lift Rw also damp rain, because both depend on mass, asymmetry and interlayer damping. Ask for the Rw with its corrections rather than a marketing description, and ask whether the manufacturer has rain noise data for the same make-up.
Laminated interlayers, and the acoustic grades
Laminated glass is two panes bonded with a polyvinyl butyral interlayer. Standard PVB adds useful damping on its own, because the interlayer shears slightly and turns vibration into a small amount of heat rather than sound.
Acoustic PVB is a softer, more viscoelastic version of the same interlayer, developed specifically to dissipate more energy. In a rooflight it does two jobs at once: it damps the drumming, and it holds the pane together if it is broken, which is the safety requirement for overhead glazing anyway. That overlap is why laminated outer glass is the single most cost effective acoustic decision available on a rooflight. You were probably specifying laminate for other reasons.
Asymmetric panes, and the coincidence dip
Every pane has a frequency at which the wavelength of sound in air matches the wavelength of bending waves in the glass. At that frequency the pane becomes acoustically transparent and the insulation collapses. That is the coincidence dip, and for a 4mm pane it sits high, around 3,500 Hz, while for a 10mm pane it sits nearer 1,400.
Two identical panes in a sealed unit have the same dip and reinforce each other’s weakness. Two panes of different thickness have dips at different frequencies, so each covers for the other. Specifying 6mm outer against 4mm inner, or 8.8mm laminated outer against 6mm inner, costs very little and is one of the two decisions that actually moves the number.
Cavity width, and the mass spring mass problem
A double glazed unit is two masses separated by a spring, and the spring is the trapped gas. That system has a resonant frequency, and below it the unit performs worse than a single sheet of glass of the same total weight. Narrow cavities push the resonance up into the range you care about.
A 16mm argon filled cavity, which is the thermal optimum in a vertical unit, resonates around 200 Hz. Widening the cavity lowers the resonance and improves acoustic performance, which pulls against the thermal choice discussed on the hub. Beyond about 20mm in a near horizontal rooflight the convection penalty starts to bite. For most jobs this is a compromise rather than a solved problem, and the interlayer is the better lever.
| Make-up | Rain noise behaviour | Thermal side effect |
|---|---|---|
| 4mm / 16 argon / 4mm | Baseline, drums freely | Thermal optimum |
| 6mm / 16 argon / 4mm | Noticeably calmer, dips separated | Negligible |
| 6.8mm laminated / 16 argon / 4mm | Clearly quieter, standard interlayer | Slight light loss |
| 8.8mm acoustic laminated / 16 argon / 6mm | The practical best on a domestic unit | Heavier, may need frame uplift |
Triple glazing, and where the third pane helps
A third pane adds mass and adds a second cavity, and it does reduce rain noise, mostly because the outer pane is usually thicker in a triple make-up. It is rarely the efficient route to quiet, though, because the money buys thermal performance you may not need and weight you have to carry. The full argument for and against the third pane, including the weight and lead time consequences, has its own page and is summarised there rather than here.
If quiet is the goal and the budget is fixed, an asymmetric double with an acoustic laminated outer pane beats a symmetric triple almost every time. If the room is a winter occupied bedroom and both matter, a triple with an acoustic laminated outer is the specification worth pricing.
Rain noise against the noise that was already there
Rooflights are often blamed for noise they merely reveal. In a room with an existing background of traffic or aircraft, adding glass overhead can lift the total, and the perceived change depends on what dominates. North Essex sits under approach and departure routes for Stansted, the A12 corridor runs the length of the county, and rooms near either already carry a background in the high thirties.
The practical consequence is that a specification chosen only for rain may underperform on the day to day. Where aircraft or traffic noise is a live issue, the Ctr corrected figure is the one to compare, since both sources sit low in the spectrum, and the same asymmetric laminated make-up serves both problems.
The frame, the kerb and flanking sound
Glass is not the only route in. Sound also travels through the frame, down the kerb and into the ceiling structure, and it leaks straight through any air gap around the opening. An air path is an acoustic path: the same unsealed junction that loses heat also carries noise.
So the airtightness detail at the perimeter, taped from the frame onto the ceiling line before the reveal is boarded, is an acoustic measure as well as a thermal one. On a lantern the ridge and hip junctions are the equivalent weak points. Where a specification has been upgraded and the room is still noisy, the flanking route is usually the reason, and it is far cheaper to close during the install.
Domed and polycarbonate units, and why they are the loudest thing up there
The twin skin polycarbonate domes fitted across Essex through the seventies and eighties are the worst performers on a roof by a distance. Polycarbonate is very light, so it has almost no mass to resist impact, and the two skins are thin, closely spaced and identical, which puts the resonance and the coincidence dip in exactly the wrong places.
People who have lived under one and then moved to a modern sealed unit describe the change as larger than any other single improvement. Swapping a crazed dome for a laminated glass unit in the existing kerb is often a single day’s work, and it is covered on the skylight replacement page.
The room underneath, and reverberation
Half of how loud something feels is decided after the sound is in the room. A modern extension with a resin floor, plastered walls, a large glazed rear elevation and minimal soft furnishing is acoustically hard, so rain energy bounces rather than being absorbed and the space rings.
The same rooflight over a carpeted bedroom with curtains and a bed in it reads as noticeably quieter with no change to the glass at all. Where the room is going to be hard surfaced, that is an argument for pushing the glazing specification up a grade, and an argument for a rug and some soft furnishing that has nothing to do with the roof.
The rooms that notice, and the rooms that never will
Position drives the decision more than product does. Directly over a bed, an acoustic make-up is worth having and blackout is usually wanted at the same time. Over a home office or a study, speech intelligibility during a call is the test, and the mid frequency behaviour matters. Over a kitchen island, a hall or a utility space, standard glazing is genuinely fine and the upgrade money is better spent elsewhere.
Height helps too. A rooflight three metres above a floor delivers less to the ear than the same unit at two and a half, and a splayed reveal spreads and slightly diffuses the sound as well as the light. None of these effects is large on its own. Together they decide whether a room is comfortable in a downpour.
External shading, and the noise it adds or removes
An external awning above the glass changes the acoustic picture and not always for the better. A taut fabric screen is itself a drum, and rain landing on it can be more audible than rain on the glass, particularly on a still night. A slatted external shutter breaks the drops up before they arrive and generally reduces the impact.
Internal blinds have very little effect on rain noise, because the energy has already entered the room by the time it reaches them. The full comparison between the two positions is set out on the hub and its shading pages, but for acoustics the short version is that shutters help, awnings are a coin toss, and blinds are neutral.
What to put on the order
An acoustic specification is contractual only if it is written before anything is made. Name the pane thicknesses individually, outer to inner, rather than accepting a total. State which pane is laminated and whether the interlayer is standard or acoustic grade. Give the cavity width and the gas fill. Ask for the Rw with its C and Ctr corrections, and for rain noise data on the same make-up if the manufacturer holds it.
Then confirm the frame will carry the weight, because an 8.8mm laminated outer pane adds real mass and some frames need uprating for it. On a large unit that can affect the structural opening, which is a survey question rather than an order one.
Acoustic performance is a decision made at order and it cannot be added to a unit afterwards.
What a realistic result feels like
An honest expectation helps. A well specified acoustic rooflight does not make rain inaudible, and any supplier suggesting otherwise is selling. What it does is move the sound from an insistent drumming that pulls your attention upward to a soft background you notice only when you think about it.
In practice that is the difference between a room that gets used on a wet Sunday and one where somebody quietly turns the television up. The upgrade is a modest share of the cost of the unit and a very small share of the cost of the extension, and it is one of the few specification choices where the improvement is obvious within days of the first heavy rain. Indicative figures for the different make-ups sit on the costs page.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →Condensation, ventilation and rooflights
Read the guide →Triple glazing in a rooflight and when it pays
Read the guide →Warm-edge spacers and where condensation starts
Read the guide →Low-emissivity coatings and argon fills
Read the guide →
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