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Answered: VELUX Window Installation

Are roof windows noisy in rain?

The short answer

They make more noise than a wall window and less than people fear, and almost all of the difference between a quiet one and a loud one is decided by the glass specification at the moment the order goes in. A standard double-glazed roof window in steady rain sits somewhere around 45 to 50 decibels measured in the room below, which is conversational level. Change the inner pane to a laminated one and that drops by roughly 5 to 10 decibels, which the ear reads as a halving of loudness. Nothing done after the window is in the roof gets close to that, which is why this is worth ten minutes of thought before the order rather than a year of irritation afterwards. Glass options across the ranges are on the VELUX window installation page.

Why a raindrop overhead sounds different from a raindrop on a wall

Two things are working against a roof window and neither of them is about quality. The first is the angle. A vertical window is hit by rain at a glancing angle, and only the horizontal component of the drop’s momentum transfers into the glass. A roof window at 30 degrees takes a much larger share of that momentum directly into the pane, and one lying nearly flat takes almost all of it.

The second is that you are underneath it. Sound radiates from both faces of a vibrating pane, and the room side faces straight down at the bed. There is no ceiling between the glass and the person listening, no cavity, no insulation, and usually a hard plastered reveal on four sides funnelling the sound into the room. A pane of glass struck by a drop behaves like a small, stiff drum skin: the impact excites it, it flexes, and it radiates that energy as an audible click. Multiply by a few thousand drops a second in heavy rain and it becomes a continuous sound rather than a series of taps.

The laminated inner pane, and the interlayer that absorbs the impact

Laminated glass is two sheets bonded either side of a polyvinyl butyral interlayer, usually 0.38mm or 0.76mm thick. It exists primarily for safety, because it holds together when broken, and safety is why it is already required as the inner pane wherever there is habitable space, a stairwell or a bath underneath a rooflight.

The acoustic benefit is a side effect and it is a substantial one. The interlayer is viscoelastic, meaning it converts bending energy into a small amount of heat rather than passing it along, and it damps the pane’s natural resonance rather than simply adding mass. In practice a 6.8mm laminated inner pane in place of a 4mm toughened one takes 5 to 10 decibels off the drumming in heavy rain, and it also broadens the frequencies at which the improvement holds. Because it is often required anyway, this is frequently the cheapest acoustic upgrade available on any glazed opening in the house. The specification detail sits with acoustic glazing and rain noise.

On most loft rooms the quietest available glass is the same glass the regulations already point at.

Decibel figures, and what a ten point drop sounds like

Numbers help here because the perception is not linear. A reduction of 3 decibels is just noticeable. Ten decibels is heard as roughly half as loud. So the difference between the two glass builds below is not marginal, it is the difference between rain you sleep through and rain that wakes you.

Glass build Approximate rain noise in the room How it reads at 3am
4mm toughened outer, 4mm inner, standard double Around 48 to 52 dB in heavy rain Clearly audible, intrusive in a bedroom
4mm toughened outer, 6.8mm laminated inner Around 40 to 45 dB Present but easy to sleep through
6mm toughened outer, 8.8mm acoustic laminated inner Around 36 to 40 dB Noticeable only in the heaviest downpour
Triple glazed with a laminated inner Around 36 to 42 dB Similar to the above, chosen mainly for the U-value
Polycarbonate dome on an old flat roof Around 55 to 65 dB Loud enough to stop a conversation

Those are indicative rather than certified figures and they move with pitch, exposure and the room below. What they establish reliably is the ranking, which does not change.

Two panes of the same thickness share a weakness

There is a detail in glass acoustics worth understanding because it explains why a thicker unit is not automatically a quieter one. Every pane has a coincidence frequency at which the bending waves travelling through the glass match the wavelength of sound in air, and at that frequency the pane transmits sound far more freely than it does either side of it. It shows up as a dip in the performance curve.

Two panes of identical thickness in the same unit share the same coincidence frequency, so their dips line up and reinforce each other. Make them different thicknesses, say 4mm outside and 6.8mm inside, and the dips fall at different frequencies, so each pane covers for the other. This is why an asymmetric build outperforms a symmetric one of the same total weight, and it is why the standard acoustic recommendation is unequal panes rather than simply thicker ones. The cavity width contributes as well, with wider cavities generally better, though a roof window’s cavity is largely fixed by the frame section.

Pitch, and why a Forest of Dean roof is quieter than a vale lean-to

Angle matters, and it works in the direction that suits the steeper roofs. At 45 degrees a raindrop strikes the glass at a slant and a good part of its energy goes into sliding down the pane rather than into flexing it. At 15 or 20 degrees the impact is much closer to perpendicular and nearly all of it goes into the glass.

Across this county that produces a real difference between one house and another. The Forest of Dean terraces at Coleford, Cinderford and Mitcheldean sit at 40 to 45 degrees on Welsh slate, and although the Forest is the wettest and windiest part of the county, the geometry works in its favour. A Victorian rear addition in a Gloucester or Tewkesbury terrace has a lean-to at 22 to 30 degrees over the kitchen, which takes the impact more squarely. And a shallow unit on a flat roof link takes it fully. Steeper roofs also shed faster, so water does not build a film on the glass that adds its own hiss. None of this changes the specification advice, but it does explain why two neighbours with the same window report different things.

The reveal and the room are the other half of it

The glass sets how much sound enters the room. What the room does with it afterwards is a separate question and it is one people forget when they specify.

A loft conversion is typically the hardest space in the house: plastered sloping ceilings, a wooden or laminate floor, minimal soft furnishing, and a small volume with parallel surfaces. Sound arriving from above bounces between the floor and the ceiling with very little absorbed. The same window over a carpeted room with curtains, a bed and a wardrobe sounds quieter, and the difference is easily worth several decibels. The reveal contributes too. A splayed, plastered reveal spreads the sound into the room, and a fabric blind fitted in side channels within that reveal absorbs a measurable part of it before it gets there. A blind is not an acoustic product and should not be sold as one, but a closed blackout blind in channels does take the sharp edge off, and where blinds are being specified anyway that is a free benefit. Blind specification is on the roof window blinds and when to specify them page.

Hail, and the sound nobody specifies for

Rain is a steady, broadband sound that people acclimatise to within a couple of nights. Hail is not. A hailstone of five or ten millimetres carries orders of magnitude more energy than a raindrop and arrives as a hard, sharp impulse rather than a soft one, and there is no glass build that makes it quiet.

Gloucestershire gets it several times a winter, usually with a squall coming up the Severn estuary on a south-westerly, and it is over in a few minutes. What laminated glass does provide is confidence rather than silence: the interlayer means the pane holds together under an impact that would otherwise be worrying above a bed, and toughened outer panes handle hail impact well as a matter of course. It is worth knowing this in advance so that the first hailstorm after an installation is not interpreted as a fault in the window.

What to put on the order when quiet matters

Four things, in order of how much difference they make. A laminated inner pane, ideally 6.8mm or an acoustic laminate at 8.8mm, which is the single biggest lever and is often required for safety in any case. An asymmetric build, so the two panes are different thicknesses. A pitch as steep as the roof allows, which is rarely a choice but is worth knowing about. And the room below considered: a carpet, a blind in channels and something soft on a wall are cheap compared to reglazing.

The two situations where this is worth pressing on hardest in this county are a bedroom in a loft conversion, where the glass is directly over the pillow, and any room under a shallow rear addition roof on the vale terraces, where the low pitch takes the impact squarely. On both, the laminated inner pane is the answer and it is one line on the order form. The wider glass discussion, including the U-values and solar control coatings that get chosen at the same moment, is on the energy efficient rooflights page, and it is worth settling all of it in one go rather than one specification at a time.

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