Secondary Glazing For Noise Reduction

Living with noise can affect both mental and physical health and for anyone living near a busy road, or near other noisy neighbours, it becomes very invasive.

Windows are a weak spot where noise can easily transfer, and the truth is that nothing can stop the glass from transferring sound. Apart from bricking up a window, which isn’t an option. Fortunately, there are options to improve your living space and find some sanctuary in your own home.

Secondary glazing is one of the most effective solutions for reducing noise that can outperform most standard double-glazed windows.

Everest are experts in noise reduction for windows and can help you to regain your peace of mind.

How does sound travel through windows?

There are two main ways that noise travels:

• Through the air
• Through other objects

As noted above, soundwaves travel through the air. If an object is in the path of a wave, the object will absorb and transmit the sound. The sound will pass through and continue to travel through the air on the other side. The reverberation of sound in a room happens when a large empty space allows soundwaves to bounce around the space and the 'echo' created can make the perceived sound louder.

Sound leakage occurs from soundwaves finding their way through gaps in solid objects such as walls.
To find the right soundproofing solution, it's important to be aware of the different methods that can be used to block out different types of noise.

Sound waves travel through both the air and other objects. They move faster and further through the air and dense objects slow and dissipate the reverberation of the wave. Which is why sound reduces by distance and through large objects.

When a soundwave reaches a window, it will pass through the glass into a room. Depending on how much the window can dissipate the soundwave is how much noise you will hear. Single-glazed windows offer very little resistance to this process. The glass is thin, there's nothing to interrupt the vibration, and any gaps around the frame let sound through directly.

It’s worth noting that even small gaps will leak sound and trickle vents can also transmit noise. Sound is like water and will find any tiny gap to get through.

Read more: Acoustic Glass Explained & How It Works

Why secondary glazing is so effective at reducing noise

The air gap is what makes the difference

The single biggest factor in acoustic performance is the distance between the two panes of glass. Standard double glazing has a cavity of around 28mm to 32mm. Secondary glazing creates a much larger cavity between the existing window and the secondary unit, typically between 100mm and 200mm, depending on the depth of the window reveal.

The most significant way to reduce sound is to disrupt the soundwave and that happens where two panes of glass vibrate at different frequencies, explained above.

At a separation of 100mm or more, the inner and outer panes act as independent barriers rather than as a single resonating unit. Sound energy has to cross the outer glass, travel through a large volume of still air, and then set the inner glass vibrating all over again. Each stage of that journey reduces the sound wave energy.

A standard double glazed window will reduce sound by up to 33dB. According to Historic England, a secondary unit with an air gap of 100mm or more can achieve sound insulation of up to 45dB.

For context, a reduction of 10dB is perceived by most people as roughly halving the loudness of a sound. A well-specified secondary glazing system can make a busy road sound like a quiet residential street.

Breaking the sound wave

The larger the cavity, the more energy a sound wave loses crossing it. But the air gap alone isn't the whole story.

Sound also loses energy each time it passes through a physical barrier. Secondary glazing creates two separate barriers separated by a substantial air mass, and that combination is what makes it so effective against the types of noise that cause most problems at home.

Traffic noise, aircraft noise, and railway noise all sit at frequencies where this double-barrier effect works particularly well. The low rumble of heavy goods vehicles, the higher pitch of aircraft engines, and the rhythmic noise of rail traffic are all significantly reduced by a properly specified secondary glazing system.

Airtightness matters

Glass and air gap account for most of the acoustic performance in a well-designed system. But any gap in the seals around the frame will let sound straight through, regardless of how good the glazing is.

Secondary glazing is only as effective as its least well-sealed point. A poorly fitted unit with gaps between the frame and the surrounding reveal will underperform significantly, not because the glazing itself is inadequate, but because sound is bypassing it entirely.

A well-designed secondary glazing system uses compression seals around the full perimeter of the unit. These create a tight, consistent barrier with no air path for sound to follow. The frame must also be accurately sized to the individual window opening. A standard unit cut to approximate dimensions and packed out to fill the gap is not the same thing, and the acoustic performance will reflect that.

Why bespoke secondary glazing performs better than DIY systems

There's a wide range of secondary glazing products available, from professional bespoke installations through to magnetic acrylic sheets and plastic film kits you can fit yourself in an afternoon.

The difference in acoustic performance between the two ends of that spectrum is significant.

Cheap DIY systems typically use thin acrylic or plastic sheet rather than glass. Acrylic is lighter than glass and resonates more easily, which means it does less to interrupt sound transmission. The fixings are designed for easy removal, which also means the seals are imperfect. And the cavity depth is often determined by whatever depth the magnetic strip or frame allows, which is rarely optimised for acoustic performance. These systems can reduce draughts and provide some thermal benefit. As serious acoustic solutions, they are limited.

Bespoke secondary glazing is manufactured to the precise dimensions of each individual window opening. The frame is fitted accurately into the reveal with compression seals around the entire perimeter. The glass specification, thickness, and cavity depth are all choices made with the specific requirements of the property in mind. The cavity can be set at 100mm or greater, where the acoustic benefit is most pronounced. The system is robust, designed for daily use, and built to last.

For noise reduction that makes a difference, the quality of the installation matters as much as the glazing specification itself. 

What types of noise can secondary glazing reduce?

Traffic noise

Road traffic is the most common source of noise complaints in residential properties in England. The low-frequency rumble of heavy vehicles, the higher pitch of car engines and horns, the continuous background drone of a busy road: secondary glazing is well suited to all of these. A cavity of 100mm or more, combined with well-sealed frames, can make a significant and noticeable difference to traffic noise levels in a room.

Aircraft noise

Properties near airports or under established flight paths experience a particular kind of noise: intermittent, high-intensity, and often at frequencies that carry through walls and windows easily. Secondary glazing with a deep cavity and acoustic laminate glass is one of the most effective solutions for aircraft noise in a residential setting. The larger the cavity, within the constraints of the reveal depth, the better.

Railway noise

Railway noise combines low-frequency vibration with higher-frequency mechanical sound. Both are attenuated by secondary glazing, though the vibration element is harder to address through glazing alone. Secondary glazing will reduce audible noise significantly; the low-frequency structural vibration that sometimes accompanies heavy rail traffic requires additional measures beyond the window.

Urban and neighbourhood noise

General urban noise, voices, music from nearby properties, the sounds of a town centre, responds well to secondary glazing because it covers a broad range of frequencies. A well-designed system installed in a property in a dense urban area or above a busy commercial street can transform the acoustic environment of a room.

Secondary glazing vs double glazing for noise reduction

Many homeowners assume that replacing their windows with double glazing will solve a noise problem. In practice, the acoustic performance of standard double glazing is limited by the size of its internal cavity.
A standard double-glazed unit has a gap of around 16mm to 24mm between the panes. At that distance, the two panes of glass are close enough to vibrate in sympathy with each other, which limits the decoupling effect.

Secondary glazing's advantage is the cavity it creates between the secondary unit and the existing window. At 100mm or more, that gap is five to ten times larger than the cavity inside a standard double-glazed unit. The acoustic performance that follows from that difference is significant.

Is secondary glazing right for your home?

Secondary glazing for noise reduction is worth considering if:

• Your property is on or near a busy road and road noise affects day-to-day comfort
• You live near an airport, on an established flight path, or close to a railway line
• You're in a town centre or densely populated urban area where general noise is a persistent presence
• Your property is listed or in a conservation area, where replacing windows is restricted or requires consent
• You want better acoustic performance without altering the external appearance of your home
• Your existing windows are original or period features you want to preserve

If you're unsure which type of noise you're dealing with or what level of reduction would make a practical difference, our guidance on noise reduction windows covers the principles in more detail.

Frequently asked questions

Does secondary glazing reduce traffic noise?

Yes. Secondary glazing is particularly effective against traffic noise because of the large air cavity it creates between the existing window and the secondary unit. A cavity of 100mm or more decouples the two panes of glass, which significantly reduces the vibration and sound energy that passes through. Precise seals around the frame prevent noise from bypassing the glazing.

Is secondary glazing better than double glazing for noise reduction?

In many cases, yes. Standard double-glazed units have a cavity of around 16mm to 28mm between the panes, which limits their acoustic performance. Secondary glazing creates a cavity of 100mm or more, which is far more effective at decoupling the panes and absorbing sound energy. For noise reduction specifically, secondary glazing typically outperforms standard double glazing.

How much noise can secondary glazing block?

According to Historic England's published guidance, a secondary unit with an air gap of 100mm or more can achieve sound insulation of up to 45dB. A reduction of 10dB is perceived by most people as roughly halving the loudness of a sound. The actual level achieved depends on the quality of the installation, the cavity depth, the glass specification, and the airtightness of the seals.

Does the air gap affect noise reduction?

Yes, it's the single most important factor. A larger gap between the existing window and the secondary unit reduces resonance between the two panes and gives sound energy more distance to dissipate. The benefit increases up to a gap of around 200mm, beyond which additional improvements are minimal. A gap of at least 100mm is generally recommended for meaningful acoustic performance.

Is glass better than acrylic for soundproofing?

Yes. Glass is heavier and denser than acrylic, which makes it more resistant to vibration and a more effective acoustic barrier. Cheap secondary glazing systems often use acrylic or plastic sheet, which resonates more easily and provides less sound attenuation. For a property where noise reduction is a genuine priority, glass is the correct choice.

Can secondary glazing help with aircraft noise?

Yes. Aircraft noise sits at frequencies where secondary glazing, particularly with a deep cavity and acoustic laminate glass, performs well. Acoustic laminate glass has a special interlayer that targets higher-frequency sound, which is characteristic of aircraft engines.