Secondary Glazing for Flats & Apartments: Soundproofing Above Shops and Tube Lines

For owners of London flats and apartments, noise rarely arrives from one direction. It may rise from a shop below, pass through an open sash window from a busy road, reverberate from a neighbouring flat, or arrive as low-frequency rumble from a nearby Tube or railway line.

This is more than an inconvenience. Persistent urban noise can interrupt sleep, reduce concentration and erode the sense of refuge that a home should provide. In mansion blocks, period conversions and apartments above commercial premises, the challenge is compounded by leasehold restrictions and conservation requirements.

The practical question is therefore not simply, “How do I soundproof my flat?” It is:

For many London properties, the answer is carefully specified secondary glazing.

Why Flats and Apartments Have Distinctive Noise Challenges

A detached house may have several layers of external wall and garden between its occupants and the street. A flat often has a much shorter acoustic path. Your bedroom may be directly above a restaurant, beside a communal entrance or only metres from a Tube viaduct.

1.1 Street-level and commercial noise

Flats above shops are exposed to a particularly disruptive pattern of sound, including:

• Early-morning deliveries and roller shutters
• Commercial refrigeration and extraction equipment
• Pedestrian conversation and late-night customers
• Taxis, buses and motorcycles stopping outside
• Waste collection and bottle disposal
• Shop alarms and security doors

These sounds are often intermittent rather than continuous. That makes them especially effective at disturbing sleep: the brain remains alert because it cannot predict the next interruption.

1.2 Tube, rail and road rumble

Tube and railway noise presents a more complex acoustic problem. The low-frequency rumble of a train can travel through the structure as vibration, while airborne noise enters through windows, vents and other weak points.

Secondary glazing is most effective where the window is a primary transmission route for airborne sound. It can substantially reduce the audible noise entering through the glazing, although severe structure-borne vibration may also require assessment of floors, walls and ceilings.

1.3 Neighbouring flats and shared structures

Secondary glazing is not a universal treatment for party-wall noise. Voices, music and impact noise from neighbouring apartments may travel through masonry, floors, ceilings or service risers.

However, many flats have a mixed acoustic problem: neighbour noise inside the building and traffic or commercial noise outside. Treating the windows removes one of the most significant external routes, allowing other measures to be assessed more accurately.

How Acoustic Secondary Glazing Works

Secondary glazing adds an independent internal window behind the existing primary window. The original window remains in place, while the new acoustic unit creates a second barrier.

The performance depends on three interlocking principles: mass, decoupling and airtightness.

2.1 Mass: 10.8mm acoustic laminate glass

Our system uses 10.8mm acoustic laminate glass, incorporating a viscoelastic PVB interlayer. In simple terms, the interlayer behaves like a microscopic damping layer between the glass sheets, absorbing part of the sound energy rather than allowing the pane to vibrate freely.

The additional mass is particularly valuable against lower-frequency urban sounds, including buses, heavy vehicles and rail movement.

2.2 Decoupling: a 100–150mm air gap

The existing window and secondary pane are separated by a precision 100–150mm air gap. This cavity acts as an acoustic buffer.

A narrow gap can allow the two panes to behave too much like one coupled system. A larger, correctly designed cavity improves separation between the barriers and reduces the amount of sound energy transferred into the room.

This is a logarithmic system: a small change in decibel level can represent a substantial change in acoustic energy. A reduction of 10dB is commonly perceived as approximately half as loud, although the result depends on frequency and listening conditions.

2.3 Airtightness: twin-compression EPDM seals

Even excellent glass and a wide cavity will underperform if sound can pass around the frame. Our units therefore use twin-compression EPDM seals to create a continuous, airtight acoustic envelope.

This matters because small gaps behave like open doors acoustically. The seals are compressed against the frame on both sides, helping to prevent flanking paths for traffic hiss, sirens, voices and commercial noise.

Expected Noise Reduction

With the correct specification and installation, our acoustic secondary glazing can reduce external city noise by up to 54dB. This is a maximum performance figure, not a guarantee that every room will experience the same result.

Actual performance depends on:

• The existing window’s condition
• The size and shape of the opening
• The available reveal depth
• The frequency profile of the noise
• Gaps around the primary frame, walls or vents
• Whether noise is airborne or structure-borne

The following table provides a useful comparison.

Glazing solution	Indicative acoustic performance	Typical suitability
Original single glazing	Limited reduction	Quiet streets and low-noise elevations
Standard double glazing	Approximately 28–32dB	General thermal improvement and moderate noise
Standard laminated secondary glazing	Approximately 35–38dB	Light to moderate urban noise
10.8mm acoustic laminate with 100–150mm gap	Up to 54dB	Traffic, sirens, Tube, rail and commercial noise

The key conclusion is clear: the glazing specification and cavity depth matter more than simply adding another pane.

Why Secondary Glazing Suits London Flats

4.1 No alteration to the building façade

Because the system is installed internally, the external appearance of the mansion block, terrace or conversion remains unchanged. Original sash proportions, glazing bars and architectural detailing remain visible from the street.

This makes secondary glazing particularly appropriate where replacement windows are prohibited, impractical or aesthetically unacceptable.

4.2 A conservation-conscious approach

Historic England describes reversible internal secondary glazing as a lower-impact method of improving historic windows, provided the original fabric is not damaged. However, listed building and conservation-area policy varies between boroughs.

For example, local authorities may have different expectations regarding Listed Building Consent, frame design, ventilation and fixing methods. Always confirm the position with your local planning or conservation team before work begins.

Our dedicated heritage and listed-building secondary glazing service has achieved a 100% conservation officer approval rate across submitted projects. This is a record of our applications and workmanship: not a substitute for checking the requirements of your particular building.

4.3 Leasehold and freeholder considerations

Planning consent and leasehold consent are separate matters. Even where planning permission or Listed Building Consent is not required, your lease may require written approval for alterations.

Before commissioning work:

1. Check the lease for clauses concerning internal alterations and windows.
2. Ask the freeholder or managing agent whether a licence for alterations is required.
3. Provide a drawing and specification showing that the installation is internal, reversible and non-structural.
4. Obtain written confirmation before work begins.
5. Keep the consent, drawings and warranty documents with your property records.

A specialist installer should be familiar with this documentation and able to explain how the proposed system interacts with the original window.

Cost Per Window for Flats

The cost of secondary glazing for flats and apartments in London depends on window size, access, finish, opening style and acoustic specification.

As a current guide, specialist acoustic secondary glazing is typically priced from approximately £450 to £1,600 per window. Larger openings, bay windows, unusual shapes, restricted access and premium finishes may sit toward the upper end of this range.

Project factor	Likely cost implication
Small, accessible window	Lower installation cost
Large sash or casement window	More glass and bespoke framing
Bay or curved opening	Additional surveying and templating
10.8mm acoustic laminate	Higher specification than standard glazing
100–150mm cavity	May require deeper bespoke framing
Listed or managed block	Additional drawings and approval support

A flat does not necessarily require every window to be treated. A professional acoustic survey can identify the dominant elevations and rooms. Many owners begin with bedrooms facing the street, a living room above a late-night premises or the elevation closest to a Tube or rail line.

For background on specifications and pricing, consult our London noise reduction guide and cost and pricing guide.

Installation with Minimal Disruption

For apartment buildings, installation etiquette matters. Neighbours may be working from home, managing young children or living under strict building-management rules.

A carefully planned installation should include:

• A pre-installation survey and access plan
• Confirmation of working hours with the building manager
• Protection for floors, furniture and shared areas
• Internal installation without scaffolding or external access
• Controlled drilling and dust management
• Removal of packaging and waste at the end of each visit
• Clear communication with neighbours where access or noise is unavoidable

Because the secondary units are fitted from inside the flat, disruption is generally limited to the installation area. The original windows are retained, and there is no need to remove the façade or alter the external elevation.

The Verdict for Flats Above Shops and Tube Lines

Secondary glazing is not a generic “extra pane” solution. For demanding London flats, the result depends on a complete acoustic system:

• 10.8mm acoustic laminate glass for mass and damping
• 100–150mm air gap for acoustic decoupling
• Twin-compression EPDM seals for airtight performance
• Bespoke internal frames that respect the existing architecture
• A survey that distinguishes airborne noise from structure-borne vibration

When these elements are correctly combined, owners can transform a flat from a room that reacts to every delivery, siren and train into a more controlled, private sanctuary.

Our company is London-only and dedicated exclusively to acoustic secondary glazing. Every installation is supported by a 25-year warranty, and our work is designed to preserve the character and value of period apartments rather than compromise it.

Actionable Guidance: Your Next Steps

If you own a mansion-block flat, period conversion or apartment above shops, begin with a structured assessment:

• Identify which rooms and elevations are most affected.
• Record when the noise occurs and whether it is rumble, voices, impact or high-frequency traffic sound.
• Check the window reveal depth to determine whether a 100–150mm cavity is feasible.
• Review your lease and contact the freeholder or managing agent.
• Confirm conservation requirements with your borough.
• Request a written acoustic specification rather than a generic glazing quote.
• Ask about warranty coverage, installation methods and post-installation support.

For a measured recommendation, arrange a free London secondary glazing survey. We will assess your existing windows, identify the principal noise paths and provide a bespoke specification for a quieter, more predictable interior.

A peaceful flat is not an indulgence. In one of the world’s most active cities, it is an intelligent investment in sleep, concentration, wellbeing and the long-term enjoyment of your home.

Frequently Asked Questions

Is secondary glazing suitable for a flat above a shop?

Yes. It can reduce airborne noise from deliveries, shutters, customers, traffic and commercial premises below when the window is a significant transmission route.

Can secondary glazing reduce Tube noise?

It can substantially reduce airborne Tube and railway noise entering through windows. If vibration is travelling through the building structure, a wider acoustic assessment may also be needed.

Does secondary glazing change the external appearance of a mansion block?

No. The system is installed on the room side of the existing window, so the external façade and original window remain unchanged.

Do leaseholders need permission?

Possibly. Lease requirements vary, and many leases require freeholder or managing-agent approval for alterations. Check before arranging installation.

How long is the warranty?

Our acoustic secondary glazing installations are backed by a 25-year warranty covering the specified system, subject to the applicable warranty terms.