Secondary Glazing for Casement Windows: A Complete Fitting & Soundproofing Guide

Original casement windows are among the defining features of London’s period homes. Their timber profiles, handmade details and historic proportions contribute significantly to the character of a property. Yet the same windows can leave bedrooms, studies and living rooms exposed to traffic, sirens, aircraft, construction and late-night activity.

For homeowners searching for secondary glazing for casement windows London, the central challenge is not simply adding another pane of glass. The solution must reduce noise while respecting the way the original window opens, preserving the existing frame and meeting the expectations of a listed building or conservation area.

This guide examines the fitting process, acoustic principles and specification choices that determine the result.

Why Casement Windows Present a Particular Acoustic Challenge

A casement window is hinged at the side and opens outward or inward. In a period property, it may also include projecting handles, stays, catches, uneven timber, slender glazing bars and reveals that are far from perfectly square.

These characteristics create several design constraints:

• An internal secondary unit must not obstruct the original casement’s operation.
• The new frame must be fitted accurately into the internal reveal rather than damaging the historic window.
• Sufficient space must be retained for a large acoustic air gap.
• The secondary unit must close tightly around its entire perimeter.
• The finished installation should remain visually subordinate to the original architecture.

A conventional replacement window may offer improved thermal performance, but replacing original casements is often unacceptable: or prohibited: in listed buildings and conservation areas. Secondary glazing takes a different approach: the original window remains in place, while an independent acoustic window is installed internally behind it.

This preserves the external appearance and creates the separation required for effective sound reduction.

The Acoustic Principles Behind the System

High-performance secondary glazing relies on three interacting mechanisms: mass, decoupling and airtightness.

2.1 Mass: using acoustic laminate glass

The heavier the glass, the more difficult it is for sound energy to make it vibrate. Our acoustic specification uses 10.8mm acoustic laminate glass, incorporating a viscoelastic PVB interlayer.

The interlayer acts rather like a microscopic shock absorber. Instead of allowing the glass to flex and transmit vibration freely, it dissipates a proportion of that energy within the laminate.

This is particularly relevant to London noise, where a room may be affected by both higher-frequency sounds: such as voices, horns and sirens: and lower-frequency energy from buses, lorries and aircraft.

2.2 Decoupling: creating distance between the windows

The original casement and the secondary glazing are structurally independent. Between them is a carefully designed cavity of approximately 100–150mm.

This separation interrupts the direct path through which sound would otherwise travel. In simple terms, the external window and internal window do not vibrate as one object.

The air gap also influences the system’s resonance. A narrow gap may provide useful draught and thermal control, but a larger cavity is generally more effective for serious urban noise when combined with the correct glass specification.

2.3 Airtightness: preventing acoustic leakage

Sound will exploit the smallest weak point. A high-specification pane cannot compensate for an open joint around the frame, a poorly closing catch or an unsealed ventilation route.

Our units use twin-compression EPDM seals. EPDM is a durable synthetic rubber commonly used where long-term resistance to weathering and compression is required. The twin-seal arrangement forms a continuous acoustic envelope around the opening.

That principle is consistent with the wider acoustic guidance applied to building elements: performance depends on the complete installed construction, not glass in isolation. Sound insulation is commonly assessed using methods described in BS EN ISO 717-1 and related test standards.

What Noise Reduction Can You Expect?

With the appropriate combination of acoustic laminate glass, a 100–150mm air gap, independent framing and continuous compression seals, our system is designed to block up to 54dB of city noise.

This figure should be interpreted correctly. Decibels are logarithmic rather than linear. A reduction of 10dB is commonly perceived as approximately half as loud, although the experience varies according to frequency, background noise and the listener.

The following table illustrates the practical distinction between common specifications:

Specification	Typical application	Acoustic assessment
Basic single glazing	Original period window	Limited resistance to external traffic noise
Standard replacement double glazing	General thermal improvement	Can help with noise, but cavity and glass specification may be insufficient for severe urban noise
Basic secondary glazing	Draught and thermal improvement	Improved separation, with performance dependent on seals and cavity
Acoustic secondary glazing	Busy roads, rail, aircraft and nightlife	10.8mm acoustic laminate glass, large cavity and airtight seals; our system blocks up to 54dB

The result will depend on the complete room construction. Walls, doors, roof spaces, ventilation openings and adjacent windows can all create secondary transmission paths. A professional survey is therefore more meaningful than relying on a glass-only rating.

How Secondary Glazing Is Fitted Behind Timber Casements

A successful installation begins with the existing window, not a standard catalogue size.

Step 1: Survey and acoustic assessment

The surveyor records:

• Window width, height and squareness
• Opening direction and hinge positions
• Handles, stays and catches
• Reveal depth and available cavity
• Condition of the original timber
• Internal blinds, curtains and shutters
• Noise sources and the rooms most affected

This assessment determines whether a hinged, fixed or lift-out secondary unit is most appropriate.

Step 2: Selecting the opening configuration

For casement windows that need regular access, a hinged internal secondary casement is generally the most practical arrangement. It opens into the room, allowing access to the original window for cleaning, maintenance and ventilation.

Where the original window is rarely opened, a fixed or removable panel may be considered. The final choice should balance acoustic performance, ventilation requirements and everyday convenience.

The design must also account for inward-opening primary casements. Where both systems could occupy the same space, the installer will specify clearances and hardware carefully to prevent interference.

Step 3: Bespoke manufacture

Each frame is manufactured to the surveyed opening. The design is adjusted to suit the historic reveal and to maximise the available 100–150mm cavity.

The heavy 10.8mm acoustic laminate glass also requires suitable hinges, catches and structural support. Hardware must keep the unit aligned over time so that the EPDM seals continue to compress consistently.

Step 4: Internal installation

The secondary frame is fixed within the reveal, normally without removing or altering the original external casement. The installer checks that the frame is square, plumb and correctly positioned before fitting the glazed unit.

The perimeter is then sealed and the hinges, catches and compression hardware are adjusted. The objective is a continuous barrier: no unintended gaps, loose corners or areas where the unit fails to close evenly.

Step 5: Commissioning and final checks

The completed installation should be checked for:

• Smooth opening and closing
• Even compression around the seals
• Clearance from handles and window stays
• Correct operation of the original casement
• Draughts or visible perimeter gaps
• Potential noise paths around vents and adjoining elements

A professional installation is not complete when the frame is merely in place. It is complete when the system operates correctly and creates the intended acoustic envelope.

Conservation and Listed Building Considerations

Secondary glazing is often considered for heritage properties because it is installed internally and leaves the original external elevation intact. This can be particularly important in Kensington, Chelsea, Westminster and other London districts where planning controls protect historic façades.

However, listed-building and conservation-area requirements vary by property and local authority. Homeowners should confirm the position with their conservation officer before work begins.

Our specialist approach has achieved a 100% conservation officer approval rate for the listed-building projects we have submitted. That record reflects careful attention to reversibility, sightlines, frame colour, fixing positions and the retention of original fabric; it should not be taken as a substitute for property-specific approval.

For broader context, Historic England’s guidance on traditional windows and their repair or upgrading explains why internal improvements can be preferable to wholesale replacement in historic properties.

Why Specialist Installation Matters in London

Casement windows can appear simple, but period construction rarely is. Reveals may be irregular, timber may have moved over time and the most noise-sensitive room may not be the one with the largest window.

A dedicated acoustic survey considers the building as a system. It identifies whether the principal issue is road noise, rail vibration, aircraft movement, nightlife or a combination of sources.

As previously noted, the strongest result comes from the interaction of:

• 10.8mm acoustic laminate glass
• 100–150mm precision air gap
• Independent secondary framing
• Twin-compression EPDM seals
• Correctly specified hinges and catches
• Attention to surrounding acoustic weak points

Secondary Glazing Noise Reduction is London’s only dedicated specialist focused exclusively on acoustic secondary glazing. Premium and heritage specifications carry a 25-year warranty, while warranty cover depends on the selected package, providing a substantially longer period of assurance on qualifying installations than many conventional window upgrades.

Actionable Guidance Before Requesting a Quote

Before arranging a survey, record:

• Which rooms are affected most severely
• The times of day when noise is most disruptive
• Whether the primary casements must remain fully operable
• Whether the property is listed or within a conservation area
• The approximate internal reveal depth
• Any existing vents, shutters or internal blinds
• Whether sleep, home working or entertaining is the principal priority

You can also review our London noise reduction guide, compare approaches in our secondary glazing comparison, or explore local guidance for Kensington, Chelsea and Westminster.

Verdict: A Reversible Acoustic Upgrade for Period Casements

Soundproofing a casement window is not a matter of adding the thickest possible pane. It is an exercise in measured separation, controlled mass and airtight detailing.

For London homeowners, bespoke secondary glazing can preserve the original timber casement while creating a quieter interior sanctuary. With 10.8mm acoustic laminate glass, a 100–150mm air gap, twin-compression EPDM seals and professional installation, the system is engineered to block up to 54dB of city noise.

The outcome is not merely a technical improvement. It is the restoration of sleep, concentration and calm behind the historic façade you chose to protect.

For a property-specific assessment, book a free acoustic survey with London’s dedicated acoustic secondary glazing specialist, or call 0207 060 1572.