Timber Acoustic Secondary Glazing for Sash Windows
Bespoke inward-opening timber secondary windows designed to retain original sash windows and provide substantial acoustic protection where reveal depth is limited.
Secondary glazing can significantly improve protection from road, rail, aircraft and other external noise, but the result depends on the existing window, glass-to-glass cavity, glazing specification, airtightness, installation and surrounding building fabric. We assess those factors before recommending a system.
OUR SECONDARY GLAZING FOR SASH WINDOWS
Installed internally behind the existing sash window, our bespoke timber secondary windows allow the original window to remain in place while adding a substantial acoustic barrier inside the room.
Each system is designed around the available reveal depth, the existing window, the noise source and the architectural character of the property. The secondary window opens inwards, providing access to the original sash for cleaning, ventilation and maintenance.
Because the installation is internal and can be designed to respect the original sightlines and joinery, it may be particularly suitable for period properties, conservation areas and listed buildings where replacing the original windows is undesirable or restricted. Planning and listed-building requirements should always be confirmed for the individual property.
A COMPLETE INWARD-OPENING SECONDARY WINDOW
We retain the original sash window and install a bespoke inward-opening timber secondary casement behind it.
Unlike lightweight secondary panels, this is a complete timber window incorporating substantial acoustic glazing, continuous perimeter seals and secure closing pressure. The frame can be installed within the existing reveal or face-fixed internally where the available reveal depth is limited.
The specification is selected according to the existing window, the glass-to-glass cavity, the available opening space and the type of noise affecting the property.
DESIGNED FOR GREATER AIRTIGHTNESS
Many conventional secondary-glazing systems use sliding panels and brush seals. These can provide useful thermal, draught and moderate acoustic improvement, but their performance may be limited where serious road, rail or aircraft noise is involved.
Our inward-opening system uses continuous compression seals around the opening section. When the window is closed, the seals are compressed against the frame to reduce the small air gaps through which sound can travel.
The final result still depends on the complete installation, including the primary sash window, glazing specification, cavity depth, perimeter detailing and surrounding building fabric.
THE FULL RANGE
Our timber secondary windows are designed around real properties, including period sash windows where only a shallow reveal or restricted glass-to-glass cavity is available.
We offer three performance levels. The appropriate specification is selected according to the existing sash window, available cavity, noise source, room use and architectural requirements.The product level does not represent a guaranteed reduction inside the room. Final performance depends on the complete primary and secondary window arrangement, glazing specification, airtightness, installation quality and surrounding building fabric.
SOUNDPROOF
Typical applications: General road traffic, neighbourhood activity and other moderate external noise.
A substantial timber secondary window designed for properties affected by general road traffic, neighbourhood activity and other moderate external noise.
The specification combines acoustic glazing, continuous compression seals and bespoke perimeter installation while retaining access to the original sash window.
SOUNDPROOF+
Typical applications: Persistent road traffic, railway noise, aircraft movements and louder commercial activity.
An enhanced specification for properties exposed to persistent road traffic, railway noise, aircraft movements or louder commercial activity.
The glazing, frame configuration and installation detail are selected around the noise profile and the glass-to-glass cavity available behind the existing window.
ULTRA
Typical applications: Demanding noise where the original window must be retained and the available reveal or glass-to-glass cavity is restricted.
Our highest-performance secondary-window specification, designed for particularly intrusive noise and properties where the original windows must be retained despite limited reveal depth.
The system uses substantial acoustic glazing, continuous compression seals and project-specific frame and installation details to make the best use of the cavity available.
Ultra specifications may require reinforced frame construction, additional structural support, face-fix installation or bespoke internal joinery. A technical assessment is recommended before the final specification is confirmed.
RECENT INSTALLATIONS
Every secondary window is designed around the original opening, available reveal depth and architectural details of the property.
These installations show how our timber secondary windows can be fitted behind traditional sash windows using reveal-fit, face-fix and bespoke joinery details.
ENGINEERED FOR SHALLOW AND RESTRICTED REVEALS
Secondary-glazing performance cannot be judged from the glass specification alone. The existing window, the distance between the two panes of glass and the airtightness of the complete installation all influence the final result.
In many period properties, there is not enough space for the deep cavity used in some laboratory test arrangements. Our secondary windows are therefore designed around the actual reveal depth and glass-to-glass cavity available in the property.
REVEAL-FIT OR FACE-FIX INSTALLATION
At this property, the original sash-window frame was flush with the internal wall face, leaving no usable reveal depth in which to position a secondary frame.
Rather than projecting the secondary window unnecessarily far into the room or removing the original sash, we designed a bespoke timber system that was face-fixed internally to the wall surrounding the opening.
This installation achieved an approximate 80 mm glass-to-glass cavity, despite there being no usable frame-to-frame reveal behind the original sash.
The before-and-after comparison shows how the new secondary window and surrounding timberwork were integrated with the existing opening.
Drag the slider left or right to compare the original opening with the completed face-fixed secondary-window installation.
REVEAL DEPTH AND GLASS-TO-GLASS CAVITY
Reveal depth is the usable space available inside the window opening. The glass-to-glass cavity is the distance between the internal face of the existing primary glass and the external face of the new secondary glass.
These measurements are not the same. The depth of the secondary frame, the position of the original sash and the installation detail all affect the final glass-to-glass cavity.
We assess both measurements before recommending the frame position and glazing specification.
MAKING THE BEST USE OF THE AVAILABLE CAVITY
A deeper glass-to-glass cavity can be beneficial, but cavity depth is only one part of the acoustic system.
Performance is also affected by:
- the condition and airtightness of the original sash window;
- the mass and construction of the glazing;
- continuous compression seals;
- closing pressure;
- perimeter sealing;
- installation quality;
- ventilation openings;
- walls and other possible transmission paths.
A shallow cavity does not automatically rule out a high-performance solution. It does mean that the secondary window must be designed carefully around the actual property rather than selected from a standard specification.
MEASURED RESULTS IN REAL HOMES
PRINCE ALBERT ROAD - 56MM CAVITY
Road traffic noise behind original timber sash windows
This Bow Quarter apartment retained its original single-glazed Crittall windows. The unusually tall openings, curved heads and listed-building context required a fully bespoke secondary-window design.
The new inward-opening timber system was positioned approximately 95mm glass-to-glass from the original glazing. It incorporated a 39 mm acoustic double-glazed unit rated Rw 49(-2; -7)dB, continuous dual compression seals and multipoint locking.
Measured inside the apartment:
- average sound level reduced from 49.2 dB to 32.2dB;
- measured average improvement of 17 dB; maximum recorded level reduced from 67.0 dB to 42.9dB;
- measured maximum-event improvement of 24.1dB.
Each measurement period lasted approximately 15 minutes and captured around 20–25 representative train pass-bys. These were comparative site measurements rather than a controlled laboratory test.
BOW QUARTER - 95MM CAVITY
Railway noise within a Grade II listed building
This Bow Quarter apartment retained its original single-glazed Crittall windows. The unusually tall openings, curved heads and listed-building context required a fully bespoke secondary-window design.
The new inward-opening timber system was positioned approximately 95mm glass-to-glass from the original glazing. It incorporated a 39 mm acoustic double-glazed unit rated Rw 49(-2; -7)dB, continuous dual compression seals and multipoint locking.
Measured inside the apartment:
- average sound level reduced from 49.2 dB to 32.2dB;
- measured average improvement of 17 dB; maximum recorded level reduced from 67.0 dB to 42.9dB;
- measured maximum-event improvement of 24.1dB.
Each measurement period lasted approximately 15 minutes and captured around 20–25 representative train pass-bys. These were comparative site measurements rather than a controlled laboratory test.
PRODUCT OPTIONS
Every secondary window is made to order. The glazing, opening arrangement, frame design and finishes are selected around the noise source, size of the opening, available cavity and architectural requirements of the property.
ACOUSTIC GLAZING
We offer a range of laminated acoustic, double-glazed and triple-glazed specifications.
The appropriate glass build-up is selected according to:
- the type and frequency of the noise;
- the performance of the existing sash window;
- the available glass-to-glass cavity;
- the size and weight of the opening sections;
- safety, privacy and thermal requirements.
Options may include acoustic laminated glass, asymmetric double-glazed units, triple-glazed units, obscure glass and safety glazing.
Laboratory ratings of available glazing units:
- Specialist acoustic double- and triple-glazed units are available with laboratory ratings of: 40 dB, 43 dB, 49 dB, 51 dB and 53 dB
- Single laminated acoustic glass options include: 6.4 mm, 8.4 mm, 8.8 mm, 10.8 mm, 12.8 mm and 16.8 mm.
BESPOKE FRAME AND OPENING DESIGN
Each timber frame is manufactured to suit the individual opening. Depending on the window proportions and available space, the secondary window can be designed with:
- inward-opening side-hung sections;
- tilt-and-turn operation; divided frames aligned with the original sash;
- reveal-fit or face-fix installation;
- bespoke sub-frames, architraves, boxing or panelling.
The opening arrangement is designed to retain practical access to the original sash for cleaning, maintenance and ventilation.
FINISHES AND IRONMONGERY
Frames can be finished to complement the surrounding joinery and interior of the property.
Available options include:
- bespoke paint finishes;
- selected RAL colours;
- matching timber trims and architraves;
- traditional or contemporary handles;
- hinges and locking hardware selected for the size and weight of the opening sections.
Where the property contains period mouldings, shutters or panelling, the surrounding joinery can be designed to integrate the secondary window more sympathetically.
technical specification download
THERMAL COMFORT, DRAUGHT REDUCTION AND CONDENSATION
In addition to reducing external noise, a well-sealed secondary window can improve thermal comfort by creating an additional glazed barrier behind the existing sash.
Continuous compression seals help reduce draughts and uncontrolled air leakage, while the additional glazing and cavity can improve internal surface temperatures near the window. The amount of improvement depends on the existing sash, glazing specification, cavity depth, frame design and quality of installation.
Selected double- and triple-glazed acoustic units are available with centre-pane Ug values as low as 0.6 W/m²K.
ACOUSTIC PERFORMANCE WHILE VENTILATING
The strongest acoustic performance is achieved when both the primary and secondary windows are closed.
The inward-opening design allows access to the original sash for cleaning, maintenance and ventilation. However, opening or tilting the secondary window reduces the acoustic barrier for as long as it remains open.
Where regular background ventilation is required, we can consider the window arrangement alongside suitable acoustic-ventilation options.
OPENING, ACCESS AND VENTILATION
Our secondary windows are designed to provide strong compression sealing when closed while retaining practical access to the original sash window.
The opening arrangement is selected around the size and proportions of the window, the weight of the acoustic glazing, the available space inside the room and how the client needs to use the original sash.
OPEN
Depending on the dimensions and design of the opening, the secondary window can be configured with inward-opening side-hung or tilt-and-turn sections.
Multipoint locking applies consistent closing pressure around the opening section, compressing the perimeter seals and reducing the small air gaps through which sound can travel.
Larger or heavier glazed sections may require divided frames, reinforced construction or project-specific hardware.
CLEAN AND MAINTAIN
The inward-opening design provides access to the cavity and the original sash window for cleaning, maintenance and repairs.
The opening arrangement and surrounding joinery are designed to allow practical access without permanently removing the secondary frame.
Where shutters, deep window boards or other internal features are present, these must be considered during the design stage.
VENTILATE
The secondary window can be opened to access and ventilate through the original sash. Selected configurations may also provide a restricted opening or tilt position for controlled ventilation.
However, the strongest acoustic performance is achieved when both the primary and secondary windows are closed. Opening or tilting the secondary window reduces the acoustic barrier for as long as it remains open.
Where regular background ventilation is required, the window design should be considered alongside an appropriate ventilation strategy rather than relying on an open window while expecting full acoustic protection.
DESIGNED TO COMPLEMENT THE ORIGINAL INTERIOR
A secondary window changes the inside of the opening, so its proportions, frame position and surrounding joinery need to be considered as carefully as its acoustic specification. Our timber secondary windows are made to suit the individual property.
We can align the new frame with the sightlines of the original sash and design the surrounding trims to complement existing architraves, panelling, shutters and window boards. The original external window remains in place, preserving the appearance of the façade while the new acoustic window is installed internally.
BESPOKE TIMBER JOINERY
Where the opening contains period mouldings or other architectural details, we can manufacture bespoke timber elements around the secondary window, including:
- architraves; sub-frames;
- window boards; panelling;
- boxing;
- trims matching existing profiles;
- painted finishes selected to complement the room.
The aim is not to disguise that a new window has been installed, but to make the completed work feel appropriate to the existing interior rather than like a generic add-on.
REVEAL-FITTED OR FACE-FIXED
Where sufficient reveal depth is available, the secondary frame can be positioned within the window opening for a discreet installation.
Where the original sash is flush with the internal wall or the reveal is too shallow, the new frame can be face-fixed around the opening. Bespoke timber sub-frames, boxing, architraves or panelling can then be used to integrate the installation with the room.
The chosen position must also provide sufficient clearance for the secondary window to open inwards.
MATCHING SIGHTLINES AND PROPORTIONS
The layout of the secondary frame can be designed to follow the meeting rails, glazing bars and divisions of the original sash window.
Careful sightline alignment helps the secondary window appear more considered and reduces the visual interruption when viewed from inside the room.
On larger or unusually shaped openings, additional frame divisions may be required to support the weight of the acoustic glazing and allow practical operation.
PERIOD, CONSERVATION AND LISTED PROPERTIES
Internal secondary glazing may be particularly useful where the original sash windows contribute to the architectural character of the building and replacement is undesirable or restricted.
A carefully detailed installation can retain the external appearance and may be designed to minimise unnecessary alteration to historic fabric.
However, listed-building and planning requirements depend on the individual property, the proposed fixing method and the significance of the surrounding joinery.
Consent requirements should therefore be confirmed with the relevant local authority or conservation officer before installation.
This is consistent with the site’s requirement to explain both the architectural implications and the limits of the proposed solution, rather than implying that every listed-building installation is automatically permitted.
SECONDARY WINDOWS BEHIND MODERN uPVC WINDOWS
Secondary windows are not limited to historic properties. They can also be installed behind modern uPVC, aluminium or timber windows where the existing units do not provide sufficient protection from the relevant noise.
Before recommending a secondary system, we assess the existing glazing, opening joints, seals, trickle vents, frame condition and installation. In some properties, repairing a defective window or addressing an open ventilation path may be more appropriate than adding another window.
Where the primary window is in good condition but its acoustic performance remains insufficient, a bespoke inward-opening timber secondary window can add a substantial second glazed barrier without removing the existing unit.
The available reveal depth, glass-to-glass cavity, inward-opening clearance and surrounding building fabric must still be assessed before the design is confirmed.
CHECK WHETHER A SECONDARY WINDOW CAN FIT YOUR PROPERTY
To assess whether a reveal-fitted or face-fixed secondary window may be suitable, send us:
- photographs of the complete window from inside and outside;
- an approximate width and height;
- a photograph showing the internal reveal with a tape measure;
- the type of noise affecting the room;
- the times when the noise is most intrusive;
- details of any shutters, panelling, window boards or other obstructions;
- confirmation of whether the property is listed, in a conservation area or subject to other restrictions.
We will review the available space, likely glass-to-glass cavity and practical opening arrangement before recommending the most appropriate next step.
For more demanding noise, unusual openings or uncertain transmission paths, we may recommend an acoustic survey before confirming the final specification.
SECONDARY GLAZING FOR SASH FAQs
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What is acoustic secondary glazing for sash windows?
Acoustic secondary glazing adds a separate internal window behind the existing sash. Our system is a bespoke inward-opening timber secondary window incorporating acoustic glazing, continuous compression seals and secure closing pressure. The original sash remains in place and can still be accessed for cleaning, maintenance and ventilation.
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How effective is secondary glazing at reducing noise?
A well-designed secondary window can significantly reduce external road, rail, aircraft and commercial noise. The result depends on the condition of the primary window, glazing specification, glass-to-glass cavity, airtightness, installation, noise frequency and surrounding building fabric. A laboratory rating for the glass does not represent the guaranteed reduction achieved inside the room.
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What does the Rw rating of the glass mean?
Rw is a laboratory rating used to describe sound-insulation performance under controlled conditions. Where we display an Rw value for an acoustic double- or triple-glazed unit, the rating applies to that glazing unit. It is not the rating of the complete primary and secondary window arrangement and is not a guaranteed room reduction.
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How much glass-to-glass cavity is needed?
There is no single cavity depth that is right for every property. Greater separation between the primary and secondary glass can be beneficial, but performance also depends on the glazing specifications, primary-window condition, seals, frame design and installation. We assess the available reveal depth and likely glass-to-glass cavity before recommending the frame position and glazing.
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Can secondary glazing be installed where there is no usable reveal?
Potentially, yes. Where the original sash frame is flush with the internal wall and there is no usable reveal for the new frame, the secondary window may be face-fixed around the opening. This can create a useful glass-to-glass cavity even though the new frame cannot be installed within the original reveal. Wall space, inward-opening clearance and surrounding joinery must still be assessed.
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Can secondary glazing be installed in a listed building?
Internal secondary glazing is often suitable for historic and listed properties because the original external window can remain in place. Consent requirements depend on the individual building, the fixing method and any effect on historic joinery or character. The proposal should therefore be checked with the local planning authority or conservation officer before work begins.
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Do the original sash windows need to be refurbished first?
Not always. Secondary windows can often be installed while retaining the existing sash in its current condition. Repairs may be advisable where the original window has decay, water ingress, loose joints, damaged glass, defective cords or serious operating problems. The primary window should remain weather-resistant and safe to use.
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Will I still be able to open, clean and maintain the original sash window?
Yes. The secondary window opens inwards, providing access to the cavity and the original sash for cleaning, maintenance and repairs. The opening arrangement is designed around the window dimensions, glazing weight, room layout and any shutters, panelling or other internal obstructions.
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Can I ventilate the room after secondary glazing is installed?
Yes. The secondary window can be opened to provide access to the original sash for ventilation. However, the strongest acoustic performance is achieved when both the primary and secondary windows are closed. Opening or tilting the secondary window reduces the acoustic barrier for as long as it remains open. Where regular background ventilation is required, the ventilation strategy should be considered alongside the window specification.
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Does secondary glazing reduce draughts and improve thermal comfort?
A well-sealed secondary window can reduce uncontrolled air leakage from the room side and create an additional thermal barrier behind the primary sash. The level of improvement depends on the existing window, glazing specification, cavity, frame design and installation. Any Ug value displayed on this page applies to the glazing unit unless expressly stated otherwise.
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Will secondary glazing prevent condensation?
No secondary-glazing system can guarantee that condensation will never occur. Risk depends on indoor humidity, room ventilation, external temperature, the condition of the original sash and air movement within the cavity. These factors should be considered as part of the design and installation.
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Can secondary glazing be fitted behind uPVC windows?
Yes, in some properties. We first assess the existing glazing, seals, vents, frame condition and likely noise-transmission paths. Where the current window is serviceable but does not provide sufficient acoustic protection, an internal timber secondary window may be considered.
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How much does timber acoustic secondary glazing cost?
Cost depends on the dimensions and number of openings, glazing specification, frame arrangement, glass weight, reveal-fit or face-fix installation, surrounding joinery and site access. Send photographs and approximate dimensions for an initial estimate. A detailed survey may be required before the final specification and installation price are confirmed.
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How is the secondary window installed?
The new timber frame is installed internally behind the existing window. Depending on the available space, it may be positioned within the reveal or face-fixed around the opening. The perimeter is sealed and the surrounding trims, architraves, boxing or panelling are completed according to the agreed design.
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AWARD-WINNING PRODUCTS & INSTALLATION
Following awards in 2020, and 2021, The Soundproof Windows was announced as the ‘Best Acoustic Window & Door Installation Company – London’ at the SME News Awards 2022.