What Is Secondary Glazing?

 

Secondary glazing creates an additional internal layer of glass behind the existing window. The original external window remains in place, while the secondary unit is fitted to the room side of the opening.

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The space between the original window and the new internal pane creates an insulating cavity. This can reduce heat loss and help prevent cold air from entering through ageing frames.

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A typical secondary glazing system may include:

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• An aluminium, timber or slimline frame

• Standard, toughened or laminated glass

• Sliding, hinged, fixed or removable panels

• Brush seals or compression seals

• Locking catches or restrictors

• Acoustic or low-emissivity glass

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Unlike conventional double glazing, the two panes are not usually sealed together as one manufactured glass unit. The wider gap between the original and secondary windows can be particularly helpful for noise reduction.

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Benefits of Secondary Glazing

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Improved thermal comfort

Secondary glazing provides an additional barrier between the room and the outside air. This can reduce heat loss through older windows and make the internal surface feel less cold.

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It is particularly useful where existing windows are single glazed or affected by minor draughts.

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Reduced outside noise

The wide air gap created between the original and secondary panes can provide effective acoustic insulation.

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Secondary glazing may help reduce noise from:

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• Road traffic

• Railways

• Aircraft

• Schools and playgrounds

• Commercial premises

• Neighbouring properties

• General street activity

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For better acoustic performance, the secondary pane may use thick or laminated glass. The glass should normally be a different thickness from the original pane so the two layers respond differently to sound vibrations.

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Retention of original windows

Historic timber and sash windows are often important architectural features. Secondary glazing allows these windows to remain visible from outside while improving comfort inside the property.

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This can make it particularly appropriate for listed buildings and sensitive conservation areas.

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Reduced draughts

Well-fitted secondary glazing can reduce cold air entering around older window frames.

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However, it should not be used as a substitute for repairing badly deteriorated timber, failed putty or significant gaps around the original window. Basic repairs and draughtproofing may still be required.

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Improved security

The additional internal pane creates another barrier between the room and the outside.

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Laminated glass, secure locks and fixed frames can provide increased resistance at vulnerable ground-floor windows. The original window, secondary glazing and surrounding frame should be considered together as one security system.

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Less disruption

Secondary glazing can usually be installed without removing the existing windows or disturbing the external finishes.

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This can reduce installation time, waste and the need for extensive repairs to plaster, brickwork or decorative features.

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Secondary Double Glazing

 

The term secondary double glazing is commonly used to describe a second internal window fitted behind an existing single-glazed window.

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The result creates two separate layers of glass, but it differs from a conventional sealed double glazed unit. Standard double glazing contains two panes manufactured as one sealed unit with a narrow gas-filled cavity.

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Secondary double glazing normally has a much wider space between the panes. This can offer strong noise reduction, although thermal performance will depend on the frame, seals and type of glass used.

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Secondary glazing may also be fitted behind existing double glazed windows where additional acoustic insulation is required.

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Secondary Glazing for Sash Windows

 

Secondary glazing for sash windows can improve comfort while preserving traditional timber frames and the appearance of the building.

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Sash windows contain moving panels that slide vertically, so the secondary system should provide suitable access for operation, ventilation and cleaning.

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Common options include:

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Vertical sliding secondary glazing

Vertical sliding panels follow the movement of the original sash windows. One or both internal panels can be moved to provide access to the existing window.

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This is often one of the most practical solutions for traditional sash windows.

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Hinged secondary glazing

A hinged internal frame can be opened like a door. This provides access to the original sash and can be useful where the window is regularly opened or cleaned.

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Lift-out panels

Lift-out secondary glazing uses removable panes that can be taken out when required.

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This can provide a discreet appearance and good sealing but may be less convenient for windows that need frequent access.

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Fixed secondary glazing

Fixed panes are suitable for windows that do not need to open. They can offer slim frames and effective sealing but must allow for cleaning and ventilation.

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Horizontal sliding systems

Horizontal sliding panels are more commonly used behind casement windows but may be appropriate where the shape or proportions of the opening make vertical sliding impractical.

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Secondary Double Glazing for Sash Windows

 

Secondary double glazing can be particularly effective where traditional sash windows contain thin single glazing and gaps around the moving sections.

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The existing sash windows may first be repaired and draughtproofed. The secondary unit then provides an additional layer of thermal and acoustic protection.

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This approach can retain:

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• Original timber frames

• Historic glass

• Traditional sash cords and weights

• Existing glazing bars

• External architectural character

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The internal frames can normally be powder-coated or painted in a colour that blends with the existing window and surrounding decoration.

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Secondary Glazing for Listed Buildings

 

Secondary glazing for listed buildings is often considered because it allows original windows to remain in place.

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Historic windows may contain handmade glass, narrow glazing bars, traditional joinery and original ironmongery. Removing them can result in the loss of important building fabric.

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A carefully designed secondary system can provide improved comfort without significantly changing the external appearance.

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Important considerations include:

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• Keeping the secondary frame visually discreet

• Matching the colour of existing internal joinery

• Avoiding damage to historic timber and plaster

• Retaining shutters and decorative linings

• Allowing access for maintenance

• Minimising visible reflections

• Using reversible fixings where possible

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Listed Building Consent may still be required, particularly where the secondary glazing affects historic fabric or important internal features.

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Approval should be confirmed with the local planning authority before installation begins.

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Secondary Glazing in Conservation Areas

 

Properties in conservation areas are not automatically prevented from replacing their windows, but local planning policies may seek to retain traditional materials and appearance.

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Secondary glazing can offer a less visually intrusive alternative to changing timber windows to UPVC or altering traditional glazing-bar arrangements.

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Where the original windows are visible from the street, retaining and repairing them may protect the character of the building more effectively than complete replacement.

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Properties affected by an Article 4 Direction may have additional restrictions, so the planning position should be checked before any alterations are made.

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Thermal Performance

 

Secondary glazing can reduce heat loss through older windows, particularly where the original glazing is single pane.

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Performance will depend on:

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• The width of the cavity

• Glass thickness and coating

• Frame material

• Quality of the seals

• Condition of the original window

• Gaps around the window opening

• Installation quality

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Low-emissivity glass may be used within the secondary pane to help reflect heat back into the room.

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Draughtproofing the original window can also improve performance. However, some ventilation may still be required to manage moisture and condensation.

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Noise Reduction

 

Secondary glazing can provide excellent noise reduction because the distance between the original and secondary panes is generally greater than the cavity within standard double glazing.

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For improved acoustic performance, the system may include:

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• A wide cavity between the panes

• Laminated acoustic glass

• Different glass thicknesses

• Tight compression or brush seals

• A securely fixed frame

• Acoustic treatment around the window reveals

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The best result is achieved when the frame is well sealed. Small gaps can allow sound to bypass the glass.

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Noise may also enter through walls, doors, roofs, vents and other openings. A specialist assessment may be useful where noise levels are particularly severe.

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Condensation and Ventilation

 

Secondary glazing can make the internal glass surface warmer, which may reduce condensation on the room-facing pane.

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However, moisture can sometimes form between the original window and secondary unit if warm internal air enters the cavity and reaches the colder outer glass.

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Good installation should therefore provide:

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• Effective seals around the secondary frame

• Suitable ventilation to the room

• Minor ventilation of the cavity where required

• Repairs to defective original windows

• Control of excessive internal humidity

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Bathrooms, kitchens and utility rooms should have suitable mechanical extraction.

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Secondary glazing should not be used to seal a room completely without considering fresh-air requirements.

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Secondary Glazing Cost

 

Secondary glazing cost depends on the number and size of windows, the system selected and the glass specification.

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The main factors include:

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• Fixed, hinged, sliding or removable panels

• Aluminium or timber frames

• Standard, toughened or laminated glass

• Acoustic or low-emissivity glazing

• Size and shape of the opening

• Curved, arched or unusually shaped windows

• Listed-building requirements

• Access and installation difficulty

• Colour and finish

• Repairs to the original windows

• Internal making good

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A simple fixed panel will normally cost less than a bespoke sliding system designed for large sash windows.

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Secondary glazing may cost less than complete window replacement, particularly where external scaffolding and extensive making good would otherwise be required. Bespoke listed-building installations can be more expensive because of the care and specialist detailing involved.

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Quotations should clearly state whether they include surveys, installation, glass specification, finishes and repairs to the existing windows.

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Secondary Glazing or Replacement Windows?

 

Secondary glazing may be the better option where:

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• Original windows should be retained

• The property is listed

• External appearance must remain unchanged

• Noise reduction is a major priority

• Complete replacement would be too disruptive

• The existing frames can be repaired

• A reversible improvement is preferred

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Replacement windows may be more appropriate where:

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• Frames are beyond economical repair

• Timber is extensively rotten

• Windows cannot be opened safely

• Security is seriously inadequate

• The property is undergoing major renovation

• Planning restrictions do not require retention

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The decision should be based on the condition and significance of the existing windows rather than cost alone.

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Secondary Glazing Materials

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Aluminium frames

Slim aluminium frames are commonly used because they are strong, lightweight and available in numerous powder-coated colours.

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They can accommodate sliding, hinged and removable panels while remaining relatively discreet.

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Timber frames

Timber secondary glazing can complement existing wooden windows and traditional interiors.

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Bespoke joinery may be suitable for listed buildings, although timber frames can be wider and require more maintenance than aluminium systems.

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Magnetic and acrylic systems

Lightweight magnetic or acrylic panels can provide a lower-cost or temporary solution.

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These systems may improve draught control but are unlikely to offer the same durability, acoustic performance or security as professionally installed glass systems.

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Choosing a Secondary Glazing Installer

 

An experienced installer should assess the existing windows, room ventilation and required level of performance before recommending a system.

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A detailed quotation should confirm:

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• Frame material and colour

• Glass type and thickness

• Opening arrangement

• Acoustic or thermal performance

• Fixing method

• Access for cleaning

• Ventilation requirements

• Planning responsibilities

• Making good

• Product and installation guarantees

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For listed buildings and period properties, ask to see examples of similar completed installations.

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Find Trusted Secondary Glazing Installers

 

Secondary glazing can improve thermal comfort, reduce outside noise and preserve the character of original windows. Correct measurement, suitable glass and careful installation are essential for achieving a discreet and effective result.

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APN Double Glazing can connect homeowners with experienced local installers offering secondary glazing for sash windows, period properties and listed buildings.

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Get in touch for expert advice and a free, no-obligation quote for secondary glazing.