Interior and finishes · Glass & Glazing
Secondary Glazing System (Independent Inner Glazing)
Explains how a second independent window differs from a sealed double glazed unit, and why the width of the gap, the ventilation of it and the openability of the inner frame are the questions that decide whether it works.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What secondary glazing is
Secondary glazing is a complete second window fitted inside the reveal of an existing one. It has its own slim frame, usually aluminium or timber, and its own glass, and it is sealed against the reveal or the existing frame lining. The original window is left in place and continues to work as before. The inner unit is made to open, slide, hinge or lift out so that the outer window can still be reached and used.
That is a different thing altogether from an insulated glass unit. A sealed unit is a factory made pair of panes sharing one frame, with a narrow hermetically sealed cavity, a spacer bar and desiccant, and it replaces the glass in the window. Secondary glazing adds a second window with a wide, deliberately unsealed gap between the two, and it changes nothing about the existing one, which is why it is so often the route taken where the original windows have to be kept.
The consequences follow from that wide gap. Two independent frames separated by a substantial air space behave differently from a sealed unit, particularly where external noise is the reason for the work. Because the cavity is not sealed, moisture management becomes a design question: which side is sealed tightly and which is allowed to breathe, how the cavity is reached for cleaning, and what happens to condensation in cold weather.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- secondary glazing
- internal secondary window
- retrofit inner glazing
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Buildings where the existing windows are being retained and cannot be replaced.
- Rooms exposed to traffic, aircraft or nightlife noise, where the outer window alone is not enough.
- Draughty single glazed sashes and casements in older houses and flats.
- Upper floor rooms in city buildings where external work would be difficult to arrange.
- Rooms being converted to bedrooms or studies where comfort is the driver rather than appearance.
Consideration
Where it is commonly considered
- Where the relevant authority requires the original windows to remain, and any change must be reversible.
- Where external noise is the main complaint and the design team has identified the window as the weak point.
- Where the existing frames are sound enough to justify keeping them and repairing them.
- Where the reveal is deep enough to receive an inner frame without spoiling the room or blocking the outer window.
- Where a tenant or leaseholder cannot alter the external appearance of the building at all.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the existing window is decayed, since a second window in front of a failing one solves very little.
- Where the opening serves as a means of escape, which is a matter for the local requirements and a qualified professional.
- Where the reveal has no room for an inner frame without cutting into linings, shutters or architraves.
- Where trickle ventilation or background ventilation would be blocked by the new inner frame.
- Where condensation between the two windows would be unacceptable and the cause has not been established.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Acoustic behaviour
- The wide separation between two independent windows is why this approach is raised where noise is the problem, but acoustic behaviour is a property of the complete tested assembly and the room, not of an inner pane on its own.
- Moisture behaviour
- The cavity is not sealed, so which side is made tight and which is allowed to breathe determines where moisture goes. This is a design decision for the specific window rather than a rule that applies everywhere.
- Appearance
- An inner frame is visible from the room and, faintly, from outside. Sightlines, the way the frame lands on the reveal and its colour all affect a room that may have been chosen for its windows.
- Installation dependency
- Fixing is into the reveal, the lining or the existing frame, and what can be fixed into is established by survey. In protected buildings the reversibility of those fixings is part of the discussion.
- Maintenance
- Both windows now need cleaning, and the cavity between them collects dust and insects, so how the inner unit opens for access matters more in use than in a brochure.
- Repairability
- The inner unit is an independent product and can be adjusted, reglazed or removed without touching the original window, which is a genuine practical advantage over replacement.
- Thermal behaviour
- An added inner window changes the way the opening behaves in cold weather, but the original window, the frame, the reveal and any gaps around them all contribute, so improvement is a whole opening question.
Exposure
Exposure and environmental conditions
- What is the actual source of the noise, and does it arrive through the glazing or through the frame and reveal?
- How much moisture is produced in this room, and how is it ventilated once the window is harder to open?
- Does the existing window suffer condensation now, and where does that water currently go?
- Is the outer window still weathertight, or does rain reach the inside of the frame?
- Does the room overheat in summer, and will an inner window make that worse?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The reveal, lining, shutter boxes and architraves determine where an inner frame can be fixed and how it will look.
- The condition of the existing window governs whether repair should come first, which it usually should.
- Background ventilation provision for the room is part of the same decision and cannot be left out of it.
- Curtains, blinds, shutters and window seats all compete for the same reveal depth and are coordinated together.
Appearance
Appearance and finish considerations
- Slim frames intrude less on a period interior, and where the frame lands relative to the existing glazing bars is very visible.
- The inner frame can be seen faintly from outside, which the relevant authority may have a view about.
- Two layers of glass produce a double reflection that reads differently from a single pane, particularly at night.
- Frame colour is chosen against the joinery rather than against the wall, or the addition announces itself.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- Is the existing window sound enough to justify keeping it, and what repairs does it need first?
- How will the inner unit be operated day to day, and will that operation stay easy?
- What happens to the cavity in a cold snap, and has the installer explained how that is managed?
- Can the inner unit be removed for redecoration of the original window without damage?
- How are the seals of the inner frame expected to age, and can they be renewed?
Upkeep
Maintenance and repair considerations
- How is the cavity between the two windows reached for cleaning?
- How often will the original window need to be opened, and does the inner unit allow it?
- What happens if dust or insects accumulate between the panes?
- Who maintains the inner unit's seals, tracks and catches once the installer has gone?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Each opening is surveyed individually, since reveals in older buildings are rarely square or consistent.
- Fixings are into sound material, which is established before the order rather than discovered on the day.
- Repairs and decoration to the existing window are completed first, because access afterwards is restricted.
- The way the inner unit opens is chosen around furniture, curtains and how the outer window is used.
- In protected buildings the fixing method is agreed with the relevant authority before work begins.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's information for the specific inner glazing system proposed.
- A survey record of each opening with the proposed opening arrangement for each one.
- Written confirmation of how the cavity is intended to be sealed and ventilated.
- Warranty terms for the inner glazing unit and for its installation into the reveal.
- Any approval correspondence with the relevant authority where the building is protected.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What problem is this meant to solve, and is the window really where that problem enters the room?
- Should the existing window be repaired before anything is added in front of it?
- Which side of the cavity will be sealed, and why that side for this window?
- How will background ventilation for the room be maintained afterwards?
- Does this opening serve as a means of escape, and who is confirming that?
- How does the inner unit open, and does that suit how the room is used?
- What does the relevant authority say about this approach in this building?
Blind spots
Commonly overlooked points
- A second window in front of a decayed one hides the decay rather than stopping it.
- Ventilation is easy to lose, and a room that was draughty can become one where moisture has nowhere to go.
- Noise often arrives through gaps, the frame and the reveal as well as through the glass, so sealing matters as much as glazing.
- The cavity needs to be reachable, and an inner unit that is awkward to open stops being opened at all.
- Curtains, shutters and window seats may no longer fit the reveal once an inner frame is in place.
What this page does not do
- Whether an opening must remain usable for escape is determined by the local requirements and by a qualified professional, never by the glazing supplier alone.
- Acoustic behaviour belongs to the complete tested assembly and the room, so no improvement can be promised from a product description.
- Blocking existing background ventilation without replacing it can create moisture problems that are harder to solve than the original complaint.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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