Article 06 · Fenestration Knowledge Series
Heritage steel windows are often blamed for draughts, condensation and poor thermal performance. In many buildings, however, the original frame is still sound, well-proportioned and worth keeping. The smarter intervention is not always replacement. Secondary glazing can improve comfort and acoustic performance while the original steel window remains in place.
That makes it especially useful where the character of a building matters, where planning or heritage controls apply, or where a repair-first approach is the responsible choice.
What is secondary glazing?
Secondary glazing is an additional glazed panel installed on the room side of an existing window. It creates a second layer of glass, usually with a sealed or ventilated air gap between the original window and the new inner panel. The existing steel window continues to form the outer weather line, while the secondary system improves the internal performance of the opening.
It is different from replacing the original pane with a new unit and different again from replacing the entire window. The distinction matters: the original frame, sightlines and external appearance can remain largely unchanged.
Why use it with heritage steel windows?
Original steel windows have slender profiles and a visual rhythm that is difficult to reproduce with a modern substitute. They may also be part of a building’s architectural significance. Secondary glazing addresses several everyday problems without asking the historic frame to do a job it was never designed to do on its own.
- More comfortable rooms: the additional layer reduces the feeling of a cold internal surface and helps limit draughts when the outer window is properly repaired.
- Better noise control: the second pane can reduce traffic, wind and neighbourhood noise, particularly when the air gap and panel seals are correctly designed.
- Retention of original fabric: the existing steel window, hardware and external appearance can be kept rather than discarded.
- Reversible intervention: a well-designed internal system can usually be removed or altered in the future with less disruption than a full replacement.
Secondary glazing only works when the original window is assessed first
Secondary glazing is not a cover-up for a failing frame. Water entry, corrosion, loose putty, failed fasteners and seized or missing hardware should be investigated before the inner panel is specified. If the outer steel window cannot close, drain and shed water properly, the new layer may hide a defect rather than solve it.
A proper assessment looks at the complete opening: the steel or metal frame, glass, putty or glazing compound, hinges, stays, locks, perimeter seal and the surrounding masonry. D&G Window Solutions takes that whole-opening view because the best result depends on the relationship between the original window and the new intervention.
What should a good secondary-glazing design consider?
The air gap and the room-side line
The distance between the original pane and the secondary panel affects performance, sightlines and how easily the window can be cleaned. The available reveal, skirting, shutters, curtains and internal trim all need to be measured before a system is chosen. A solution that performs well on paper but blocks a handle or looks oversized in a narrow reveal is not a successful heritage intervention.
Glass type and safety
The inner pane should be selected for the actual risk and use of the room. That may mean safety glass in a vulnerable location, a laminated option where retained fragments matter, or a specification aimed at acoustic improvement. The correct choice depends on panel size, access, impact risk, ventilation and the condition of the original glass. Our guide to how modern safety glass works explains why toughened and laminated glass behave differently after breakage.
Openable panels, ventilation and maintenance
Some rooms need access to the original window for cleaning, painting and emergency ventilation. Hinged, sliding or lift-out secondary panels can make that practical, but the operating method must suit the reveal and the way the room is used. Hardware should be robust, discreet and serviceable rather than treated as an afterthought.
Condensation and moisture movement
Condensation is a building-physics issue, not simply a glass issue. Moisture levels, room ventilation, the temperature of both panes and the condition of the outer frame all affect the result. Secondary glazing can change the temperature balance around the original window, so the design should be considered alongside ventilation and any existing damp or drainage problems.
Secondary glazing versus replacing the window
Replacement may be justified when a frame is beyond economic repair, when the opening has been badly altered, or when a project has requirements that cannot be met by retaining the original assembly. But replacement is not automatically the best-performing or most sustainable answer. It can remove historic material, alter external sightlines and create a new interface with the surrounding wall.
Secondary glazing is often a better fit when:
- the original frame is substantially sound or repairable;
- the exterior appearance must be preserved;
- the building has heritage, conservation or design constraints;
- the owner wants improved comfort without a major external building intervention; or
- the existing window has valuable wavy, crown or drawn glass that should remain visible.
Our earlier article, Why Heritage Steel Windows Should Be Repaired Before They Are Replaced, sets out the repair-first questions that should come before any replacement quotation.
When secondary glazing is not the right answer
No single system suits every opening. It may be unsuitable where the reveal is too shallow, where the original window cannot be made weather-tight, where internal access is impossible, or where the proposed panel would interfere with shutters, blinds, escape routes or required ventilation. It also needs careful detailing in rooms with persistent damp.
That is why a site survey should come before a product choice. The correct recommendation may be repair alone, secondary glazing, new high-performance glazing in a retained frame, or a carefully matched replacement. The answer should follow the condition of the building rather than a sales category.
A repair-first path for heritage steel windows
- Survey the opening: record frame condition, glass, hardware, drainage, reveals and the surrounding wall.
- Make the outer window work: repair corrosion, glazing, seals and hardware so the original assembly can close and drain correctly.
- Set the brief: identify the real priorities—comfort, noise, safety, ventilation, appearance or conservation.
- Design the inner layer: select the panel type, glass, air gap, access method and finish around the existing opening.
- Review the whole result: confirm operation, maintenance access, condensation risk and the visual relationship between old and new.
If the project needs new work as well as conservation, D&G Window Solutions also designs and installs made-to-measure steel windows and metal windows and custom aluminium windows and doors where a new opening is appropriate.
Keep the frame, improve the room
Secondary glazing is not a compromise between preservation and performance. When it is designed around a repaired original window, it can be a precise way to keep the character of a building while making the room more comfortable and usable. The right answer depends on the frame, the glass, the building and the people who use it.
For an assessment of existing steel or metal windows, see our heritage refurbishment and mechanical servicing service. To discuss a project or request a quotation, contact D&G Window Solutions.
Next in the Fenestration Knowledge Series
Next Saturday’s chapter will look at how to choose the right glass for steel windows—balancing safety, daylight, thermal performance, heritage character and the frame’s existing limitations.