Summary
Secondary glazing is a second layer, usually made of glass or lightweight polycarbonate sheet, that is fixed to the inside of the existing windows. This can be highly appropriate for traditional buildings when there is heritage value in retaining the existing glazing.
Options range from simple DIY single pane panels which can be secured with magnetic tape, to more sophisticated double glazed units which allow opening.
Benefits of secondary glazing include:
- Less disruption than replacing windows.
- Preservation of the original window to comply with heritage requirements.
- Reduction of heat losses – by 58% for secondary glazing on a traditional double hung sliding sash window – B&NES Energy efficiency, retrofitting, and sustainable construction SPD (2022).
- Improved acoustic performance.
Drawbacks include the need to make repairs to make the existing window good first, which can be costly.
Cost/difficulty
LOW/MEDIUM COST – The cost will depend on a number of variables. A typical installation, with two single glazed low-e glass panels, the lower panel of which is sliding, costs in the region of £400 for a 1.8m x 1.2m window. According to data from BEIS, the estimated cost of secondary glazing is £110 per m2 – B&NES Energy efficiency, retrofitting, and sustainable construction SPD (2022).
LOW/MEDIUM DIFFICULTY
Applicability in Letchworth
Applicable – No specific requirements.
In Letchworth, windows contribute significantly to the historic identity and value of the homes. They are also an important factor in the appearance of groups of houses and the street scene.
The Heritage and Modern Character Area and Design Principles state that:
Repair of original windows with materials matching the original design is generally preferable, which can be assisted by the introduction of secondary glazing.
Insights from ASBP
“As an alternative to UPVC, metal and timber framed secondary glazing units are available which have less visual impact and can better match the aesthetic of the existing window. In the case of timber, they tend to also have lower embodied carbon.”
Case study
St. Alphege’s Presbytery
An in-depth secondary glazing case study from Bath Preservation Trust.
Links to further information
- The Surrey Home Energy Improvement Guide (2025), page 50
- Energy Efficiency Guide for Letchworth Garden City residents (2024), page 5
- HCCSP Retrofitting Your House Guide (2024), page 24
- B&NES Energy efficiency, retrofitting, and sustainable construction SPD (2022)
- Energy Efficiency and Historic Buildings – Secondary Glazing for Windows (2016)
Image credits – Selectaglaze, Bath Preservation Trust
This guidance is part of Windows and Doors
See more guidance notes on this topic below: