Guide to secondary glazing

For period properties or home's where replacing single glazing isn't an option, secondary glazing can offer a solution.

By Hugh Metcalf | 10 September 2020

If retrofitting glazing to improve energy efficiency, sometimes secondary glazing is a more appropriate choice than replacing windows entirely.

bronze secondary glazing in a period property - green homes grant - grand designs

Image: Another benefit of secondary glazing is that it can form part of an increased level of security, especially in an historic property. Photo: Architectural Bronze Casements

When is secondary glazing appropriate?

“Secondary glazing can improve comfort and reduce heating demand in heritage properties without causing too much disruption. They can also have great impact on reducing noise from outside,” explains architect Robert Prewett of firm Prewett Bizley.

“Ask the questions: are the existing windows repairable and are they original? Frequently, original windows are repairable as the timber they were made from is durable. How will the installation affect the usability of the window? This will depend on size and secondary system but also whether the spaces it serves relies on the window for ventilation,” he continues.

Consumer site MyGlazing.com and the Glass and Glazing Federation (GGF) offer advice about the performance and qualities of secondary glazing, with answers corroborated with research, product testing and product data from GGF Members including Roseview Windows and Pilkington UK.

How does secondary glazing compare to replacing windows?

The good news is that secondary glazing when placed in close proximity to the primary window can offer good thermal improvements. Often with listed buildings double glazing cannot replace the original window design and specification (which is usually single glazed). Here, secondary glazing units can provide impressive results as an alternative.

The thermal performance is not as high as that of a full double or triple glazing replacement. This is because of the huge advance in sealed insulating that double and triple glazing units have, trapping gases in between the panes. These gases have a lower thermal conductivity than air combined with warm edge spacer bars in between. However, within a project’s constraints, secondary glazing can provide an effective solution.