If retrofitting glazing to improve energy efficiency as part of the Green Homes Grant, 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

As part of the Government's Green Homes Grant, energy-saving measures for single glazed windows are eligible for funding as a secondary measure.

With funding available for both replacing single glazed windows with energy efficient double glazing and installing secondary glazing, it's down to individual homeowners to make the best choice for their specific properties, but when is secondary glazing 

"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. 

With this in mind, the Grand Designs magazine team posed some key questions to consumer site MyGlazing.com and the Glass and Glazing Federation 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.

Read more: Green Homes Grant: replacing single glazing

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 double and triple glazing units that have 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.

Read more: How to reduce the carbon footprint of your build

One the most common ways of measuring thermal performance in building materials is the U value, which measures how easily heat can pass through a material. Materials that allow more heat to escape from a building have higher (i.e. worse) U-values. Materials that let less heat pass through them have lower (i.e. better) U-values.

Solely based on glass U-values the table below is a reasonable measure of the difference on different window types:

U Value chart for windows - green homes grant - grand designs

Information provided by MyGlazing.com

It’s important to note that the figures above do not factor the individual performance of frames, seals and insulation around the frames, the U values of the glass are based on typical performances.

Low emissivity (low E) coatings for glazing are microscopically thin, transparent coatings (even thinner than a human hair) that reflect long-wave infrared energy (or heat!). So instead of heat escaping through the glass, a low E coating will reflect heat back into the home. In secondary glazing hard coat Pilkington K Glass™ is used to create the same effect.

The table shows the difference in thermal insulation from a single pane of glass through to a triple glazing unit with two panes of low E glass. As you can see the difference from single glazing with the addition of secondary glazing approximately doubles energy efficiency.

Conservation area properties and listed buildings use secondary glazing as a solution to improve energy efficiency because once installed, it improves energy bills immediately. Installing double glazing or triple glazing, increases thermal performance even more significantly, by roughly 100% from secondary glazing.

Secondary glazing is also a less expensive option (approximately 50% less on a supply and install) and is often an easier or temporary solution for those homeowners who want to improve their energy efficiency but may not have a huge budget.

What is the average cost of secondary glazing?

 exterior of house with traditional windows and secondary glazing - grand designs - green homes grant

Image: Pilkington's Spacia is a super fine double glazing unit, that can be used a replacement windows in period properties to get around planners, and can act as a secondary glazing while retaining the original frames. 

Cost varies depending on the project. Secondary glazing can be bespoke, coloured and shaped to suit the properties they are found in. Designed to cover the existing window style and be hardly visible, the variation in styles these days even includes fully tilt out to clean units. On average, a typical secondary glazing window is around half the price of a double glazing window. The installation of secondary glazing is also better for acoustics, whether it’s keeping sound out or in.

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When should you use secondary glazing over replacement windows?

 The noise cancelling effects of secondary glazing are particularly impressive. Secondary glazing is used extensively throughout the country to combat noise pollution. It is often fully subsidised as part of government schemes to improve living conditions in particular situations such as airport flight paths and housing in proximity of railways or highways.

Secondly, in conservation areas or listed buildings where restrictions are in place. Many older public buildings like museums, schools or town halls have secondary glazing installed because the owners or local councils may have to retain the external appearance of the building. Well-installed secondary glazing is virtually invisible from the external of such a property and delivers the advantages of energy efficiency and noise reduction.

Secondary glazing also offers an added layer of security and the internal finishes that can be achieved with secondary glazing are now of such a high quality, they can be a real feature of the interior design.  

What are the drawbacks of secondary glazing?

 Secondary glazing is not as energy efficient as a new double or triple glazing window though it does improve energy efficiency where there are restrictions. One drawback that is sometimes mentioned is the maintenance and cleaning of secondary glazing. It can mean twice as much work when cleaning, especially when double and triple glazing can also feature self-cleaning glass. However, most modern secondary glazing panels are removable and the weight of a single glazed panel is much lighter to handle when doing so.

It has been mentioned that in some situations, secondary glazing can resolve a problem with condensation. Providing an extra barrier of glass can help regulate the extreme temperature clash of cold glass pane against warm centrally heated air though it should always be well ventilated to reduce the likelihood of condensation. Secondary glazing is not the best energy efficient glazing solution, because the seals are not as airtight as those on double and triple glazing. In addition, the thermal performance of secondary glazing is very much dependent on the condition of the prime single glazed window on a building’s exterior.

 

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