Secondary Glazing is fairly recent in the context of buildings and architecture. It was only in the 19th century that we started to use secondary glazing. Double glazing is now the norm in new homes, with some even benefiting from triple glazing. However, there are still plenty of homes lacking double glazing. In some cases, this can be due to concerns about altering the character of historic buildings. Secondary glazing can be used to provide an extra layer of insulation to existing windows, avoiding the need to replace windows in order to add double glazing. The character of the property can be maintained while the insulation and energy efficiency are improved.

The history of secondary glazing is interesting and can give us a good understanding of how secondary glazing came to be what it is today.

What Is Secondary Glazing?

Secondary glazing is the use of a window system, which is installed on the internal side of a window. It’s completely independent of the window itself, allowing it to be installed without changing the original window. This makes it ideal for older properties, especially as secondary glazing can be either fixed or installed as openable or removable units. When secondary glazing is installed with openable panels, they can be casements or sliding sashes, which allow access to the original windows so that they can be cleaned or to allow for ventilation.

Secondary Glazing vs Double Glazing

Secondary glazing and double glazing may sound similar, and they are in some respects, but they’re not exactly the same. Double glazing is the installation of windows featuring two panes of glass that are separated by an air gap for insulation. The air gap can be filled with inert gas and the glass can also be coated, both of which help to improve insulation even more.

Installing double glazing in a property where there is only single glazing typically means replacing the windows. This often alters the appearance and character of the property, which isn’t ideal (or even permitted) for many historic properties. Therefore, secondary glazing is often a better option as it is removable and installed internally to avoid altering the external character of the property.

Secondary Glazing in the 19th Century

In the 1800s, we began to see some of the first uses of secondary glazing. Some buildings featured a second double-hung sash window or solid panels. These were fitted with counterbalanced weights in the space under the window. This type of secondary glazing was used to help improve the insulation of both heat and sound. Also in the 19th century, we can see examples of wrought iron casements, as well as timber-hinged casements in northern and central Europe.

Secondary Glazing in the 1960s

Although there are early examples of secondary glazing, it didn’t develop into modern secondary glazing techniques until the middle of the 20th century. Modern double glazing was invented in America in the 1930s. However, secondary glazing can often be much easier to install and can be the best choice for older buildings too.

Aluminium secondary glazing gained some popularity in the 1960s. This is because the aluminium extrusion industry, which had been busy during WWII, began expanding into other markets, one of them being various building products. Aluminium was, and still is, a good choice for secondary glazing systems because it’s lightweight, strong, can be formed into different shapes, and resists corrosion. Some of the leading providers of secondary glazing would also start to offer double glazing as it became more popular.

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Secondary Glazing in the Later 20th Century and 21st Century

Both secondary glazing and double glazing started to become even more popular throughout the second half of the 20th century. In the 1970s, energy crises made secondary glazing more appealing. It offered a way to help homeowners conserve heat to help control their energy bills.

As double glazing became more popular, secondary glazing manufacturers and sellers started to focus more on older buildings. Due to secondary glazing being a great choice for helping to conserve historic properties, it became the preferred option for many. Historic England provides comprehensive advice on secondary glazing and how and when it can be used – for more information, download their full article HERE.

Energy efficiency and environmentally friendly choices have become more important than ever in building construction and conservation. Secondary glazing continues to offer a way to meet modern standards for thermal insulation and reducing carbon emissions.

Secondary Glazing Today

Today, secondary glazing can be an excellent alternative to double glazing. It offers insulation to a building without having to replace the windows completely. This makes it ideal for listed properties or properties in conservation areas, where there might be limitations on how the exterior or interior of the property can be altered.

Secondary glazing systems can be removed if necessary, and they also don’t alter the appearance of the building exterior. This means that installing secondary glazing is often the better choice over installing new windows, which can require you to install expensive windows that closely match the previous windows or are sympathetic to the character of the building. Another benefit of secondary glazing over double glazing is that secondary glazing systems insulate sound better.

Secondary glazing can also be preferable over draught-proofing, providing better thermal performance.

Today’s secondary glazing systems offer a choice of materials and styles. Aluminium is still a popular choice for secondary glazing frames, and timber is often considered as an option too.

There are several options for secondary glazing systems so they can be chosen to fit the requirements of the building. Options include sliding systems, lift outs, hinged systems, and removable, fixed, and shaped systems.

Sliding systems

Sliding systems are available as either horizontal or vertical systems. They are a good choice when ventilation is required. Horizontal sliding systems have two or more panels that slide within the frame on rollers or gliders. Vertical sliding systems have two panels and sometimes use spring balances. Tilt-in vertical sliding systems allow sashes to hinge inwards so that they can be cleaned more easily.

Lift outs

Lift outs can be removed when needed so that they can be cleaned. They are a good option for windows that are fixed or don’t need to be opened often, as well as for windows with unusual shapes.

Hinged systems

Hinged secondary glazing systems are often chosen when the whole window needs to be covered to prevent sightlines on the secondary unit. They come in single or double-leaf designs and are useful for large panes. They allow access to the original window for cleaning or ventilation, but are often fitted with restrictor stays or can feature multi-point locking for safety and security.

Fixed systems

When no access is needed to the window, fixed systems can work well. However, it’s important to think about how to maintain the glass and cavity. Vents can be installed to help prevent condensation between the window and the secondary glazing.

Shaped systems

Secondary glazing systems can be shaped to fit various types of windows so that they match the original style. Although some limitations exist, this option means that any system can be fitted while keeping the building’s original character in mind.

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The Benefits of Secondary Glazing

Secondary glazing offers a range of benefits, both on its own and when compared to other options such as double glazing or draught-proofing.

Thermal insulation

One of the main reasons people choose to install secondary glazing is to improve thermal insulation in a building. Single glazing is typically not very efficient in this respect, but secondary glazing can result in significant improvements. Secondary glazing can help to improve thermal performance with an air gap between the original window and secondary glazing and by helping to prevent air leakage.

Noise insulation

Secondary glazing is also a good option to help improve acoustic performance. When soundproofing is required, secondary glazing can make improvements to noise insulation. A secondary window and airspace between the two windows can make a big difference. Additionally, acoustic glass or thicker glass can help to provide sound insulation.

UV light protection

UV light can be damaging to furniture or other interior features. Installing secondary glazing is an excellent way to control ultraviolet light when a film is used on the glazing. The special film absorbs the UV light to prevent it from causing damage.

Improved security

Secondary glazing is also useful for improving security. When a secondary glazing unit is installed, it creates an additional barrier into the property. This makes it more difficult for anyone to break in or trespass. This improved security is useful when the building use is altered or any time it might be necessary to make the property more secure without altering the original windows.

Secondary glazing has been around for a while, but it’s still extremely useful. It has become one of the best options for making improvements to older properties.