Emily Page of window fabricator Stedek, charts the evolution of energy-saving windows, from early innovations to low carbon solutions for future homes.

Windows might only make up a tiny part of the average building – but when it comes to sustainability, their impact is huge.

A brick wall, for example, has a fraction of the U-value that a window provides. However, since few of us fancy living in brick boxes without any light, we have to live with the fact that glass is inevitably less thermally efficient than other types of structure.

The good news is that window technology has advanced drastically over the last 40 years. It wasn’t all that long ago, historically speaking, that single-glazed timber framed windows dominated the market. 

While aesthetically pleasing, they present significant sustainability challenges. Together with single panes of very basic glass, they unsurprisingly offered very poor energy efficiency, and created major thermal weaknesses in the structure of the building.

This was followed by the first wave of aluminium windows – a world away from the modern, high-performance aluminium windows on the market today. Still largely single-glazed, they offered a much more durable option than timber – but as a metal, aluminium’s thermal conductivity posed a problem. Early aluminium windows struggled to retain heat, leading to higher energy bills and carbon footprints.

The double-glazing boom

The ‘80s brought two significant shifts – the development of double glazing, and the rise of PVCu.

Double glazing dramatically improved thermal efficiency. And PVCu did the same – advanced profiles with multi-chambered designs reduce energy consumption for heating and cooling. They were also durable, cost-effective and recyclable, making them an excellent choice.

Innovations in glazing

However, while the frame material plays a crucial role, the glass itself significantly impacts a window’s sustainability. 

Advancements in glazing technology have unlocked new possibilities for energy-efficient windows. Low emissivity features a special coating that reflects long-wave thermal radiation (heat) back into the building, reducing heat loss in winter. 

Solar control glass allows visible light to pass through while reflecting infrared radiation from the sun, keeping the interior cooler in the summer months.

Replacing air in the gap between panes with argon gas which provides lower thermal conductivity, further enhancing insulation.

Small components – big difference

Spacer bars also play a crucial yet often overlooked role in window performance. These thin strips separate the panes of glass within a double or triple-glazed unit. 

Traditionally, metal spacer bars were used – however, these brought the same problem as aluminium windows. They conducted heat, creating a “thermal bridge” that reduced the effectiveness of the glazing.

More advanced warm-edge spacers, made from materials like foam or composites, have significantly lower thermal conductivity. They minimise heat transfer between the panes, enhancing the overall performance.

Great looks, great performance

During that initial double glazing boom, there weren’t many who’d say they preferred the look of PVCu to timber.

PVCu windows were more durable, cost-effective and easier to maintain – but when it came to aesthetics, they were extremely basic. You could have any colour you liked, as long as it was white.

In choosing them, homeowners and housebuilders were having to make a trade-off – worse looks for better performance. Today, that trade-off is no longer necessary. Manufacturers have made huge strides, allowing them to deliver high-performance windows that also provide exceptional aesthetics.

It means that the window products available today offer better energy efficiency than they’ve ever done – without compromising on performance in other vital areas too.

Emily Page is director of Stedek