U-volution of an industry

U-values indicate how much heat escapes from a window. This simple measure has been the subject of progressive improvement in recent decades. As both the sector and the end-consumer have become more aware of sustainability and energy efficiency, window technology has evolved to meet the twin considerations of cost and environmental impact. Mark Gajda at Rehau explores where we have been, where we are now, and what the future holds for the energy-efficient window.

Anything from animal hide to paper was used to fill windows. Glass was first used by the Romans, and then much later the rise of industrial plate glass gave us the first truly modern windows.

Throughout the 1950s, 1960s and 1970s the standard UK house featured single-glazed windows, with just one pane of glass separating the indoors from the element. Insulation was very limited, with typical U-values in excess of 5.5W/m²K. Thick curtains and shutters were essential to try and keep in as much warmth as possible.

It wasn’t until the mid-1980s that double-glazing really took off. The addition of a second pane of glass, with an insulating layer of air in between, boosted heat retention, making it much cheaper and easier to heat homes. U-values were down to some 2.7W/m²K meaning some 50% less heat was escaping, with noise reduction and security also bolstered by the extra layer.

The following decades saw additional advancements – the use of inert gases in double-glazed sealed units, advanced coatings and frames – which brought U-values down even further, to as low as 1.5W/m²K.

In 2002, Part L of Building Regulations came into force. Windows for British dwellings now had to have a U-value of 2 W/m²K or below. Then in 2004, the introduction of Window Energy Ratings (WER) provided a new way to assess and compare the energy efficiency of windows, using a scale from A+ to G.

While this had its advantages, the grading system is based on heat loss and solar gain for a single location. The same window can have different ratings depending on which part of the country it is installed in, due to different geographical and climatic conditions. Group schemes such as the BFRC Simple Energy Licence (SEL) were then introduced giving windows design greater flexibility within a WER band, simplifying simulations required and speeding up the calculation process.

Going further than all previous standards is Passivhaus, which is an advanced energy efficiency concept that has now become a worldwide standard. Passivhaus creates buildings with ultimate energy efficiency and comfortable living through superior indoor air quality and thermal comfort. With its increased popularity, it presents a challenge to architects, engineers and specifiers.

One of the pioneers, Dr Wolfgang Feist, explained the standard in these words: “The heat losses of the building are reduced so much that it hardly needs any heating at all. Passive heat sources like the sun, human occupants, household appliances and the heat from the extract air cover a large part of the heating demand. The remaining heat can be provided by the supply air.”

Improved glazing options and the development of multiple chamber profiles now make it possible to produce windows with better heat retention than ever before. Windows made using the Rehau Geneo 6 chamber profile can achieve a U-value as low as 0.7W/m²K. Passivhaus-certified, these profiles mark a new level of efficiency helping to minimise the energy needed to obtain and retain warmth.

Rehau’s Geneo profiles are made from a high-tech fibre reinforced composite materials RAU-FIPRO, created by Rehau and is also fully recyclable. This material results in the brand manufacturing PVC window and door profiles that boast high stability, torsional stiffness and static properties – previously not possible without the addition of steel reinforcements.

Rehau’s Geneo profiles are among the most energy-efficient profiles available for windows ranging from low energy houses to Passivhaus standards.

Rising living costs, climate change, renewed focus on security, and the rise of the connected home, all mean the window of the future will have to be more energy efficient, tougher and smarter than anything we’ve seen so far.