Window Energy Ratings or U-values?

Back in 2005, the Glass & Glazing Federation (GGF) took over the British Fenestration Rating Council (BFRC) and then gained Government acceptance of the then new, Window Energy Rating scheme (WER’s). At the time these ratings became a means of compliance with Building Regulations. As all WERs are based on the same standard sized window, and enable direct comparisons across various manufacturers' products, this helps customers in product selection. By combining a window’s solar gain benefits, its air permeability and thermal efficiency in a simple calculation offers an easily understood ‘banding’ of performance, the result of which is something that the wider public already understands from the sale of domestic electrical goods. As we now move to higher performance windows and the need to thermally enhance our complete building fabric, are WER’s still the best measure?

With the next announced change in Building Regulations just two years away, when the Industry will be expected to offer window U-values of below 1.0 will this mean that the current WER banding becomes mostly obsolete? When first launched, the WER scheme offered ratings across a band of A to E, more recently the band is running with two further ‘A’ bands of ‘A+’ and ‘A++” which recognise higher performance windows. Will bandings need to be realigned if we decide to continue to use them in the future?

In reality, the choice of window performance on WER’s alone is flawed. When the energy performance label was introduced for electrical goods back in 1995 products bought and used across the UK would perform very much the same in any home, the same cannot be said for WER’s. Windows chosen on a WER will be made to measure, in various configurations, installed in various constructions, located in different geographic locations and in various orientations. Windows facing South will realise a high solar gain, whilst those facing North will experience very little so, the ‘best’ WER performance window chosen may not be the best performing in use and therefore not the most cost effective choice.

It is well understood today that a holistic optimisation of the complete building fabric is more beneficial in terms of energy and environmental savings than a component-by-component approach. The provision of a calculated window U-value for a given configuration can then be used in the formulation of a building's thermal performance, a WER cannot.

Homes in the future will have to be built to and refurbished to a very high thermal standard which then require very little energy to heat, indeed, some high performance buildings could actually generate more energy than they need, producing excess ‘green energy’ which can then be used elsewhere.

There are various schemes and specifications that enable architects to construct or renovate buildings to meet very high thermal standards whilst maintaining a healthy internal environment. Passivhaus is a well known specification which is rapidly becoming the specification of choice for architects. Based on a ‘fabric first’ approach, the complete building fabric must be carefully designed with high insulation materials taking great care to eliminate cold bridging either inside a building element or where various building elements interface. One of the challenges of good fabric design within Passivhaus, is the window frame interface to the structural reveal of a building, particularly in an existing building.

Whilst not all our future buildings will be built to the Passivhaus specification, the basis of this specification is logical and there will be alternate specifications agreed upon. Scotland has already agreed that in a couple of years all new homes will be built to their version of a Passivhaus standard.

So, whilst there has been a swing in the past to use WERs as a simple tool of energy performance, the industry will swing back to using the U-value as this measure will be needed to design good thermally efficient spaces within buildings. In future, it is not going to be acceptable to simply place a high performance window into a thermally inefficient reveal. Not only will this window not perform as expected, but it’s also simply not cost effective.

Whilst WERs are basically a tool for the public to be aware of energy performance of various products, we can see that in the future window replacement will be coupled together with building structure thermal improvements, this will likely be seen in new building regulations that will stop the simple replacement of windows in an existing structure. Far from restricting sales of windows and doors for home improvement companies, there is huge potential for these companies to rise to the needs of the country to help improve all our building stock to meet our commitment to being carbon neutral by 2050.