Somewhere over the rainbow

Justin Davies, general manager for The Window Store explains the jargon behind window and door energy ratings and asks whether triple glazing should be a genuine consideration when looking at glazing options.

Know your ratings!
August saw the number of signed Green Deal plans rise to 132, up from 36 the previous month. Energy minister, Greg Barker said that the figures showed “genuine consumer interest” in the scheme. So with energy bills creeping ever higher with no end in sight, and a hint towards higher council tax and stamp duty for those living in inefficient and leaky homes, there’s no escaping the pressure to make our homes more energy efficient.

Glass and glazing plays a vital role in the efficiency of a property – from windows and doors to conservatories and glass extensions. Many people will be au fait with the sliding ‘rainbow’ scale more commonly associated with household appliances. Stating the energy efficiency rating of a product, with the dark green coloured band being A (the best rating) down to dark red, G (the lowest rating and therefore the least energy efficient), what many people don’t realise is that this rating is also applicable to both windows and doors.

There are a number of factors to look out for when establishing the environmental performance of a door or window – the energy index and the u-value; these combine with air leakage and in some instances, solar factor, to give either a window energy rating (WER) or a doorset energy rating (DSER). Remember these ratings relate to the whole unit not just one individual component.

To understand DSERs and WERs, people must first get to grips with U-values. A U-value is an independently verified measure of heat transfer through a building element, in this instance a door or a window. The lower the U-value the better the material/product performs as a heat insulator, less energy is lost, and the more efficient and economical the unit.

Energy index
The Energy Index (kWh/m²/year) is one of the most important values, as it sets out the amount of energy lost or gained by a window. The value is displayed in terms of kilowatt-hours per square metre of glazing per year. Energy lost is displayed as a negative figure, and energy gained by a positive number. So the higher the number, the greater the energy conserved – and the more money saved on energy bills.

Improved rating with Low E
Significant proportions of glazing can have a dramatic impact on the energy and thermal efficiencies of a property; even something as simple as an entrance door with between 5 per cent and 20 per cent glass. One solution is to install Low E glass instead of standard glazing. Low Emissivity glass, Low E for short, is a more energy efficient glazing alternative. It dramatically reduces heat transfer and reflects interior heat back into a room. Low E glass is also recommended for properties that are north or east facing as a greater proportion of heat loss would be expected.

Triple decker
Triple glazed windows can achieve a U-value of 0.8W/m²K or less and can reduce heat loss by nearly a third more than existing double glazing. Already a popular choice in colder climates like the Sweden and Norway, triple glazing is slowly starting to build momentum in the UK. There are two key factors that could cause interest to be piqued this side of the North Sea – the change in the UK’s weather with more extreme, erratic bouts of rainfall, snow and sun, and the continued rise in energy costs.

Exceeding the current ‘A’ WER, triple glazing achieves 0.8 U-value, well ahead of the equivalent typical double glazed option of 1.5 for an A rated window. This means for the average homeowner the thermal insulation of triple glazing is currently unrivalled.

Steamy windows
Insulation is currently the easy-win of energy efficiency but by wrapping buildings in insulation and not upgrading the windows to a similar thermal efficiency it is essentially turning the windows into cold spots. At night these cold spots draw heat out of the room and off the residents causing condensation to run down the glass unit. So for an insulated property to perform to its full potential, the thermal performances of the glazing and window frame have to be of a similar standard.

Everyone needs good neighbours
As well as its thermal properties, triple glazing is also known for its sound reduction. The additional pane of glass, combined with the argon gas filling work to boost the acoustic performance of the window, reducing the amount of sound leaking into the property. This is ideal for those living close to roads, railways or in busy locations.

Glass trends
It’s hard to watch a home improvement or renovation programme without someone including a glass ‘structure’ to link the old and the new, or removing the rear face of the existing property or extension and popping in a glazed wall. With these expanses of glass the thermal performance of the building could potentially be reduced, while also having a detrimental impact on security. Triple glazing would not only offer insulation benefits but the additional pane of glass also gives added security against glass break-ins.