VELUX calls for new 'energy balance' rating to replace focus on U-values

30 April 2009

VELUX is calling for a review of the current focus on U-values (thermal heat loss) when measuring a building's energy efficiency. It is urging architects to look beyond thermal heat loss when selecting materials, especially when considering the use of glazing.

The debate around triple glazing, and whether this should always be preferable to double glazing, is causing much controversy within the sustainability sector. Based on U-values, triple glazing should always take precedence, due to its potential to further minimise heat loss. However, VELUX believes that simply focusing on obtaining a good U-value is a shortsighted approach and is at the expense of a building"s potential heat gain and daylight admittance.

Kevin Brennan, Head of Sustainability, VELUX Company Ltd, comments:

"The quest for low energy buildings is just and timely, but we would urge building designers to take a more holistic approach to their energy strategies. To simply base decisions around achieving a good U-value, without taking into consideration the benefits of solar thermal gain and good day-lighting to reduce energy consumption in the first instance, is missing the point.

"The drive towards triple glazing could actually have a detrimental effect on the ultimate energy efficiency of a building. Glazed areas have three main properties: thermal heat loss (u-value), thermal heat gain (g-value) and daylight admittance (tv- value). Focusing on just the stated U-value of any window will only measure one of these factors (heat loss), while ignoring the benefits of the other two."

Instead of a focus on U-values, VELUX is calling for the introduction of a new 'energy balance' rating, which takes into consideration not only a building's potential heat loss, but the opportunity to maximize on its potential heat gain and use of natural daylight, offering building designers much more than a simple heat loss metric.

Brennan, adds:

"The primary purpose of glazing is to allow natural sunlight into a building, to both heat space and provide daylight. However, a triple glazed window can actually reduce daylight by approximately 11 - 15%. Equally, it can reduce the passive heat gain by 25%. When the sun is our most abundant source of renewable energy, it does not make sense to ignore its potential. Instead, it would seem sensible to use triple glazing on Northerly facades, where natural daylight is at its least, and double glazing on Southerly facades where the natural renewable energy of the sun can be captured to reduce artificial heating and lighting demands.

"A building should breathe, be naturally lit and have adequate natural ventilation for comfort and health. The use of triple glazing should be considered as part of this strategy but the U-value should not be relied upon as the only metric of performance."

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