Researchers discover that our windows are cleaner than previously thought – Good news for energy efficiency

Researchers discover that our windows are cleaner than previously thought – Good news for energy efficiency

Britain’s windows are cleaner than they were originally thought to be, researchers have discovered. The finding is important because it will allow architects to design buildings that are more energy-efficient.

The research has been carried out by a team led by Peter Tregenza and Steve Sharples in the School of Architecture at the University of Sheffield. The project was funded by the Engineering and Physical Sciences Research Council.

An important factor in building design is the amount of daylight that will enter through the windows. To calculate this, one of the things that architects must take into account the likelihood of dirt building up on the glass, obscuring some of the light.

‘The amount of light coming through a window can be reduced by as much as half,’ says Professor Tregenza. ‘A lot of this loss is due to things like overhangs and blinds, but a significant amount can be lost because of accumulation of dirt on the window pane.’

Building designers use guidelines to estimate the likely effect of dirt on the penetration of light. However, the information for the guidelines is several decades old – dating from before the Clean Air Act, when there was much more industrial pollution in the atmosphere.

To get an up-to-date idea of how dirty our windows are, the Sheffield researchers selected around 500 windows across the city. Using illuminance meters they measured the amount of light coming through the windows before and after they were cleaned.

‘We found that overall the amount of dirt was much less than the current codes of practice allow for,’ says Professor Tregenza. ‘Windows were much cleaner than we had been led to believe.’

A surprising finding was that dirt on the inside of the window was at least as important as that on the outside – and in some types of building much more important. ‘In one gym which had a swimming pool the windows were absolutely filthy inside,’ says Professor Tregenza. ‘Clearly there was a lot of condensation, with droplets containing particulate matter. We found that around 30 per cent of the light was being prevented from coming into the building.’

Another significant finding was that cleaning the outside of the windows had little impact on the transmission of light because most windows get a good washing from rain. ‘It seems that dirt does not continue to build up indefinitely,’ says Professor Tregenza. ‘If a window starts off spotlessly clean and is then exposed to a normal atmosphere, you get a reduction of light penetration of about five per cent within maybe a month. After that it stays more or less constant, with the rain washing off the dirt as it accumulates.’ This is contrary to received wisdom which says it is necessary to clean windows regularly to prevent significant blocking of light.

‘I think our main finding is important,’ says Professor Tregenza. ‘If the amount of dirt on windows is much less than the codes suggest, it means that effectively windows let in more light than designers predict. This in turn means that it may be possible to reduce the size of windows while retaining an appropriate amount of daylight. This would allow buildings to be better thermally insulated and therefore more energy efficient.’

Based on the results of the work, the researchers have compiled new guidelines o