I am trying to undersatnd the use of Low/E glass in double glazed units. The WERS rating indicate a 25% reduction in the U value by using a Low/E sheet. My understanding is that the improved performance comes from the fact that it reflects heat in the long wavelength region back into the house.

The question is, which sheet should have the Low/E film? I know the low/E film should face the gap so it is protected, but should it be on the inside sheet or the outside sheet? My supplier says it should be on the outside because we are in a hot climate (Melbourne) but I am not so sure.

I have looked further at it, and it seems that the U value is not affected by the direction. The film reflects from the back just as well as from the front. What does change is the Solar Heat Gain. Effectively the Low/E film absorbs heat when sunlight falls directly on the window so the Low/E sheet will be warmer than the plain sheet. In winter you would like the warmer sheet on the inside of the unit, in summer you want it on the outside sheet.

My understanding is that in Europe the Low/E sheet goes on the inside to give better winter performance, but in Australia they put it on the outside to give better summer performance.

My feeling is that this is rather simplistic. If you are spending more on heating than cooling, you should put the Low/E sheet on the inside. Also, If your windows are protected from the sun in summer (e,g external blinds or eves) you should put the Low/E sheet on the inside.

Can anyone confirm whether my logic is basically correct and what others do in cold and warm climates?

Hi
The surface position on which the low-E glass will be placed is dependent on the climate. Ideally for warm climates where summer heat reduction is a priority, the coating should be on surface 2 for maximum performance (Note: the surface number is counted from the external surface of the outer pane [1], right through to the external surface of the inner pane [4]). The coating on surface 2 minimises heat gain because it reduces heat build up in the air gap and heat transfer to the interior.

But it works both ways, in both positions, just more effective in the positions mentioned. So really it should have two u-values, one for each location.

Ed