Bit by bit, we are being encouraged to switch from double to triple glazing. You may not have noticed yet but the pressure is on to improve the energy performance of windows and we are now reaching the stage where mere double glazing will no longer be enough.
However, opinion is divided over the real value of triple glazing.
Triple glazing is widely used in cold climate countries like Sweden and Norway, and U-values as low as 0.8W/m2k can be achieved. But, to get a window with such a low U-value, you have to not only switch to triple glazing but also insulate the frame itself, as well as using more expensive manufacturing techniques – the gas krypton tends to be used, instead of argon.
Now, though a U-value as low as 0.8W/m2k sounds very impressive, the additional energy you are saving is minuscule whilst the payback time for triple glazing (like double glazing) is high compared to other energy-efficiency improvements. But, there is a little more to it than this.
The key benefits are really to do with comfort. If you insulate the walls, roof and floor of a house, and you ignore the glazing, you end up with cold spots surrounding the windows at night, which cause draughts, draw heat away from you if you sit next to them, and result in streams of condensation running down the panes. So, in essence, the standard of glazing has to match the standard of the insulation elsewhere in the house, so that the warm wrapping around the house performs consistently.
Which is where triple glazing comes in. Because if double glazing makes a modern house more comfortable to live in, triple glazing makes it even more so.
The physics involved here have been worked out in Germany by the PassivHaus Institute. It has shown what happens to surface temperatures on various forms of glazing when it gets really cold outside, and the internal air temperature is designed to be at 21°C:
So you can see that whilst a double glazed window is perfectly adequate, a triple glazed one is just that much more comfortable, because it hangs onto heat just that little bit better. So whilst triple glazing may make little difference to your heating bills, you will notice the difference inside the house.
To make things more complex, windows behave rather differently to walls and roofs in that, when the sun is shining, they are capable of absorbing heat. In fact, the very best double glazed windows are already capable of being net heat contributors over the course of a heating season. In contrast, triple glazed windows slightly reduce the heat absorption characteristics of a window.
To reflect these complexities, the British Fenestration Rating Council has devised a scheme for the energy labelling of windows, from A down to G. The top rating is reserved for windows that are reckoned to absorb as much heat as they lose, and they include both double and triple glazed windows.
The upshot of this is that there are many who argue that triple glazing simply doesn’t make sense in a climate like ours. Triple glazing is more costly to produce, produces much heavier sections and has an embodied energy approximately 50% higher than double glazing.