Triple glazing

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.

Opinion is divided. Triple glazing is widely used in cold climate countries like Sweden and Norway, and the ultra-low energy PassivHaus standard requires triple glazed windows with a U value of no more than 0.8. 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 0.8 sounds very impressive, the additional energy we 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.. 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:

• Next to a single glazed window, the internal surface temperature is around 1°C.
• Next to a double glazed window (2000 vintage), the surface temperature is around 11°C.
• Next to a modern, energy-efficient double glazed window, the surface temperature is 16°C.
• Next to a triple glazed window, with a centre-pane U value of just 0.65, the temperature is 18°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. The PassivHaus standard promotes the use of triple glazing for precisely this reason – i.e. comfort – although it also states that the frames themselves have to be insulated, and the windows need to be mounted in the correct location within the wall assembly.