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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.
The rough and ready method of comparing the energy performance of windows is to use the U value measurement, just as we do with walls, floors and roofs. Traditional windows, with a single pane of glass in them, have a U value in excess of 5. Double glazing used to score over 3, but, over the years, the manufacturing process has undergone a number of improvements and currently the Building Regulations insist that any window you install today should have a U value no worse than 1.6.
These improvements have been brought about by the introduction of:
Now, a U value of 1.8 may look good when compared to what we have been fitting relatively recently, but compared to the U value demanded for walls – currently less than 0.3 and due to fall even lower – you can see that windows remain weak spots in the overall thermal efficiency of a building envelope. Hence the tremendous pressure to improve their performance even further.
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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.
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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.
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.
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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.
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.
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.
So why are we even thinking of going triple glazed? It’s all to do with the Code for Sustainable Homes, which the Government is using as a roadmap for us to move towards building zero-carbon homes by 2016. The Code more or less requires us to adopt the ultra-low-energy PassivHaus standard which demands windows with a U value no higher than 0.8 — triple glazing with insulated frames.
Window manufacturer Scheiwiller in Switzerland is producing quadruple glazing (i.e. four panes; three cavities). There are, however, technical problems with simply adding more and more panes of glass: not only does the resulting window become ridiculously heavy, but the additional panes stop light coming through. In fact, in terms of energy efficiency, there may even be a fall-off as the amount of sunlight the window can absorb is reduced.
It is more likely that future developments in window technology will evolve around new coatings, or phase change materials which absorb heat in sunlight and release it at night. Look out for GlassX, produced by another Swiss company, which is already manufacturing a product that does this.
An alternative option that might make more sense is to revisit the traditional practice of drawing curtains across windows after dark. It may be low-tech, in comparison with glazed cavities filled with krypton, but it’s something of a natural British habit and it does cut down on heat loss. Perhaps it’s time we paid attention to improving the heat retention characteristics of curtains and blinds, rather than continuing to engineer glazing units to ever lower and lower U values.