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 now the manufacturing process has undergone a number of improvements, current 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:

  • Wider cavities between the two glass panes (16mm is the optimum)
  • Low-emissivity coatings being added to the glass to stop heat escaping
  • The cavity being filled with an inert gas, usually argon
  • Designing out cold bridges, such as aluminium spacers, surrounding the glazed units

A U-value of 1.8 may look good but compared to the U-value demanded for walls (currently less than 0.3) you can see that windows remain weak spots in the overall thermal efficiency of a building. Hence the pressure to improve their performance even further.

Should we switch to triple glazing?

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 Uvalue 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. Also, the gas krypton tends to be used instead of argon leading to higher manufacturing costs.

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. These cause draughts, draw heat away from you if you sit next to them, and result in streams of condensation running down the panes. So 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.

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 more comfortable. So whilst triple glazing may make little difference to your heating bills, you will notice the difference inside the house.

Heat Absorption

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. 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.

Will it stop there?

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 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.

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