Triple Glazed Windows: Do They Make Sense?
Triple glazed windows are becoming ever more popular for today’s windows suppliers – particularly in the aluminium space – but long-standing concerns about the real benefits outweighing the perceived costs are still on homeowner’s minds.
Whether you’re designing a new build or looking to renovate an existing home, glazing massively impacts the aesthetic impact of a home, but it is also crucial to consider the thermal efficiency of the different options.
What is Triple Glazing?
As might seem obvious, triple glazing contains three panes of glass within a sealed frame, just as double glazing contains two. Between each pane is a pocket of air or inert gas, such as argon; argon is heavier than air and works as an insulator for both noise and heat.
Is Triple Glazing Better than Double Glazing?
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.
- Single glazed windows have a U value in excess of 5
- Double glazed windows used to score over 3 but as a result of improvements in the manufacturing process (more below), Building Regulations insist that any window you install today should have a U value no worse than 1.6
- The Passivhaus standard requires triple glazed windows with a U value of no more than 0.8 but there are some suppliers who claim to achieve just 0.5
The U value demanded for walls is currently less than 0.3 so 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.
Manufacturing Improvements in Glazing
Improvements have been brought about by the introduction of:
- Wider cavities between the two glass panes 16mm is the optimum distance
- 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
Benefits of 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.
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.
- Thermal comfort levels: while 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 (the surface temperature of a modern, energy-efficient double glazed window is 16°C in a room heated to 21°C where triple glazed can offer 18°C).
- Acoustic performance and noise reduction: double-glazed windows can achieve an acoustic performance in the range of Rw32. This can be improved into the low 40s with some types of triple glazing — the difference on a busy road between a good night’s sleep and a disrupted one.
- Reduced risk of condensation: the lower U values of triple glazed windows helps towards minimising internal condensation issues as the heat is kept inside the building and typically stops the cold external temperature reacting with internal warm air resulting in condensation. (However, the main cause of internal condensation is high internal humidity levels (tumble dryers/wet rooms and lack of adequate purge/ventilation)).
Negatives
- Can be more costly
- Their heavier frames can damage walls if they’re not properly supported
- They have to be installed correctly, otherwise their performance can be compromised
How does Triple Glazing Impact Heat Absorption and Solar Gain?
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.
How Much Does Triple Glazing Cost?
Although triple glazed windows long held the reputation of costing almost double the amount of double glazed, European manufacturers such as Velfac and Internorm are so busy creating triple-glazed windows for their European customers that it’s a bit of a pain to create double glazing for us Brits. As a result, the extra price of the glass can often be measured in the low single percentage points — if at all.
What’s more, the increase in energy performance is significant and well known with Passivhaus designs boasting energy bills as low as £120 per annum, more than enough to recoup the initial investment after enough years.
Will Triple Glazing Add Value to my Home?
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.
With the government increasingly under pressure to reduce carbon emissions and global warming being ever more important in the public’s conscience, properties that reduce energy usage through build design or upgraded features (such as windows and doors) are looked on favourably in this day and age.
(MORE: How to Add Value to Your Home: 20 Genius Tips)
Should I Switch to Triple Glazing?
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.
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.
Further Innovations in Glazing
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.
