Triple Glazed Windows: Do They Make Sense?

Triple glazed windows are becoming an ever more popular option from 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.

It’s a tricky decision that faces self-builders when they come to specify their final finishes and, as triple glazed windows cost around 20% more than double, why should anyone choose it? Advocates point out that its more about quality and comfort and that self-builders should consider triple glazing for the same reasons they install underfloor heating: it’s just better.

Here’s what you need to consider when weighing up the pros and cons of if triple glazing is worth it.

What is a Triple Glazed Window?

In short, and 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. 

The third pane of glass located half way between the inner and outer panes of double glazing creates two air locks which improves the energy performance of regular double glazing by around 50%.

Energy efficiency is further improved by variables such as the type of air or gas used in the space between the panes, warm edge spacer bars around the perimeter to reduce thermal bridging and different coating on the glass to reduce energy loss from the inside. The frames themselves also have a large bearing on the overall performance (as well as the insulation). Look out for insulated frames that have a good airtightness rating.

Is Triple Glazing Better than Double Glazing?

Energy Performance

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.

Glass manufacturers have mastered the art of coating and tinting glass with all manner of finishes which can keep the heat in, unwanted sunlight away, reduce glare and even self-clean, too. The net result of this glass engineering means the U values of glazing has been reduced dramatically. 

• Single glazed windows can have a U value of around 5.0W/m²k
• Double glazed windows used to score over 3 and can now achieve 1.4 (Velfac have 1.36). As a result of these improvements in the manufacturing process, Building Regulations now 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

How Much Do Triple Glazed Windows 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.

The energy savings from switching from single to double glazing are considerable, but the difference between a U value of say 1.4W/m²k (double glazing) and around 0.8W/m²k (triple glazing) is much less and would account for perhaps 5% improvement in the overall energy performance of a new house. (Windows conventionally account for around 20% of the overall heat loss.) Translate this saving into pounds sterling and you might expect the switch to triple to save you between £20 and £40 a year on heating bills depending on the size of your house.

It doesn’t add up in terms of conventional payback calculations; this is not a scenario in which you’ll see the additional cost of triple glazing ‘paid back’ in 10 to 15 years against the savings made of your energy bills.

However, the increase in energy performance is significant and well known with whole Passivhaus designs (rather than just switching to triple glazed windows in a regular house) boasting energy bills as low as £120 per annum, more than enough to recoup the initial investment after enough years.

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

Considering Window Ratings, 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.

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.

However, triple glazing advocates tend to argue that crude calculations of initial costs versus U values and energy bills. The big plus to triple glazing is that it adds an intangible comfort factor to life indoors in the colder months which, once experienced, is something most self-builders are willing to pay more for.

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.