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Double glazing works by trapping air between two panes of glass. This trapped air creates an insulating barrier which provides three functions. First of all it prevents heat loss or gain through the window because the barrier insulates the room from differing air temperatures at either side of the glazing unit.

Heat Loss Reduction – Understanding U-Values

This is where triple glazing comes in because if double glazing makes a modern house more comfortable to live in then triple glazing makes it even more so.


The physics involved here has 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 difference between different glazing alternatives

The energy efficiency of windows is measured with a ‘U value’ and the lower this value the better.

Single Glazing has a U value of 5, older double-glazing about 3 and new modern double glazing a u value of 1.6, which is mainly due to improved methods of double glazing compared to older models.

These improvements have been made through optimisation of the window cavity, the use of low emissivity coatings, using inert gas to fill the gap, and using aluminium spacers or ‘cold bridges’.

All this has brought down the u value of modern windows considerably, so does that make triple glazing worth it, if double glazing is now so much better? Well, walls have a u value of around 0.3, so it is clear that windows have a long way to go before they are no longer a weak point in the efficiency of the building.

In Scandinavia, triple glazing is now pretty much standard, with modern triple glazing taking the u value of the windows down to around 0.8. This is a considerable improvement on even modern double glazing. Lots of new homes are now being built with triple glazing and it is generally acknowledged as being better, but compared to top double glazing, the payback period is very similar.


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