As you might expect, the Norwegians, Swedes and North Germans have been leading the way in improved window insulation because they get long hard winters. How do you measure the insulation performance of windows? There is an international standard which rates the window, not just the glass, but also the frame, for insulation, and this is called the U-value. The better the insulation the lower the number.
From over 5 to 3, and then to 1.2
The U-value for a traditional single-pane window was over 5.0, and by 2000 double glazing was achieving values of 3.0, which was a big improvement. However, in the interests of reducing energy consumption, the EU and European governments are demanding lower values, led by the Scandinavians and Germans.
Up to about the year 2000, double glazed windows had fairly narrow voids between the two panes of glass, so although the improvement in insulation was significant, people living in cold climates wanted better insulation. The result was that the distance between the two panes was increased, the cavities were filled with gas, and then coatings that reduced the transmission of heat were applied to the glass. With these improvements, the U-value has been reduced to about 1.2.
However, the German PassivHaus standard requires a U-value of 0.8, a major reduction over the best double glazed windows. It may be unnecessarily low for the mild Irish climate. To reduce the U-value of windows to 0.8 you need triple glazing. However, triple-glazed windows that meet this standard are quite a bit more expensive than double glazed windows.
One of the reasons for the demand for improved insulation is that people would like a more uniform comfort level in the home. People don't like the fact that when they stand before a window, it is colder than when they stand near a wall. It is the old story: when our comfort improves we take it for granted, and want more. Of course, a better comfort level can be a sales feature for a new house, or an older house being sold.
Here are some figures that show the difference in the air temperature near the inside of a window:
Single-glaze window 1 deg C
Double-glazed window (made in about 2000) 11 deg C
Double-glazed window, latest design 16 deg C
Triple-glazed window 18 deg C
There is very little gain in comfort with triple glazing over double glazing, but a very big gain with double glazing compared with single-glazing. It can be argued that as it is coldest at night, the use of heavy full-length curtains can do almost as much as triple glazing to improve comfort in the home when it is coldest.
Even so, there is definitely a trend toward the use of triple glazing in the northernmost countries in the EU, mainly as a result of changes in legislation and building regulations, which are demanding better insulation of windows.
Whether you should choose triple glazing over double glazing depends on a number of factors such as your energy costs, how comfortable you believe your house is, and whether you live in a cold area or a milder one. However, there is extra cost both in the triple-glazed units themselves and also in the amount of energy needed to produce the windows. Some experts doubt whether these costs outweigh the advantages of triple glazing over double glazing.