Latest Windows Keep Their Cool

If you’re in the market for new windows, you’ve probably noticed that the subject isn’t as simple as a wood frame and a few panes of glass--and it hasn’t been for some time. Over the last few years, however, advances in technology have shown that windows can play an even more important role in conserving energy than thought before.

Back around 1980, people who replaced their old windows with double-glazed units were at the forefront in the fight to save energy. Those who opted for triple-glazed windows, the limit of technology at the time, were considered energy fanatics.

But even they were forging a weak link in the energy chain of their homes because those triple-glazed units had insulation values of about R-3, while the walls that held them had R values that ranged from 11 to 19. In many well-insulated houses, therefore, the windows were simply holes in the wall where vast amounts of energy escaped.

In fact, the average home loses about 25% of the energy it consumes through heat loss at windows.

Today, things are different. Most windows are still energy drains, but there are units available that are much more energy efficient than the glazing systems of the past. Some windows have insulation values as high as R-8. But these currently account for a very small part of the window market.

Plain double-glazed windows make up the bulk of the market, but, fortunately, most window manufacturers and just about all the better-known companies sell some version of high-performance glazing.

Manufacturers are able to offer better insulated glazings because of the development of low-e (low-emissivity) coatings. Basically, a low-e coating does a good job of reflecting radiant heat--the kind of heat given off by bodies, furniture and some heating systems.

When applied to a window, the coating reflects the heat back into the home, raising the R value of the window. Some low-e coatings are also used to reflect outside heat radiated by the street and buildings.

Some coatings are bonded to the glass, others are incorporated into the glass and still others are applied to a film that’s suspended between two panes of glass. There are pros and cons for each type.

For example, coatings bonded to the surface of the glass, called soft coat, offer the best insulating properties. However, coatings that are part of the glass, called hard coat, are more durable.

The type of coating probably doesn’t mean all that much to the homeowner buying new windows. What does mean a lot is that the low-e coatings are invisible and windows that contain them have insulation values of about R-3.

That means that a double-pane window with a low-e coating has the same insulating properties as a triple-glazed window, but the low-e window is about 50% lighter.

To increase the thermal effectiveness of low-e windows, manufacturers can fill the air space between the panes with an inert gas, usually argon, but others are used. These gases are more effective than air at decreasing heat loss through conduction. When combined with low-e coatings, gas-filled double-glazed windows can have an insulation value of around R-4.