Energy Saving Features of Insulated Gas-Filled Replacement Windows

If you need replacement windows for your home you are going to be confronted with a dazzling array of options and sales pitches. Before you go shopping for replacement windows you will want to understand the key features and terms listed below.


Most replacement windows have several layers of glass. Two layers are common but some go to three and even four layers. The dead air space in between the layers helps reduce the flow of heat through the window. Usually there is a spacer around the edge of the panes of glass that hold all of this together.

IGU (insulated glass unit) is simply the term used to describe this package of layers of glass, spacers, inert gases and glass coatings. The frame of a replacement window, in turn, goes around the IGUs.

Low-e Glass

Low-e is short for low emittance. It means one or more of the panes of glass in the window is coated with metal oxide or metal. The coating is extremely thin so you don’t normally see it. You might notice a slight tint with low-e windows but for the most part they are transparent.

They work by reflecting heat back off the glass. Sounds pretty simple doesn’t it? But the complicated part is when you start to think about which way you want the heat to be reflected.

In really hot climates you want the heat to stay outdoors right from the start. You want the heat of the sun to be immediately reflected off the outside of the windows. In really cold climates you want just the opposite. You want the sun to shine through your windows and bring all the heat with it. Then you want the window to keep the heat in by reflecting it back inside your home.

In Florida, for example, you want what is called low solar gain low-e glass. It helps keep the inside of your home cool by keeping the sun’s heat out. If you live in northern Canada, on the other hand, you want high solar gain low-e glass. It helps keep your home warm by letting the sun’s heat in and keeping it there.

Some window manufacturers have even come up with coatings that they claim can do both. These coatings are called spectrally selective. They keep heat out during the summer and let the sun’s heat in during the winter.

For many applications, low-e glass is the most cost effective option for replacement windows. However, make sure you discuss it with your contractor or installer to get the best-fit low-e glass for your climate.

Argon Filled

The inert gas argon has been used in windows for a long time. It is cheap, safe and a significant way to increase the insulation value of multi-pane windows.

The air between the layers of glass in a window is replaced with argon. Argon is a better insulator than air so there is less heat loss through the window. Also, since argon is heavier than air it will reduce the convection loss of heat inside the window.

The performance of argon filled replacement windows will vary with other factors but generally you will see a 10 to 30 percent improvement going with argon over air.

Krypton Filled

Krypton is another inert gas that is sometimes used in replacement windows. It is better than argon but it is also significantly more expensive.

The increased performance of krypton is usually not worth the increase in price. However, since krypton requires less thickness to be effective it is often used in very thin windows. Sometimes argon and krypton are used together to give a better performance/cost balance.

For most people, the best combination for replacement windows is double glazed IGUs, argon filled, with low-e glass optimized for climatic conditions.

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