Gas fill

Window manufacturers fill the space between window panes with gas to improve thermal performance. 

FAIRBANKS — The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life. 

If you are window shopping this summer, you might notice many window manufacturers advertising models with a gas fill. 

Filling the space between window panes with a heavier-than-air gas improves the thermal performance of the window. The most common types of gas include argon and krypton.

The reason for adding gas is that air currents can form in the thin space between window panes. 

In a process called  convection, the air next to the warm side of the window heats up and rises to the top of the window. 

As this air is pushed toward the cool side of the window, it transfers heat to the cooler glass pane and then falls to the bottom of the window. This flow of air up the warm side of the window and down the cold side transfers heat through the window. During the winter, you can lose a lot of heat this way.

Some windows are filled with atmospheric air, which is composed mostly of nitrogen. 

However, manufacturers also use heavier gases such as argon to improve the thermal performance of windows. Argon atoms are heavier than nitrogen, so they are slower to form a convective current within the window. Krypton is even heavier than argon but also more expensive. Both gases are nontoxic, clear and odorless.

However, the gas fill is really only part of the big picture. 

Other factors also matter when it comes to cold climate window performance: the quality of the seal, the number of panes and the type of spacers between the panes. 

For overall window performance, look at the U-value, which measures how well a window conducts heat, taking into account heat transfer through the panes as well as the frame.

If you are familiar with the R-value used to describe walls and insulation types, you can think of a window’s U-value as the inverse. The lower the U-value, the better. 

A good window for Interior Alaska has a U-value of 0.2 or lower (the equivalent is an R-value of 5).

Alaskans also might want to check out CCHRC’s Certified Alaska Tough program before shopping for windows. 

This program certifies windows that meet stringent insulation and air-tightness standards as “Alaska Tough,” indicating they are the best-suited windows for our climate. 

For more information on the program, including the standards windows must meet and current certified models, visit

Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at or 457-3454.