Some are made of thin-walled steel and have a thermal break. Others are made of silicone foam or butyl rubber. With new edge spacers, however, pay particular attention to warranties against seal failure, which results in fogging and loss of any low conductivity gas-fill. Choose windows with long warranties. Window Frame and Sash Construction Window frame and sash construction has a big impact on energy performance.
Wood is still the most common material in use, and it insulates reasonably well.
Aluminum has been used extensively, particularly in the western part of the U.S., but unless a thermal break is incorporated into the design, aluminum frames conduct heat very rapidly and are therefore inefficient. Another important property is the air-tightness of windows. Windows vary dramatically in how effectively they block infiltration. Consumers should recognize that the manufacturer's quality control at the factory and care during shipping can have a big impact on the window's air tightness at a site. Furthermore, it is senseless to invest thousands of dollars in new windows only to have amateurs install them in your home.