Steel frames are not common in modern windows of residential buildings because of their high cost; however, because of their great strength, a relatively thin steel frame can support even a heavy window. This allows thin—almost invisible—frames. The frame may not protrude farther than the surface of the “surround” which finishes the opening in the wall. Thus, steel frames offer a clean, uncluttered, and simple appearance. In addition, steel can support largest IGUs for picture windows.

Although steel is not as conductive as aluminum, it is too conductive for a high-performance window unless the inside and outside parts of the steel frame are not connected. Just as with high-performance aluminum frames, high-performance steel window frames are “thermally-broken” by a polymer connecting the inside steel and outside steel layers of the frame. This polymer connection is incredibly strong and durable.

As with an aluminum window frame, steel frame long-term durability is assured. Even UV radiation will not degrade steel window frames. However, steel frames are among the most expensive.