Steel windows offer many efficiency benefits, most notably their resilience and resistance to warping and degradation. Because energy efficiency for windows is, in large part, determined by how tightly and reliably they stay sealed against the elements, this is a major advantage for steel. Of course, the other component of window energy efficiency is resistance to heat moving through the frame material itself. While you might expect steel to be lacking here, thermally broken steel windows eliminate this flaw and allow for incredibly efficient windows that will stay efficient.