When selecting a window, door, or skylight for energy efficiency, all you have to do is look for the ENERGY STAR. Since ENERGY STAR criteria are based on whole-unit performance, including the frame, you can simply select a certified product that rather than searching for the materials, components, or technologies that are most energy efficient. ENERGY STAR certified models are available with frames made of vinyl, wood, fiberglass, composite, metal, and various combinations of these materials.