Fiberglass:
Window frames made from fiberglass are similar to vinyl in that they are durable and have air cavities, yet fiberglass is a stronger material than vinyl. When these cavities are insulated, the fiberglass frames are thermal better than wood or insulated vinyl frames. Engineered thermoplastics, which is used in automobiles and appliances has also been used to manufacture windows. Like fiberglass, it also has structural advantages over vinyl.