Fiberglass

Window frames can be made of glass-fiber-reinforced polyester, or fiberglass, which is pultruded or extruded into lineal forms and then assembled into windows. These frames are dimensionally stable and have air cavities (similar to vinyl). When the cavities are filled with insulation, fiberglass frames have thermal performance superior to wood or vinyl (similar to insulated vinyl frames). Because the material is stronger than vinyl, it can have smaller cross-sectional shapes and thus less area. Another polymer-based approach is to use extruded engineered thermoplastics, another family of plastics used extensively in automobiles and appliances. Like fiberglass, they have some structural and other advantages over vinyl.

Usually these high performance frames are used with high performance glazings. The window properties shown below assume fiberglass frames that have cavities filled with insulation.