Window frames can be made of glass-fiber-reinforced polyester, or fiberglass,
which is 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.