Fiberglass window frames are a relatively new type of frame made of fiberglass composite material. Some of these window frames are hollow; others are filled with fiberglass insulation.

Most are prefinished with a white or brown polyurethane coating. These window frames are manufactured using a process called “pultrusion.” Glass matting and continuous glass strands are pulled through a tub filled with resin.

The window frames are then formed in a preformer and pulled into a heated die. A chemical reaction hardens the material. Because of this process, there are some limits to the design flexibility that can be achieved with Fiberglass.

Fiberglass is an excellent insulating material. In addition, fiberglass window frames do not warp, shrink, swell, rot, or corrode. They do not soften and degrade under the thermal conditions that most windows are exposed to.

Fiberglass window frames are also relatively strong and durable, and can hold a large expanse of glass. Because they have to be painted, fiberglass windows are not nearly as maintenance free as vinyl windows.

Since fiberglass windows have not been available for that long, they do not have the track record that other materials have. They also tend to be more expensive than other framing materials for the same level of quality and performance.