There are advantages and disadvantages to all types of frame materials. Overall, vinyl, wood, fiberglass, and some composite frame materials provide greater thermal resistance than metal.
Aluminum or Metal Frames
Although very strong, light and almost maintenance free, metal or aluminum window frames conduct heat very rapidly. Because of this, metal makes a very poor insulating material. To reduce heat flow and the U-factor, metal frames should have a thermal break—an insulating plastic strip placed between the inside and outside of the frame and sash.
Composite window frames consist of composite wood products, such as particle board and laminated strand lumber. These composites are very stable, they have the same or better structural and thermal properties as conventional wood, and they have better moisture and decay resistance.
Fiberglass window frames are dimensionally stable and have air cavities (similar to vinyl). When these cavities are filled with insulation, they offer superior thermal performance compared to wood or vinyl (similar to insulated vinyl frames).
Vinyl window frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. PVC is a very versatile plastic with good insulating value. Vinyl window frames also do not require painting and have good moisture resistance. However, at high temperatures, they may expand and warp; at extremely low temperatures, they may crack. Also, if sunlight hits the material for many hours a day, colors other than white may tend to fade over time.
Insulated vinyl frames are also available. Unlike standard vinyl frames, their hollow cavities are filled with insulation. This makes them thermally superior to standard vinyl and wood frames. Usually these high-performance frames are used with high-performance glazings.
Wood window frames insulate well, but they also expand and contract according to weather conditions. They can also be quite heavy and thicker than other frames. This can make storage difficult, reduce the view out the window, and reduce the amount of natural light in the room. Wood frames also require the most maintenance. There are, however, aluminum- or vinyl-clad wood frames that reduce maintenance requirements.