Improving the thermal resistance of the frame can contribute to a window's overall energy efficiency, particularly its U-factor, which measures how well the window insulates. In general, the range for windows is from 0.20 to 1.20. The lower the U-factor, the better the window insulates. There are advantages and disadvantages to all types of frame materials, but vinyl, wood, fiberglass, and some composite materials provide greater thermal resistance than metal.
Aluminum or Metal
Although very strong, light, and almost maintenance-free, metal or aluminum window frames conduct heat very rapidly, which makes metal 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
Composite window frames consist of composite wood products, such as particleboard and laminated strand lumber, and some are mixed with polymer plastics. These composites are very stable, have the same or better structural and thermal properties as conventional wood, and have better moisture and decay resistance.
Fiberglass
Fiberglass window frames are dimensionally stable and have air cavities that can be filled with insulation, giving them superior thermal performance compared to wood or uninsulated vinyl.
Vinyl
Vinyl window frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. Vinyl window frames do not require painting and have good moisture resistance. The hollow cavities of vinyl frames can be filled with insulation, which makes them thermally superior to standard vinyl and wood frames.
Wood
Wood window frames insulate relatively well, but they require regular maintenance, although aluminum or vinyl cladding reduces maintenance requirements. Metal-clad wood frames may have slightly lower thermal performance.
Source: Department of Energy
