Frame material and design variations primarily impact the overall Window U-factor. Low conductivity frames made from wood, wood with vinyl or aluminum cladding, vinyl, and various composites help lower the U-factor. The frame has less impact on the SHGC, but a small frame profile means more glass area and more glass area means higher SHGC (and more view area). The different types of windows frames and their key advantages are as follow:
- Wood Frames
- Wood frames are strong, durable and the most expensive option. In addition to durability they have good appearance and thermal performance. The addition of cladding systems offers lower maintenance as well. The traditional window frame material is wood, because of its availability and ease of milling into the complex shapes required to make windows. Wood is favored in many residential applications because of its appearance and traditional place in house design. From a thermal point of view, wood-framed windows perform well with frame U-factors in the range of 0.3 to 0.5 Btu/hr-sq. ft-°F. Wood is not intrinsically the most durable window frame material, because of its susceptibility to rot, but well-built and well-maintained wood windows can have a very long life. Paint protects the exterior surface and allows an easy change in color schemes.
- Hybrid Frames
- These frames use two or more frame materials to produce a complete window system. The wood industry has long built vinyl- and aluminum-clad windows to reduce exterior maintenance needs. Split-sash designs may have an interior wood element bonded to an exterior fiberglass or vinyl element. These combinations can provide better-performing products at lower cost. As manufacturers increasingly turn to hybrid frame designs it becomes more important for a homeowner to learn about all the materials options from the perspective of maintenance requirements and interior finishes. It does then become increasingly difficult for code officials and builders to estimate the thermal properties of such a frame from simple inspection. The best source of readily available and up to date information is the NFRC label that provides the thermal properties of the overall window system.
- Vinyl Frames
- Vinyl frames are inexpensive and low maintenance. They have good thermal performance, but do require different installation techniques. Plastics are relative newcomers as window frame materials in North America. Vinyl, also known as polyvinyl chloride (PVC), is a very versatile plastic with good insulating value. Vinyl window frames do not require painting and have good moisture resistance. Because the color goes all the way through, there is no finish coat that can be damaged or deteriorate over time -- the surface is therefore maintenance-free. Some vinyl window manufacturers are now offering surface treatments like laminates (wood veneer, paintable/stainable, maintenance free) and coatings. These products increase color selection and surface appearance options. Recent advances have improved dimensional stability and resistance to degradation from sunlight and temperature extremes.
- Fiberglass Frames
- These high-performance frames are more expensive when compared to vinyl frames, but when the cavities are filled with insulation, fiberglass frames have thermal performance superior to wood or vinyl (similar to insulated vinyl frames). The material is also stronger than vinyl, so it can have smaller cross-sectional shapes and thus less frame area and more window area.
- Aluminum Frames
- Aluminum frames are strong, durable, low maintenance, generally low profile and normally the least expensive option. They have a highly conductive solid frame that means a higher U-factor, but thermal breaks serve to reduce conductivity a bit. Aluminum window frames are light, strong, durable, and easily extruded into the complex shapes required for window parts. Aluminum frames are available in anodized and factory-baked enamel finishes that are extremely durable and low-maintenance.
For additional information on this topic, please consult the Frame Types and Operator Types sections of the Efficient Windows Collaborative website.