A window’s overall performance is largely determined by the glazing, but the frames represent 10–30% of the total window area, and the frame materials play a significant role in performance. When choosing new windows, consider the thermal, maintenance, aesthetic, cost, and other qualities of the window frames. U-values of various frames are shown in the table below.

Window Frame U-Values

Frame Type


Insulated fiberglass, insulated vinyl


Wood, vinyl


Aluminum-clad wood, reinforced vinyl


Aluminum with thermal break


Aluminum without thermal break



Light weight, durability, and low maintenance made aluminum window frames popular from just after World War II through the mid-1980s. But thermal performance was poor, and aluminum frames were later eclipsed by wood and vinyl. Modern aluminum window frames perform better, thanks to a thermal break; the frame consists of two pieces, joined by a less conductive material, such as plastic. Even with a thermal break, however, aluminum frames don’t meet current energy codes in very cold climate zones.


Wood frames have long been favored for their aesthetic appeal and good insulating properties. Wood is less durable than other materials, but with regular paint and maintenance, a wood window frame can last a long time. Wood is considered a high-end choice, because competing products tend to cost less. In recent decades, vinyl, enameled metal, or aluminum cladding have been added to the exterior of wood windows to improve their weather resistance.


Vinyl frames have good insulating value; require little maintenance; and are resistant to impact, abrasion, corrosion, air pollutants, and termites. Vinyl frames are more stable and durable nowadays than they once were, but vinyl still expands and contracts with temperature changes more than wood or aluminum does; look for heat-welded joints and/or interior webs (rather than mechanically joined frames) for stress resistance. Insulated vinyl frames offer even greater thermal resistance and are typically used with high-performance glazing.

New Materials

Several recent developments have further expanded the choice of window frame materials:

  • Extruded composite frames, which incorporate reclaimed sawdust and wood scrap in a polymer binder, are stable and more durable than wood alone.
  • Fiberglass frames are dimensionally stable and have good insulating value. They are more expensive than vinyl, but less expensive than wood.
  • Engineered thermoplastics have some structural advantages over vinyl, but they are more expensive and haven’t captured much of the market.

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