Aluminum or Metal Window Frame
Aluminum or metal frames for windows are a durable and low maintenance option. They tend to conduct heat too well through the metal, but their energy efficiency is improved with thermal breaks in the frame to reduce conduction of heat. What To Look For Look for windows with a thermal break built into the frame - an insulating strip placed between the inside and outside of the frame and sash. The insulating value of windows is usually rated by the whole window U-factor and the lower the value the better. Energy Star rated windows should have a U-factor of 0.27 Btu/hr-sq ft °F or less for Northern climates.