What energy-efficient features should your replacement windows have? According to the Efficient Windows Collaborative (http://www.efficientwindows.org), a group of insulation and window manufacturers that comply with federal energy requirements, new windows should have low-e coatings, which let in visible light but block radiant heat losses to cut heating bills.
They should have solar control, or spectrally-selective, coatings to block solar heat gain to save cooling energy but let in visible light. The windows should have insulated frames. Metal frames without insulation are the least efficient. Vinyl, insulated vinyl, fiberglass, and wood frames are more efficient.
The invisible gas filler in a double-pane window is critical to energy efficiency. Instead of plain air, high-efficiency models use argon or krypton gas, which conducts very little heat and helps the window’s insulating properties.
The material used to create the separation between the two panes of glass, called a thermal break, was traditionally metal. New materials insulate better and make the overall window more efficient. In a warm climate, retrofit films can be applied to windows to reduce solar gain and cut cooling costs.