Types and Parts

The technologies that make windows efficient go beyond just the glass panes – they are found between the glass and in the frames as well. NFRC’s ratings system evaluates all these parts of a window to determine its energy-efficiency rating: 

Opening Mechanisms

Windows typically open or close by swinging in and out, sliding up or down, or sliding horizontally within a frame.


Single-glazed windows have one pane of glass and typically cost less. Double-glazed windows have two panes, and balance cost and efficiency. Triple-glazed windows use three panes and are the most efficient.


Metal and wood frames are popular and usually efficient. Frames made of PVC, composite materials and hybrid alternatives are also efficient and typically require less maintenance.   

Gas Fills

Windows with more than one pane have a fixed gap between them, and filling that space with a safe, odorless, and colorless gas boosts efficiency. In cold climates, the gas reduces heat loss through the glass. In hot climates, it reduces unwanted heat gain.


These small parts fit between multiple panes of glass and hold them in place. Using specific materials and shapes can boost efficiency.

Low-Emissivity Coatings

Transparent coatings on the surfaces of glass add efficiency because they can be tailored to cold, hot, or moderate climates to help maintain interior temperatures and comfort.