Types and Parts

The technologies that make windows efficient go beyond just the glass panes – they are found in the frames and inside them as well, including the internal spacers that hold the glass in place. NFRC’s ratings system evaluates all these parts of a window to establish its energy-efficiency ratings: 

Operator Types

Traditional operable window types include the projected or hinged types such as casement, awning, and hopper, and the sliding types such as double- and single-hung and horizontal sliding. This section on Operator Types describes how these typical windows work.

Frame Types

The material used to manufacture the frame governs the physical characteristics of the window, such as frame thickness, weight, and durability. It also has a major impact on the thermal characteristics of the window. This section on Frame Types describes the performance impact of different frame materials and how they influence the total window performance.


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.

Low-e Coatings

Low-emissivity (Low-E) coatings are transparent metallic layers, thinner than human hair, applied to the surfaces of glass. This coating prevents heat from entering buildings during the summer months and escaping from them during the winter.

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.

A variety of window technologies can improve window energy efficiency, including gas fills, low-E coatings, and high-performance frame options. How these technologies affect a window’s energy performance depends on the sum of all parts. This is where whole window energy ratings help, accounting for the combined effect of glazing, spacers and frame (thermally improved). The only reliable way to determine whole-window energy properties are the ratings certified by the National Fenestration Rating Council (NFRC). In most jurisdictions across the United States, building energy codes require that windows bear the NFRC label so that the code compliance of their energy ratings can be verified.