When considering window options, there are a few rating systems that can help you understand the efficiency of one window versus another.
To make it easy for homeowners, the window industry uses a rating called the U-factor. U-factor indicates how much heat flows through an entire window assembly, and accounts for each material and the ratio of glass to frame. The lower the U-factor, the more energy-efficient the window.
When selecting replacement windows, you may also want to consider additional ratings—Solar Heat Gain Factor (SHGF) and Visible Transmittance (VT).
SHGF, expressed as a number between 0 and 1, indicates how much solar radiation gets through the window. In areas where heating is the main requirement, use windows with a higher SHGF to let more radiation in. To avoid air-conditioning, use windows with a lower SHGF to keep more radiation out.
VT, expressed as a number between 0 and 1, indicates how much visible light gets through the window. If daylighting a space is important, use windows with a higher VT to let more light in.
Virtually all replacement windows have a label designed by the National Fenestration Rating Council (NFRC) that indicates the U-factor, SHGF, VT, and an air-leakage rating for that particular window.