Interactive guide to understanding window energy ratings, U-factor, SHGC, and how to choose the most efficient windows for your home.
Window performance is measured by four key metrics that determine how well a window insulates, controls solar heat, and maintains comfort. The U-factor measures the rate of heat transfer through the window — lower values indicate better insulation. A U-factor of 0.20 or below is considered excellent for cold climates like Chicago and Toronto, while values up to 0.30 are suitable for milder regions. The Solar Heat Gain Coefficient (SHGC) measures how much solar radiation passes through the glass, with values ranging from 0 to 1. In northern climates, a higher SHGC (0.25–0.40) can help capture free solar warmth in winter, while southern climates benefit from lower SHGC to reduce cooling costs.
Visible Transmittance (VT) indicates how much natural light the window lets through. Higher VT values mean brighter rooms with less need for artificial lighting. Air Leakage (AL) ratings measure how much outside air enters through the window assembly, with lower values indicating a tighter seal. Together, these four metrics give you a complete picture of a window's energy performance. Look for ENERGY STAR certification and NFRC labels when comparing products to ensure you are getting independently verified performance data.
Choosing the right combination of these ratings depends on your climate zone, the orientation of your windows, and your energy priorities. South-facing windows may benefit from higher SHGC to capture passive solar heat, while west-facing windows often need lower SHGC to prevent afternoon overheating. Our Window Performance Lab lets you explore these tradeoffs interactively, helping you select the ideal glass package for every window in your home.