How do I interpret the U-factor and performance of windows?

U-factor is a measure (W/ m²·K) of the rate of non-solar heat transfer through the window. The lower the U-factor, the greater a window’s resistance to heat flow, the better its insulating value, and the more energy efficient it is.

Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1, with values typically ranging from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits.

Visual Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1, with values generally ranging from 0.20 to 0.80. The higher the VT, the more light you see.

See our glossary for more energy efficiency terms.