U-Value (or U-factor) is the rate of heat or, perhaps more accurately, thermal transfer through the product measured in BTU/h*ft2*°F. The lower the U-Value, the better the insulating value of the window.
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. The lower the SHGC, the less solar heat the window transmits.
Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1. The higher the VT, the more light you see.
Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows.
Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf·m/ft².