Two organizations, the American Architectural Manufacturing Association (AAMA) and the National Fenestration Rating Council (NFRC), test the performance and quality standards for replacement windows.
The NFRC established standards for thermal performance.
U-factor: This is a measure of heat loss. The lower the U-factor the better.
SHGC (Solar Heat Gain Coefficient): Once again the lower the better.
VT (Visible Transmittance): The higher the visible transmittance, the more daylight is transmitted. A high VT is desirable to maximize the amount of daylight entering the home.
The AAMA established standards for structural integrity, air infiltration, water infiltration resistance and forced entry resistance.
Structural Integrity is the ability to withstand wind and other dynamic pressures. This is rated in design pressure (DP). The higher a product's rating design pressure, the greater the amount of wind or pressure the window will withstand.
Air Infiltration tests involve the equivalent of 25 mph wind. The lower the air infiltration rating the better.
Water Infiltration tests a window's ability to resist water leakage through the window frame, sashes, joints, weather-stripping, etc. No leakage is permitted under this test criteria.
Forced entry is tested by a certified lab technician using many different tools to simulate breaking into a home through the window. Next, windows and doors are subjected to hundreds of pounds of force to try to lift or push them open while in the locked position.