Strength Performance:

Design Pressure (DP):

The Design Pressure Rating represents the maximum positive and negative wind load that a window or door can experience without breaking, deglazing or permanently distorting. The test results are valid for the size unit tested and smaller. DP Rating is expressed in positive and negative pounds per square foot (+/- psf). The test is conducted at 150% of the rating achieved. i.e. a DP +/- 45 psf rated window was tested to a structural load of 67.67 psf.

Performance Grade (PG):
A product only achieves a performance grade rating if that product complies with all requirements of the NAFS standard for a certain design pressure corresponding to a maximum expected wind velocity regime at the given geographical location. This is the link between design pressure and performance grade - but the latter goes further by encompassing:

a) Structural performance: The pressure applied for structural performance verification is 150 percent of design pressure for doors and windows and 200 percent for unit skylights and tubular daylight devices. This is a test that assesses any permanent damage associated with the assembly;

b) Water penetration resistance: The water penetration resistance tests pressure, which simulates the force behind wind-driven rain, is generally based on 15 percent of the design pressure for R, LC, and CW performance classes and 20 percent of the design pressure for AW performance classes;

c) Air infiltration resistance;

d) Uniform load-deflection test;

e) Forced-entry resistance (if applicable)

 

Here you can see a couple examples of a vinyl window with a low Performance Grade (PG) that has bowed: