Understand, Verify, and Specify Appropriate Wind Ratings for Windows

Standardized wind-driven rainwater penetration tests are conducted with wind pressures that are only 15 to 20 % of the design wind pressure. A code-minimum fenestration product is based on a test wind pressure that corresponds to about a 35 mph wind speed—equivalent to a common thunderstorm gust. Consequently, such doors and windows are likely to leak during significant wind events, even when they are properly installed and flashed.

There are several important implications from this “disconnect” between water penetration tests and design wind pressures:

1. Good window flashing practices (Chapter 4) aren’t just “extra credit.” They provide a necessary belt and suspenders type of approach to limiting (or mitigating) water infiltration through and around windows. Of particular importance to address this issue is the use of pan flashing to remove water that penetrates the window unit itself, not necessarily the flashing of it.
2. At a minimum, builders and designers should verify that glazing in windows and doors meets requirements for the locally applicable wind pressure loading (design wind pressure). Window labeling and certification should indicate the appropriate wind pressure rating.