1992 was the year that changed everything. That was the year Hurricane Andrew blew through South Florida and wreaked havoc to the tune of $25 billion. Building officials blamed much of the destruction on wind pressure leaking in through broken windows and doors, causing roofs to blow off and walls to collapse. To reduce the potential of future damage resulting from big blows, the code was changed to fortify buildings against wind penetration, thus the advent of impact resistant window.
The code basically breaks down the method to meet the higher window load requirements in two ways: the window itself, and its installation. It says that the impact resistant window must be able to withstand the “large missile impact” of a 9lb 2×4 hitting the window at a speed of 34 miles per hour without penetrating the window. The glass may break, but the opening must not be penetrated. In addition, the windows and doors have to withstand 9,000 cycles of positive and negative air pressure, conditions in a real hurricane that could pull a regular windows right out of its frame.
To meet the penetration restriction, window manufacturers all use a similar type of impact window glass that’s much like a heavy duty car window. Florida’s best windows and doors manufacturing process is basically the same as it is with most window manufacturers: their design consists of a clear plastic interlayer sandwiched between two panes of glass. Two types of interlayer are used: Polyvinyl butyral (PVB) and SentryGlas Plus (SGP). The SGP is more rigid and less likely to tear than PVB. For that reason, some manufacturers use it in their larger windows, or those windows designed for the highest wind zones.
The glazing system is also an important factor: a silicone sealant that may be strong enough to hold the glass in place in a Zone 3 impact test, but would likely fail in a Zone 4 test, causing the window to fly out of its frame. Windows in the higher test category would instead use something more like a construction adhesive for its glazing system. “The difference, says Kurt Mendez of Florida’s Best, is between Elmer’s glue and super glue.”
Window manufacturers also reinforce the frames in order to meet requirements. Lawson Industries uses a one piece sill plate (this also helps to prevent water leaks into the home) and interlocking meeting rails which ties the sash together when shut. Heavier locks also place a role in meeting impact tests, as they can be critical on certain types of windows, such as casements. For a casement window, the only thing holding the window sash in place under negative pressure is the hinge on one side and the lock on the other.
Today, virtually all of the major window manufacturers make a high-grade, impact-resistant window that has been certified to meet protective standards. The current gold standard in testing is the Miami-Dade County hurricane impact test. Remember, it is your life and your property. Essentially all of Florida, and all the coastal regions of the United States should prepare for possible wind speeds of 110 mph or greater.