In hurricane-prone regions, builders have choices when it comes to toughening their windows.
When Hurricane Andrew slammed into South Florida in 1992, it wiped out thousands of homes. It also set in motion a procession of code upgrades designed to make homes tougher. One significant change in the 2000s was a requirement to protect window openings in hurricane-prone regions.
Why toughen windows? There are several reasons. One is that if windows are breached in a storm, water intrusion does costly damage to building interiors. Beyond that, when windows fail, wind enters the building and pressurizes the interior. That can lead to a cascade of structural failures that results in the destruction of the entire building. Windows can be the weak leak in the chain and the root cause of a total loss.
The International Building Code (IBC) and International Residential Code (IRC) now include maps that depict the areas close to the coastline that are termed “Wind Borne Debris Regions.” These are locations where the design wind speed is fast enough to pick up objects such as framing members, roof tiles, or tree branches and carry them through the air. When heavy objects crash into typical windows, they tend to destroy the windows and set off that chain of destructive events. So in wind-borne debris regions, the code requires window openings to be protected.
Window Protection Options
The code provides several allowable methods for protecting window openings. The least expensive is to provide precut wood structural panels that can be applied over the openings when a storm approaches. This method requires the panels to be cut to size and predrilled for fastening and the attachment points to the building to be permanently affixed to the structure.
The building can also be fitted with permanent storm shutters. Or, the building can be equipped with impact-rated windows that have been tested to withstand the impact of wind-borne debris.
The method of supplying wood structural panels has the advantage of low cost. Besides that, proponents argue that applying panels over the window can protect the window itself from damage, avoiding the cost of window replacement in case of a storm. Impact-rated windows protect the building from damage, but in the event that they’re needed, the windows themselves are sacrificial: They are likely to be damaged and will probably have to be replaced.
On the other hand, building the protection into the windows themselves avoids the practical problem of installing temporary shields as a storm approaches. This is attractive to non handy homeowners who may not be up to the task; and when a storm is approaching, it can be hard to find the labor to prep a house for a hurricane impact. Many coastal homes are second homes or rental properties, and the owners might not be around to take care of storm preparations when needed.
Weighing these factors, many production builders choose the temporary panel solution. But spec builders and custom-home buyers are more inclined to choose impact-resistant windows.
So what’s an impact-resistant window? To meet the code, samples of the window unit need to have passed laboratory testing under ASTM Standard E1996. That standard specifies a test regime in which window assemblies, including the attachment to the structure, are bombarded by a 9-pound 2×4 missile fired out of a test gun at 34 miles per hour (that is, 50 feet per second). The window is blasted at three locations, and to pass, it has to remain able to protect the opening. The 2×4 can make a hole, but it has to be smaller than 3 inches in diameter. After the cannon assault, the window is subjected to thousands of pressurizations and depressurizations to ensure that it could withstand hurricane-force wind pressures even after a “large missile” impact.
An impact-resistant window can shatter and still pass the large missile test if the window remains largely intact through the 9,000 pressure cycles. The glass fragments tend to adhere to the plastic interlayer, which is considered intact if the size of any rips that develop during the test total less than 1/16 inch wide by 5 inches long.
An impact-resistant window can shatter and still pass the large missile test if the window remains largely intact through the 9,000 pressure cycles. The glass fragments tend to adhere to the plastic interlayer, which is considered intact if the size of any rips that develop during the test total less than 1/16 inch wide by 5 inches long.
Windows designed to pass this test have beefy frames. As for the glazing, it’s a sandwich made up of two layers of glass fused to a tough plastic membrane in the center. In practice, the glass typically shatters during the test. But the plastic typically remains intact, preserving the integrity of the opening protection.