Impact vs. Double-Pane Windows

Double-pane windows become energy savers in calmer climates -- and in some cases, a combination of both types are implemented.

Impact-Resistant

Impact windows are technology’s answer to the need for sturdy windows that resist high and violent wind conditions -- such as hurricanes, tornadoes, severe thunderstorms and human attempts to break window glass by force. Windows become a key point-of-entry during extreme weather, and for human entry via break-ins to homes and buildings. Impact-resistant windows have multiple layers of glass bonded together to a plastic interlayer, and installed in sashes and frames reinforced to provide durability and strength to resist breaks and shattering.

Double-Pane

A double-pane window has two sheets of glass and a gap of air in between the panes. Double-pane windows increase the energy efficiency of homes, because the panels set up a barrier to reduce heat loss in winter and heat gain in the summer. The panes are typically separated by a gap that measures from ½-inch to ¾-inch. In most cases, the gap is filled with argon or krypton gas to further increase the energy efficiency of the window system. The argon or krypton gas is injected into the air gap to insulate the window and block air from penetrating from the outdoors to the indoors. The sides of the glass that face each other in the center can also be coated and glazed to reduce solar emissions and ultra-violet rays, to further increase the energy efficiency of a double-pane window.

Similarities

Impact and double-pane windows are stronger than conventional single-pane windows. Because they incorporate more than one pane, they're more energy efficient. Heat loss is less when the weather is cold, and heat gain is less when the weather is warm. Both types of glass can be used for everything from conventional see-through windows to doors, skylights and sliding patio doors.

Ratings and Testing

Double-pane windows are labeled with information relative to their energy efficiency, following standards set by the National Fenestration Rating Council. The Miami-Dade County hurricane impact test is the current gold standard for testing impact glass. Various window brands are tested for their ability to withstand large and small missile-like projections. In the large missile tests, the glass is exposed to 9-pound pieces of lumber projected at the glass at a speed of 50 feet per second. In small missile testing, the windows are exposed to the impact of 10 ball bearings, hitting the glass at 80 feet per second. Glass must pass both tests for installation in new homes in the South Florida area of the United States.

Usage Comparison

A double-pane window is more of a choice to improve energy efficiency, while impact-resistant windows are primarily used for safety reasons. Having impact-resistant windows is a requirement in certain states. At the time of publication, impact-resistant windows are required in new constructions in the states of Alabama, Connecticut, Delaware, Florida, Georgia, Maryland, Massachusetts, Mississippi, New York, North Carolina, Pennsylvania, Rhode Island, South Carolina, Texas and Virginia.