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Tech Set 7: Wind-Resistant Openings

     

Keeping your family safe is your number one concern. This includes their safety from hurricanes and other high wind events.

Although older homes are more vulnerable to high wind-related damage, even properly designed new homes remain susceptible to severe damage if wind penetrates the building's facade through broken doors or windows. In addition to damage to the walls and contents of a home by wind-driven rain and flying debris, the roof of a home can be blown off and walls can be damaged when high winds enter and pressurize the interior.

However, openings in a building's envelope, including windows, entry and garage doors, needn't be the weak point of a building's exterior any longer, as technologies have evolved that allow reasonably priced protection of these openings.

1 Impact-resistant doors
2 Reinforced garage doors or single-car openings
3 Hurricane shutters or impact-resistant glass
4 Windows with high-performance glass
 















Details: The Wind-Resistant Openings Tech Set

Impact-Resistant Doors

Impact-resistant doors have been tested and labeled for their ability to withstand wind-born projectiles. They often include impact resistant glass, although the method of attachment also affects a door's impact-resistance. Make sure to look for appropriate labels designating the tested characteristics of an impact-resistant door.

Sliding glass doors are larger and more vulnerable to wind and debris than most doors and windows. Use impact-resistant glazing where possible, or at the very least install hurricane shutters.

Reinforced Garage Doors or Single-Car OpeningsDoor.jpg

Often, due to its large size and the relatively weak materials of its construction and hardware, the weakest opening point in a home is a garage door. Garage doors can now be constructed, tested, and rated for impact and wind resistance. The marginal cost of a rated garage door is only $200 to $300 over a door without wind-resistant features.

A retrofit kit to strengthen an existing two-car garage door will cost about $300. A kit usually consists of a vertical post that is placed between the roof and the concrete floor, although other systems also exist.

Single-car openings are more resistant to strong winds than two-car garage doors.

Hurricane Shutters or Impact-Resistant Glass

Shutters can be made of many materials like wood sheathing, acrylic, or steel panels, which have passed the appropriate tests and are properly fastened to structural frames.

Metal hurricane shutters are easily installed on most existing homes. In some designs, hurricane shutters can be electrically rolled down to protect the home.

Impact-resistant glass is optimal for windows not easily fitted with hurricane shutters or those that are hard to reach. Impact-resistant windows are made from glass laminated with composites that provide enough strength to allow windows to withstand high winds, projectiles, or even bullets. This impact-resistant glazing can reduce the risk of window failure and personal injury or property loss during tornadoes, hurricanes, and explosions. When struck, laminated glass may crack or shatter, but the glass fragments tend to adhere to a plastic layer and stay in place.

Cost Comparison of Impact-Resistant Materials:
Based on Figures Compiled by FLASH
2,250 sq. ft Home
Plywood Shutters $743
Temporary Panel Shutters $4,084
Accordion Shutters $5,940
Swinging Shutters $16,707
Electric Roll-up Shutters $22,275
Impact Resistant Glass $14,850

Results from the Interactive Shutter Tool.

Windows with High-Performance Glass

Special glazings can be used to both make windows impact resistant (as mentioned above) and more energy efficient. Low-E and solar control low-E (also called spectrally selective) coatings can be used to boost the energy efficiency of windows. Low-E double pane windows, most common in cold and moderate climates, are more energy efficient than clear windows because the low-E coating reduces heat loss through the window.

Solar control glass, also called Low E2, is a good glass for hot climates because, in addition to improving the insulating ability of windows, it also limits solar heat gain by blocking passage of infrared and some ultraviolet rays. Solar control glass allows a higher level of visible light to pass through a window with less solar heat gain reduction than tinted window coatings.

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Implementing the Wind-Resistant Openings Tech Set

Five steps to design, specify and install storm-resistant windows.

1. Consider Wind Zone and Exposure

Building codes contain maps detailing basic wind speeds that can be expected in any area of the United States over a 50-year mean recurrence interval. These are the starting point for calculating an exterior opening's exposure to pressures from high winds. In addition to design wind speed, a building is rated with an exposure classification which indicates the level of sheltering around the building. Exposure classifications range from A (city center with tall buildings surrounding) to D (flat, unobstructed area exposed to wind flowing over open water).

Because vast expanses of open land or water allow unobstructed wind movement, the best practice is to site windows on walls where natural landscape features, like tree buffers or dunes, protect them from direct wind. If tree buffers are created, plantings should be located farther away from the structure than their expected height at maturity.

Obstructions on a building's façade, like bay windows and cantilevered decks, create vortexes for wind movement. Where these architectural features are present, builders should ensure that cladding and structural attachments have been properly designed and attached.

2. Identify Window Products That Work for the Region

Hurricane Shutters A. Performance Classification

The building codes require all windows to meet wind-driven rain conditions under the testing standard "Voluntary Specifications for Aluminum, Vinyl (PVC), and Wood Windows and Glass Doors," AAMA/NWWDA 101/I.S.2. The standard establishes five classes of windows based on the (wind) design pressure (DP) that the window was tested to, as detailed in column 2 of Figure 2. A DP of 40 is equal to a 155-mph wind. Determine necessary design pressures from the Table of Equivalent Wind Velocities.

B. Impact Resistance

In addition to the force of wind and wind-driven rain, homes are often struck by airborne debris from compromised structures and landscaping during storms. Therefore, building codes also require that window and door products installed in homes in areas where winds exceed 110 miles per hour meet tests for impact resistance. The tests mimic the window, door, curtain wall, or protective covering being struck by gravel traveling at 80 feet per second (called small missiles) or a 2x4 stud traveling at 50 feet per second (called a large missile) and undergoing repeated strikes. The Florida Building Code (FBC) requires that windows in high wind zones located within 30 feet of the ground meet the large missile test criteria and those higher than 30 feet from ground level meet the small missile test criteria.

C. Energy Efficiency

In addition to resistance to high wind, windows should be selected for energy efficiency and function. The easiest way to select the most energy-efficient window for a climate is to choose one bearing the ENERGY STAR© logo. In the absence of that designation, windows can be selected based on their thermal and other properties. The National Fenestration Rating Council (NFRC) operates a voluntary labeling program which provides efficiency information to consumers.

3. Size Windows for Egress

Bedrooms and habitable sub-grade basements require windows sized for escape or entry (by rescue personnel) in the event of a fire. The International Residential Code (IRC) requires a minimum opening width of 20 inches with 5.7-square-foot minimum free area at a sill no higher than 44 inches from the floor. Depending on the manufacturer, this requirement can usually be met with a 26x48 or larger double-hung unit. It is important to consider that inoperable hurricane shutters, like homemade plywood coverings that are applied from the exterior, will impede quick exit or entry when in place, as will shutters that require power to operate if the power source goes out during a storm.

4. Follow Manufacturer's Installation Instructions

Ultimately the day-to-day and disaster mitigating performance of window and door components depends on competent installation. Manufacturers provide detailed installation instructions. In addition, the American Architectural Manufacturers Association (AAMA) has developed an Installer Training Qualification program for windows and doors that is based on ASTM E-2112, Standard Practice for Installation of Exterior Windows, Doors, and Skylights. Through the program, installers are trained and tested, then issued AAMA installation certification. This and trade contractor quality assurance programs assure builders that the units have been professionally installed.

5. Install Shutters and Other Temporary Coverings

Shutters or other temporary coverings can provide impact resistance to windows and other openings. Miami-Dade County has a searchable database of these products that comply with the FBC.

FEMA has created fact sheets for homeowners, such as Install Shutters or Plywood Window Covers, and Against the WindPDF document - Adobe Acrobat required. The Federal Alliance for Safe Homes (FLASH) also has a publication on DIY opening covers.

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Code Considerations

The International Residential Code (IRC) requires exterior windows and doors to be designed to resist the design wind loads specified in Table R301.2(2) adjusted for height and exposure per Table R301.2(3) and that they be tested and labeled. Labels must include manufacturer, performance characteristics, approved inspection agency, and compliance with the requirements of either AAMA/NWWDA 101/I.S.2, or the new version, AAMA/WDMA 101/I.S.2/NAFS, Voluntary Specifications for Aluminum, Vinyl (PVC) and Wood Windows and Glass Doors.

Ten years after Hurricane Andrew caused an Cloud estimated $26 billion in property damage that displaced 250,000 people, Florida became the first state in the nation to create building codes that addressed the extreme wind conditions experienced during tropical storms in coastal regions. The Florida Building Code (FBC) requires that buildings be designed to withstand design pressures (DP) that are a function of wind zones mapped for mainland Florida and the height and exposure of the structure. The International Residential Codes (IRC) of 2000 and 2003 and some Gulf Coast states' adoption of the IRC, as well as the pending FBC addition of additional wind zones in the Florida Panhandle, will place all coastal states under similar wind design prescription.

Per the FBC, protection of exterior windows and glass doors from windborne debris in buildings located in hurricane-prone regions is required in Miami-Dade and Broward Counties. A Miami-Dade County Notice of Acceptance (NOA) for impact-resistant products is one way to ensure that a window has been tested and meets all the requirements for hurricane protection in the highest wind velocity zones. The Florida Building Commission recognizes Miami-Dade NOAs as approved products.

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Additional Resources

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