Windows Have Impact-Rated Glass or Protective Coverings

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Windows Protected by Hurricane Shutters.
Windows Protected by Hurricane Shutters.
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Protect exterior glazed openings (windows and glass doors) against wind pressures, windborne debris, hail, and heat during hurricanes, wildfires, and severe storms by installing impact-resistant glazing or by installing hurricane shutters or storm panels to protect the glazing. In wildfire-prone areas, use dual-pane, tempered-glass windows.

In areas susceptible to wind-borne debris,

  • Determine if glazed opening protection is required 
  • Determine the most suitable type of opening protection 
    • impact-resistant glazing (the only option for skylights)
    • hurricane shutters
    • storm panels
    • shutters and panels
    • qualified opening protection systems must pass ASTM E1996 and E1886 impact tests for large missile “D.”
  • Properly install products in accordance with manufacturer installation instructions and all applicable building codes including any specific product approval requirements.
  • Inspect shutters and panels annually

In wildfire-prone areas,

  • Determine if glazed opening protection is required.
  • Determine the most suitable type of opening protection.
  • Properly install products in accordance with the manufacturers’ installation instructions and all applicable building codes including any specific product approval requirements.
  • If replacing windows or installing shutters is not an option, suggest deployable coverings.
  • Remind the homeowners to manage vegetation and combustible materials around the home.
  • Remind the homeowners to close operable windows before evacuating a home due to a wildfire threat, if there is time to do so..
 

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and EPA Indoor airPLUS.

 

Description

In natural disasters, windows are the most vulnerable elements in the building envelope. Broken windows leave the home vulnerable to the entry of windborne rain and wildfire embers, smoke, and debris. The wind itself can be a destructive force if it enters the home through a broken window and pressurizes the building, blowing out additional windows, doors, or walls.

Windows can be protected by shutters or storm coverings as described below. Or the windows themselves can be made of more break-resistant glass.

Different types of glass are available, including annealed, tempered, laminated, and impact resistant. Annealed glass is the “softer” glass most windows are made of. Tempered glass is glass that has been subjected to high temperatures followed by rapid cooling, which compresses the surface and edges of the glass making the tempered glass up to five times stronger than traditional glass. If tempered glass does break, it does not break into large shards like regular glass, but instead shatters into small pebble-sized bits without dangerous edges that can cut or damage.

Laminated glass is created by bonding together two or more panes of annealed glass with a thin layer of film or vinyl in between. This inner layer works as an adhesive film that holds the glass together should it break or crack.

Impact-resistant windows may use both tempered and laminated glass and the panes are often installed in stronger heavy-duty frames, allowing the windows to withstand the blunt force of strong winds, windborne debris, and fire.

Impact-resistant windows are sometimes made with an exterior pane of standard (annealed) glass and an interior pane of laminated glass (two sheets sandwiching a vinyl layer). On impact, the exterior pane may break but the interior pane will crack into small pieces but stay adhered to the vinyl layer.

Hurricane Resistance

During a hurricane or other extreme storm, windborne debris such as tree limbs, building materials, and patio furniture can shatter unprotected windows and glass doors, exposing the building and allowing wind and water into the house. In addition to damaging household contents and interior finishes, the wind pressure can break the house apart from the inside by blowing off the roof and the water can lead to significant damage to the house (FEMA P-762).

Failure of Roof Structure from Pressurization Due to Window Failure During a Hurricane.
Figure 1. Failure of Roof Structure from Pressurization Due to Window Failure During a Hurricane. (Source: FEMA 488).
 

To protect exterior glazed openings (windows and glass doors) against windborne debris, install impact-resistant glazing or install hurricane shutters or storm panels to protect the glazing. Several options are available: windows and doors with impact-resistant glass, permanently installed hurricane shutters, and removable storm panels.

Shutters are always in place and ready to be closed. The primary types of hurricane shutters are rolldown, accordion, Bahama/Bermuda, and colonial shutters. Consider shutters that can be operated from inside for hard-to-reach windows.

Hurricane shutter styles include colonial, Bahama, roll-up, and accordion shutters.
Figure 2. Hurricane shutter styles include colonial, Bahama, roll-up, and accordion shutters. (Source: FEMA Technical Fact Sheet No. 6.2).
 

 

Panels have permanently attached hardware for quick installation and are normally installed and taken down as needed and stored in a garage or shed. Storm panels are commonly constructed of corrugated steel, aluminum, polycarbonate, or fabric, and are attached using a combination of permanently installed tracks, bolts, and wingnuts. The hardware must be permanently mounted and corrosion resistant for quick installation. Panels that are clear (polycarbonate) or translucent (many fabric panels) allow natural light and visibility. Panels should be labeled for quick installation.

Figure 3 shows example attachment details for a metal storm panel. The panel is installed in a track permanently mounted above and below the window frame. The shutter is placed in the track and secured with wing nuts to studs mounted on the track.

. A metal storm panel is installed in a track permanently mounted above and below the window frame and secured with wing nuts to studs mounted on the track

Figure 3. A metal storm panel is installed in a track permanently mounted above and below the window frame and secured with wing nuts to studs mounted on the track. (FEMA 499 Technical Fact Sheet No. 6.2).

 

Impact-resistant windows and doors require no action before or after a storm and are particularly suitable for hard-to-reach windows or when occupants are expected to be away during a storm. Impact-rated glass is made of a laminated sheet sandwiched between two glass panels. Upon breaking, they crack in a spider web pattern without shattering of glass into the house. Impact-resistant windows can have aluminum, vinyl, or wood frames like any other standard windows. 

Impact-rated windows cost more than non-impact rated glazing, but there are several convenience and safety factors to consider which can make impact-rated windows the preferred option regardless of the added cost: they are always on, they don’t need to be installed or closed in an approaching storm, homeowners won’t be caught off guard if a storm comes up suddenly, they don’t block the view, they don’t need to be stored or carried up and down ladders, and they are in place even if homeowners are away when a storm occurs. The practical solution may be to install a combination of options, for example storm windows for hard-to-reach windows, rolldown shutters on first-floor windows, and fabric panels to protect sliding glass doors. In addition to protecting people and the house and contents, protected glazing adds value to the property and may help qualify for a homeowner’s insurance policy or discount.

Building codes require impact-resistant glass, shutters, or panels for new homes located in “Windborne Debris Regions.” Ask the local building department if your house is in a windborne debris region or if local requirements exceed those of the national code. Generally, opening protection is recommended in “Hurricane-Prone Regions,” particularly where the design wind speed velocity is 130 mph or greater. (See the Compliance tab for wind region terminology.)

Impact-resistant products must be rated and labeled in accordance with ASTM E 1996 and ASTM E 1886 or AAMA 506.  As an exception, wood structural panels (i.e., plywood) at least 7/16-inch thick are acceptable if installed in accordance with specific code criteria (2018 IRC R301.2.1.2). Other local standards may require specific product approval (e.g., TAS 202 for Broward and Miami-Dade Counties). Older “clamshell” awning-type shutters may not be rated for impact resistance.

The Insurance Institute for Business and Home Safety® (IBHS) offers guidance, best practices, and voluntary construction standards and programs for building in disaster-prone areas including hurricane and high-wind zones. The IBHS FORTIFIED HOME™ standard is designed to make homes more resilient and durable; guidance is available for New Construction and Existing Homes in Hurricane zones and High-Wind zones. There are three levels of FORTIFIED Home: FORTIFIED Roof™ focuses on the roof; FORTIFIED Silver focuses on roof overhangs, opening protection, gable ends, and attached structures; FORTIFIED Gold focuses on tying all components of the structure together. For houses located in hurricane zones, the FORTIFIED Home standard requires that all windows, exterior doors, and skylights be impact rated or protected with qualified protection systems. (They must pass ASTM E1996 and E1886 impact tests for large missile “D.”)

Wildfire Resistance

In wildfire-prone areas, glazed openings (windows and doors with windows) are among the most vulnerable features of a home. If a window shatters during a wildfire, airborne debris and embers can enter the home catching fire to objects inside.

During a wildfire, the main failure mode of exterior glazing is the glass shattering due to heat exposure. Radiant heat or direct flames can cause temperature differentials in the glass resulting in thermal stresses that lead to fracturing and failure of the glass.

Glass breaks as a result of temperature differences (and the resulting stresses) that develop between the glass at the center of the window on the exterior surface (surface #1) and the glass around the edges that is protected by the framing material when a window is exposed to the heat of a fire. If this temperature difference is large enough and occurs quickly enough, cracks will develop at pre-existing flaws at the edge of the glass and propagate inward (Fire in California – Windows, UCCE 2009). See Figure 4.

 

 

 

Figure 4. Radiant heat from wildfires can crack windows by heating the exterior surface (#1) causing it to expand and crack when exposed to wildfire (Source: Fire in California: Windows, UCCE 2009).

 

According to the Insurance Institute for Business & Home Safety (IBHS), a window may break after only 1 to 3 minutes of exposure to direct flames or heat, depending on the type of glass. Single-pane windows, which are common in older homes, are extremely susceptible to breaking in wildfire conditions.

Dual-pane windows that have a tempered outside pane provide better protection against wildfires than single-pane windows. In addition to better fire-resistance, homeowners may enjoy the energy savings, thermal comfort, and noise attenuation that dual-pane windows have to offer. Dual-pane windows are less effective, however when the outer pane is not tempered. According to research by Dr. Jack Cohen and Dr. Vyto Babrauskas, when exposed to heat, a tempered glass window performs roughly four times greater than a single-pane annealed glass window and roughly two times greater than dual-pane annealed glass windows as shown in Figure 5 (Quarles et al. 2010, UCANR).

States and local ordinances vary on what types of windows are required. For example, California code requires that windows be dual-pane and that at least one pane be tempered glass.

 

 

Figure 5. Likely amount of time and radiant heat exposure at which different types of window glass would break at 30 feet from the edge of a wildfire (Source: Fire in California, USCE 2020).

Tempered glass does come at a premium and will reportedly add 15% or more to the cost of the window (UCANR). A window containing tempered glass can be identified by the etched “bug” on the corner glass as in Figure 6.