Windows

Bottom line

Radiant heat during a wildfire can crack annealed windows. Multiple pane windows with at least one tempered pane can mitigate this risk. Operable windows must have 1/16 inch metal screens with no gaps or tears.

Window type

Single pane

Single pane glass windows can be identified by looking at the area where the window or pane comes in contact with the frame. Another technique is to look for an undistorted reflection; single pane glass reflects light like a mirror from most exterior viewing angles. Laminated safety glass, where an adhesive layer joins together two panes of glass, is considered a single pane for assessment purposes.

Regardless of the glass type, single pane glass windows are more vulnerable to breaking when exposed to radiant heat during a wildfire due to the temperature difference between the part of the glass protected by the frame and the exposed glass. If this temperature difference is large enough, and occurs quickly enough, cracks will develop as pre-existing flaws at the edge and propagate inward. Cracked glass can fall out of the frame, which becomes an entry point for embers.

To mitigate single pane windows, there are several options:

• Replace with a multiple tempered glass pane window assembly

• Install noncombustible shutter covering

Multiple panes

Multiple pane or multi-pane glass windows are typically pre-assembled as a unit and then installed into the window frame assembly in a factory. Many are insulated glass units (IGUs) with an inert gas (e.g., argon) in the space between the panes for energy efficiency purposes. From a wildfire perspective, non-insulated and insulated multiple pane windows perform the same. Multi-pane windows offer more protection from radiant heat and flame contact exposures regardless of the glass type.

If visually inspecting the area where the window meets the frame isn’t possible, a window that reflects objects with a “visual echo,” indicates multiple panes because the object is being reflected twice by slightly offset window panes.

Multi-pane windows perform better than single pane or laminated glass windows regardless of the glass type, and multi-pane windows with at least one tempered pane or, ideally, with two tempered panes offer the best resistance to radiant heat exposure. However, if radiant heat exposures during a wildfire are hot enough and long enough, even multi-pane tempered glass windows can fail.

Glass type

Annealed / Laminated

This is the most common glass type in windows, including most multi-pane windows. Annealed glass is not marked with any indication of its type. Because annealed glass does not undergo an additional heating and cooling cycle that tempered glass does, it is more susceptible to cracking when exposed to radiant heat. Once the glass cracks, breaks, and falls out, the window becomes an entry point into the structure for burning embers and flames.

Laminated glass is a type of safety glass that consists of two panes of annealed glass with a polymer adhesive layer between them. When broken, the polymer layer acts to hold the fragments together and does not fail as readily as non-laminated annealed glass does. Laminated glass is most commonly used in automobile applications but can also be found in structures. It is commonly the inner (interior) pane in a skylight (with the outer pane being tempered glass).

In structures located in wildfire prone areas, annealed and laminated glass windows should be replaced by windows containing multi-pane tempered glass in windows whenever possible.

Tempered

Tempered glass is 3-5 times stronger than annealed (non-tempered) glass and better resists radiant heat exposure that can crack and break annealed glass windows and allow embers to enter a structure. Tempered glass windows are typically 15-20% more expensive than annealed glass windows depending on the type of window and must be custom ordered to exact size because, unlike annealed glass, tempered glass cannot be cut after fabrication.

To identify tempered glass, look for a marking, typically in one of the corners of the window that indicates that it is tempered. In modern buildings, tempered glass is required on all windows adjacent to exterior doors and in windows that have a lower edge within 18 inches of the floor.

Plastic / Plexiglass

While it’s uncommon to find acrylic (plexiglass, Plexiglas or Lucite) panes in windows, they do exist. Plastic is not an effective insulator, scratches easily, and some types have a tendency to discolor or yellow over time when exposed to UV rays in sunlight. When exposed to radiant heat or flames, they can ignite and burn. Replace all plastic windows with multiple tempered pane glass windows.

Screen or shutter covering

Screens or shutters mitigate some radiant heat exposure to windows as long as they provide full coverage of the window and there are no screen gaps present. Screens are typically metal framed with 1/16 inch metal mesh and primarily designed to keep insects from entering the structure while allowing ventilation and often cover only the operable section of a window. The fine metal mesh both absorbs and reflects radiant heat, which shields the window behind it from the full exposure. However, this is only effective if the screen covers the entire window area; a partial screen is not sufficient to mitigate radiant heat exposure risk.

Shutters, if deployed prior to radiant heat exposure, significantly mitigate the radiant heat energy that a window would receive by blocking infrared energy from reaching the glass. Shutters can be made of combustible (e.g., wood) or noncombustible (e.g., metal or fiber cement). Noncombustible offers the greatest reduction in risk exposure from embers and radiant heat, especially to other large structures facing the window being assessed located less than 50 feet away.

Shutters run the risk of not being deployed in a hasty evacuation scenario and should be deployed on red flag days or when an evacuation may be required. Motorized shutters should have a backup power supply (e.g., battery or generator) or a manual operating mode. Panel-style shutters designed to be inserted into grooved channels on either side of windows must be lightweight enough to be deployed by occupants quickly. Traditional hinged shutters that latch securely once deployed offer ease of use. It is a good idea to test deploying shutters on an annual basis.

Screen or shutter gaps present

Indicate whether there are any gaps in the window screen or shutter larger than 1/8 inch, which diminish the mitigation value against radiant heat or ember exposures. Common examples of gaps are screens or shutters that do not sit squarely in or around the window frame, that have tears in the screen, or gaps in the shutters.

Window frame material

At this time, there is no building code requirement nor available scientific research that indicates window frame materials increase or decrease a structure’s the vulnerability to wildfire destructionof when exposed to wildfire. As a result, our assessments do not document the window frame material.

Vinyl framed windows generate the most questions and have previously been banned by some jurisdictions (e.g., San Diego County in the 1990s) based on research by Fred Mowrer (now at Cal Poly, San Luis Obispo). However, as indicated later in this section, the issues identified in the research have since been addressed, so vinyl framed windows installed since then and with metal-reinforced locking rails are not a significant wildfire risk factor.

While on a sabbatical from the University of Maryland, Fred Mowrer conducted a study at NIST that demonstrated the vulnerability of vinyl framed windows, particularly single or double hung windows. His research found that vinyl framed windows without metal reinforcement at the meeting rail deformed when exposed to a radiant heat exposure far less than that needed to break the glass, which caused the glass panel to fall out of the frame. Subsequent research (conducted elsewhere) demonstrated that use of metal reinforcement in the meeting rail (horizontal interlock) of hung windows was an effective mitigation strategy to overcome the vulnerability observed by Mowrer. This later research led to the recommendations in the window type and glass type sections.

Based on this research, vinyl framed windows were then banned in San Diego County. Subsequent research by Dr. Steve Quarles and others showed that windows that incorporated metal-reinforcement in the horizontal interlock addressed the deformation issue. Larger vinyl framed windows were already manufactured with metal reinforcements due to wind strength and other structural requirements, but some smaller windows at the time the research was published lacked the metal reinforcement.

While drafting the first version of ICC Chapter 7A, the American Architectural Manufacturers Association (AAMA, now called FGIA) indicated that all windows certified by them would have the metal reinforcement. For this reason, vinyl framed windows are compliant with Chapter 7A, and San Diego County rescinded its ban.

Like other frame materials, the glass vinyl framed windows is the most vulnerable component and may crack/break when subjected to a radiant heat or flame contact exposure. Some non-structural components of a vinyl window may deform, resulting in damage to the window. More expensive window frame materials (e.g., vinyl-clad wood or metal) address the deformation vulnerability, but they are not required by the building code.