When windows 'fail' during a wildfire, fire can move from the outside to the inside of the building, using the failed window as the pathway. Windows fail when the glass breaks and embers and/or flame enters the home, or the frame ignites, both resulting in the ignition of materials in the house. Glass can break as a result of extreme heat on the window, either radiant or flame, or potentially from the impact of wind blown debris.

Glass breaks as a result of temperature differences (and the resulting stresses) that develop between the glass that you can see (i.e., look through) and the glass 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 quick enough, cracks will develop as preexisting flaws at the edge of the glass and propagate inward. Theoretically it would be possible to polish the edge of the glass, thereby minimizing the number of edge flaws, and therefore making the glass less vulnerable. Whether this could be a long term solution in terms of maintaining the polished edge during the processing in making the window (inserting in the frame, etc.) hasn’t been determined.

As shown in this figure, the all surfaces of the panes of glass in multi-pane windows are numbered, from outside to inside. In this case, for a dual pane window, the surfaces are numbered 1 through 4.

Screens will absorb some of the radiant energy, resulting in less energy being absorbed by the glass. The temperature difference between the protected and exposed glass will be reduced, thereby reducing the vulnerability of the glass. Shutters would provide similar protection (and would also provide protection against the impact of wind blown debris). Screens won’t protect against from a flame impingement exposure that could occur if any near home vegetation, or other combustible materials, ignites.

Remembering how each surface of glass is numbered in windows, studies have shown that reflective coatings applied to the #1 surface (the exterior surface of the outer pane) improve the performance of glass since less energy is absorbed (more energy is reflected).

An extreme radiant exposure resulted in the failure of this window – including both pieces of glass in a dual pane window, and the vinyl clad screen.

An example of glass failure during window testing conducted at the University of California Fire Research Laboratory. The window was subjected to a flaming exposure from a propane diffusion burner.

An example of frame failure during window testing conducted at the University of California Fire Research Laboratory. The window was subjected to a flaming exposure from a propane diffusion burner.

Research has shown that the horizontal separator (interlock) in single or double hung vinyl windows can deform at low radiant exposures, resulting in gaps forming between the separator and glass unit. The gap allows embers or flames to enter the building. If the separator is reinforced with an aluminum (inner) cross piece, research has also shown that this deformation won’t happen.

Vinyl windows have this reinforcement member for other reasons (to resist wind loads, facilitate attachment of hardware, life and safety reasons, including resisting breaking and entering). Vinyl windows that are certified to comply with the AAMA/WDMA/CSA 101/I.S.2/A440 Standard/Specification will include reinforcement in these critical frame/sash members. Look for labeled products indicating certification.

The vinyl deformed in this fixed window as a result of a radiant exposure from a neighboring home that had ignited and was ultimately destroyed. The glass did not break, or fall out as a result of the deformation in the vinyl frame.

This is an example of an exposure from a radiant panel. The level of exposure is controlled by moving panel closer to or further away from test wall.

This slide shows the radiant energy generated during a crown fire research study (International Crown Fire Modeling Experiment, 1997-2001, Northwest Territories, Canada ). This figure is from a paper published by Dr. Jack Cohen. The breaking point for different annealed and tempered glass was complied by Dr. Vyto Babrauskas. They are based on publications from a number of researchers, and represent average values.

The radiant exposure was measured about 30 feet from the edge of the fire. In this figure, the y-axis is the measured radiant energy (kW/m2) and the x-axis is time (seconds). Relative to the measurement point, the radiant energy increased and decreased quickly as the fire front went by. Whether glass in a window will break (or combustible siding will ignite) will depend on the radiant energy level, and the exposure time. It is unlikely that tempered glass would have broken given the short exposure time at levels greater than 45 kW/m2.

Note the improvement in performance when tempered glass is used. Tempered glass is about 4 times as strong as single pane annealed, and about 2 times that of a dual pane annealed glass window.

A window with annealed glass. The large piece of glass in this broken window indicative of annealed glass.

A window containing tempered glass. Note how tempered glass breaks into small chunks.

Tempered glass will have a ‘bug’ etched on the corner of the glass. This can be used to confirm the presence of tempered glass in a window.