Devastation caused by wildfires is enormous. Whipped by hot, dry, double-digit Santa Ana winds gusting sometimes over 85 mph, the October wildland fires in Southern California burned over 500,000 acres and destroyed at least 1,500 homes. Four years earlier, the 2003 Cedar Fire in San Diego County consumed 3,000 other homes.

The National Institute of Standards and Technology has gathered its resources to attempt to learn how to mitigate airborne fire threats. Researchers, with the help of Japanese colleagues, have built a firebrand generator in the Fire Research Wind Tunnel Facility at the Building Research Institute in Tsukuba, Japan. The machine is being used to study the way firebrands ignite structures. The device allows for the generation of controlled and repeatable firebrands that can be adjusted to be representative of typical firebrands produced from burning vegetation.

Recently, fire engineers used the NIST firebrand apparatus to observe the mechanism of firebrand penetration through building vents fitted with screens. This is the first time such observations have been made. Engineers generated a controlled firebrand shower onto a structure erected in the wind tunnel facility. A gable vent on the front face of the structure and three different sized steel screens installed behind the gable vent were unable to block firebrands from penetrating the openings. The scientists saw the firebrands burn until the glowing embers fit through the screen openings and ignite fires inside the structure.

The results demonstrated the need to design building vents both in Japan and the United States that can resist firebrands.

The researchers hope this work will give architects new ideas to incorporate into home designs, making them more resistant to firebrand ignition in the path of fires in wildland urban interface zones. The research could also lead to new building codes and standards to protect homes in the United States and Japan.

Usually, fires in interface zones are caused by spotting — fires that propagate away from the main fire line when airborne chunks of burning wood and vegetation are carried randomly by fierce winds swirling in fire areas. These firebrands are entrained in the atmosphere and may be carried by winds over long distances. Hot firebrands ultimately come to rest and may ignite surfaces far removed from the fire, resulting in fire spread.

The NIST research is providing another critical piece in understanding the complex firebrand problem, which falls into three main areas: firebrand generation from vegetation and structures; subsequent transport through the atmosphere; and the ultimate ignition of fuels after firebrand impingement. Of these processes, only firebrand transport has been well investigated.

Until this NIST work, models generally have assumed firebrand sizes to perform transport calculations, since little quantitative data existed with regard to firebrand size or firebrand mass produced from vegetation and structures. The general lack of knowledge of the type of firebrands that are produced as well as the type of materials that may be ignited has hampered further understanding of this problem.