Why Ember Storms Are A Major Wildfire Threat: IBHS Wildfire Testing

Buildings are ignited by embers and flames during wildfires. Flying embers and wind-blown, ground traveling burning debris are by far the most prevalent attack mechanisms threatening structures during a wildfire. CAL FIRE identified embers as the major cause of home loss. Potter and Leonard (2011) reported that “well over 90% of houses were ignited in the absence of direct flame attack or radiant heat (exceeding 12 kW/m2) from the main fire front.” Hence, embers cause a great deal of damage, whether directly or indirectly. Direct ignition by embers happens when embers land and accumulate on combustible components of the building and ignite these components or when embers enter the building through openings and ignite interior materials. Small-scale (Richter et al., 2022) and full-scale experimental studies (Cohen, 2000; Davis, 1990; Hedayati et al., 2022; Suzuki et al., 2017) demonstrate that accumulated embers are capable of directly igniting common combustible building components. Embers can also penetrate through openings and ignite the combustibles, such as furniture, inside a building. Indirect ignition by embers occurs when embers accumulate on combustibles near a building and ignite these materials. The resulting flames ignite a component of the building by radiant heat and/or direct flame contact. These spot fires typically have notably lower intensity relative to the main fire front (Potter & Leonard, 2011), but under favorable conditions, these spot fires can spread to nearby structures. It is important to note that only tall, thick flames can radiate sufficient heat to ignite buildings, while smaller flames need to be close or in contact with the building to cause ignition (Himoto, 2022). After wildfires, unburned tree canopies are commonly found in close proximity to areas destroyed by a crown fire. These unburned areas are commonly referred to as “islands.” This pattern contrasts with the 6 Insurance Institute for Business & Home Safety damage pattern often seen in floods where areas within the flood zone are contiguously destroyed. Fire spreads when the conditions for combustion—the fire triangle—are met along its path. Combustion requirements in the forest apply equally to WUI fires where structural and wildland fuels are present (Cohen, 2000). Reducing the vulnerability of buildings to embers and flames would result in “islands” of homes and buildings in the WUI within the path of a wildfire. Although experimental and numerical studies providing information about ember storms in the WUI exist, predicting the path and accumulation position of embers is challenging. During flight, embers follow an unpredictable flight path influenced by building features and environmental parameters. The geometric characteristics of buildings, density of buildings in a community, and characteristics of receptive fuel beds affect ember accumulation, as does the shape of the embers, fuel from which the embers were generated, and local wind patterns, particularly wind vortices. Learn more at https://ibhs.org/risk-research/wildfire/.