Each year, fires in the wildland-urban interface (WUI)—the place where homes and wildlands meet or intermingle—have caused significant damage to communities. To contribute to firefighter and public safety by reducing the risk of structure ignition, fire blankets for wrapping a whole house have been investigated in the laboratory and prescribed wildland fires. The fire blankets aim to prevent structure ignition (1) by blocking firebrands to enter homes through vulnerable spots (gutters, eaves, vents, broken windows, and roofs); (2) by keeping homes from making direct contact with flames of surrounding combustibles (vegetation, mulch, etc.); and (3) by reflecting thermal radiation from a large fire within close range (adjacent burning houses or surface-to-crown forest fires) for a sustained period of time. In the laboratory experiment, two-layer thin fabric assemblies were able to block up to 92% of the convective heat and up to 96% of the radiation (with an aluminized surface). A series of proof-of-concept experiments were conducted by placing instrumented wooden structures, covered with different fire blankets, in various fires in ascending order of size. First, birdhouse-sized boxes were exposed to burning wood pallets in a burn room. Second, wall-and-eave panels were exposed to prescribed fires climbing up slopes with chaparral vegetation in California. Finally, a cedar shed was placed in the passage of the prescribed head fire in the Pine Barrens in New Jersey. The experiments demonstrated both successful performance and technical limitations of thin fire blankets. The key success factors in protecting the WUI structure are (1) the fire blanket's heat-blocking capability, (2) endurance under severe heat-exposure high-wind conditions, and (3) proper installation. Additional studies are needed in the areas of advanced material/layer development, blanket deployment methods, and multi-structure protection strategies.

Introduction

Background

Housing development in the wildland-urban interface (WUI), i.e., the place where homes and wildlands meet or intermingle, is growing (U.S. Fire Administration, 2002; Radeloff et al., 2005, 2018; Hammer et al., 2007; Stewart et al., 2007; Stein et al., 2013; Kramer et al., 2018). Between 1990 and 2010, the WUI was the fastest-growing land use type in the United States, and 97% of new WUI areas were the result of new housing rather than increases in wildlife vegetation (Radeloff et al., 2018). WUI fires have caused significant damage to communities (Cohen, 1999; Mell et al., 2010; Stein et al., 2013). The magnitude of the fire damage is increasing as well. Major wildfires California in 2018 caused over $12 billion in property damage (Evarts, 2019). In 2018, the largest (the Mendocino Complex Fire burned 459,123 acres), most destructive (18,804 structures were destroyed in the Camp Fire), and deadliest (86 deaths in the Camp Fire) wildfires in modern California history have occurred at the same time (Cal Fire, 2018; Verzoni, 2019). Like urban conflagrations a century ago, wildfire in urban and suburban settings poses one of the greatest fire challenges of our time (Grant, 2018).

The WUI fire problem can be thought of as a structure ignition problem (Cohen, 1991; Mell et al., 2010) and an effective approach to mitigating the problem is to reduce the potential for structure ignition (Cohen and Stratton, 2008). Thus, if the structure ignition is prevented, WUI fire damage can be reduced and the safety of the public and firefighters will be improved. In a wildland fire, firebrands/embers (i.e., burning branches, leaves, or other materials) are lofted and carried by the wind and start distant spot fires. The cause of the initial structure ignitions in a WUI community is predominately due to exposure to firebrands (embers), generated by a wildfire or burning structures, and/or the heat flux from flames. Post-fire studies (Leonard, 2009; Maranghides and Mell, 2009; Morgan and Leonard, 2010) suggest that the firebrands are a major cause of structural ignition of WUI fires in the U.S. and Australia. A case study (Cohen and Stratton, 2008) revealed that burning homes and surrounding vegetation ignited adjacent homes initiating a “domino effect” of home destruction without wildfire as a major factor. Most of the homes (193 out of 199) destroyed and damaged ignited homes in two ways: (1) from spreading through surface fuels within the residential area that contacted homes and/or from firebrands and/or (2) from thermal exposure directly related to burning residences from structure flames and firebrands. Cohen and Stratton (2008) also concluded, “Firefighters were overwhelmed in their attempt to prevent the residential fire spread due to multiple homes burning simultaneously. However, more homes would have burned without their intervention.” Another case study (Maranghides and Mell, 2009) found that firebrands ignited at least 60% of the destroyed structures in the WUI community. The likelihood of a structure's ignition is dependent both on its physical attributes (e.g., roofing material, decks, and vents) and the fire exposure conditions (e.g., magnitude and duration of heat flux from flames and firebrands).

Potential structure ignitions due to uninterrupted fire spread through vegetation to the structure were also reported in the perimeter of the community (Maranghides and Mell, 2009). Thus, the location of the structure in the WUI development community (perimeter or interior) is also an influencing factor (Maranghides and Mell, 2009). Mell et al. (2010) emphasized the research needs to characterize the exposure conditions and the vulnerability of a given structure design or building material when subjected to a given exposure. Butler (2010) pointed out that in the past, it had been stated that, at least for crown fires, radiant energy transport dominated the energy exchange process (Albini, 1986). More recently, laboratory and field studies indicated that convection might be just as critical to the energy transport as radiation (Anderson et al., 2010; Finney et al., 2010; Frankman et al., 2010). In the international crown fire modeling experiments in 1999 (Putnam and Butler, 2004; USDA Forest Service, 2009), one of the fire shelter testing showed that an average heat flux was measured at 80 to 100 kW/m², while peak heat flux was over 200 kW/m², and maximum (environment) temperature exceeded ≈1,300°C. Ignition of structures by burning vegetation (crown fires) is also possible (Cohen, 1999; Evans et al., 2004). In more recent fire spread experiments (Morandini et al., 2007), the peak heat fluxes measured during the four experiments increased in the range of 39–112 kW/m² with flame front size in the field (5 m × 5 m to 30 m × 50 m).

To mitigate risks of ignition of homes, there are resources available to homeowners (Cal Fire, 2006; Ahrens, 2010; Quarles et al., 2010; Stein et al., 2013; ICC, 2018; NFPA, 2018). NFPA 701 (2018)—Standard for Reducing Structure Ignition Hazards from Wildland Fire provides a methodology for assessing wildland fire ignition hazards around existing structures, residential developments, and subdivisions. The risk-assessment and risk-reduction guidelines can use the concept of home or structure ignition zone [NFPA 701, 2018] or defensible space (ICC, 2018) to categorize the recommended treatment of structure and vegetative fuels (Mell et al., 2010).

The role of structure-to-structure fire spread in WUI settings has not been given as much attention as vegetative-to-structure fire spread, which is valid for WUI communities with sufficiently low housing density (Mell et al., 2010). Post-fire analysis found that structure-to-structure fire spread played a key role in the overall fire behavior, and heat fluxes from both the flame fronts and firebrands produced by structures were instrumental in maintaining fire spread to surrounding structures and vegetation (Mell et al., 2010). Mell et al. (2010) pointed out a need to assess the effectiveness of the guidelines across a range of WUI fire setting (e.g., housing density, terrain, vegetative fuels, winds, wildland fuel treatments) and exposure conditions (heat flux from flames and firebrands generated by burning vegetation or burning structures). The 2018 Camp Fire in California swept through and destroyed the town of Paradise, possibly by the “domino effect” in structure-to-structure fire. In residential developments and subdivisions with relatively high housing density with limited space surrounding homes, the implementation of the ignition-risk reduction guidelines may not be feasible. Therefore, there is an urgent need to implement technology-based solutions that can diminish ignition vulnerabilities of structures to firebrand showers and heat flux from flames, including structure-to-structure fire spread in high housing density.

While wildfires can rage for days, weeks, or even months, the duration required to protect homes by fire blankets may range widely from minutes to hours, depending on various factors, e.g., housing density, terrain, vegetative fuels, winds, heat flux from flames, and firebrands. In a relatively low housing density, a critical period can be several minutes during a wildfire front passes. Airborne embers or other materials from burning vegetation pose a threat to ignite a house for a much longer time, an order of 30 min before and after the spreading fire front. In a relatively high housing density, e.g., suburban community or urban setting, neighboring burning houses must threat the ignition of the structure for over an hour, possibly hours, if there is no intervention by firefighters.

Conventional measures in practice to prevent the structure ignition include the application of aqueous fire suppressants and retardants in the forms of foams, gels (USDA Forest Service, 2007), or water sprays, to the structure and/or surroundings prior to the arrival of the wildland fire front. Aerial firefighting using aircraft is also conducted to combat wildfires by dropping water or flame retardant. The advantage of these liquid spray coatings is that they can be applied to the structure parts with complex shapes (including decks, eaves, fences, etc.) and vegetation. The drawback is that they need water (at least 30 psi for ground operations), and spray application is difficult under windy conditions, and foams can be blown away by the wind before the wildfire front arrives. These coatings lose effectiveness with time as a result of water evaporation. Although gels are more effective than foams or water against thermal radiation exposure, their effectiveness decreased significantly even within an hour.

By contrast, more effective and long-lasting means of thermal shielding may be fire blankets, a.k.a. structure wraps. The U.S. Forest Service has occasionally been using the structure wraps to protect historic cabins from wildfires (Kuruvila, 2008; Miller-Carl, 2008; Backus, 2013; Gabbert, 2013; Montanez, 2014; Anon, 2018). Anecdotal evidence and technical know-how on the application of cabin wrapping have been accumulated over the last two decades. A typical description of the structure wrap in the news articles is “the wraps are similar to ones firefighters use for personal safety on the job, though they are thicker and the Forest Service says they are not exactly fireproof (Stephen, 2014).” Despite a common functionality between the fire blanket and the fire shelter as thermal insulation, the design goals (e.g., the interior temperature limit and the content endurance) in protecting a building structure are very different from those for a human body. Unfortunately, scientific research has rarely been conducted.

Literature Review

Fire blankets have been used for both fire suppression and protection. The literature on fire blankets is scarce probably because the basic research has not been fully conducted and the R&D efforts have mainly been made sporadically at manufacturers without dissemination of test results other than the specifications of final products. A few specifications available are: ASTM F 1989 (2005), the British Standards BS EN 1869 (1997); British Standards BS 7944 (1999), and the General Services Administration's procurement specifications (General Services Administration A-A-50230, 1987; General Services Administration A-A-54409, 1991; General Services Administration A-A-54629, 1992). More importantly, there have been no adequate performance-based standards and ongoing third-party certification to those standards specifically designed for fire blankets. As a result, fire blanket industry voluntarily used related compliance standards for flammability tests of blankets or fabrics such as ASTM D 4151 (2001) or NFPA 1144 (2004). In early 2007, the American National Standards Institute adopted ANSI/FM 4950 (2007), a performance-based standard for welding curtains, blankets and pads. Fabrics used for hot work operations such as welding and cutting are also commonly known as fire blankets. The performance of fire blankets for protection of stored ammunition was studied (Tewarson et al., 2001; Hansen and Frame, 2008).

Despite their easiness in handling compared to fire extinguishers, fire blankets have been used for smothering relatively small incipient fires only. They are generally not recommended to be used for a liquid fire or lab equipment as it can cause the fire spread, although some products are claimed to be useable for cooking oil fires. The old fire blankets, made of asbestos, were excellent at putting out fires. However, asbestos blankets were banned because of health hazards, and non-combustible glass fiber was chosen as a substitute material. For general purposes, including personal and burned victim protection, fire-resistant-treated cotton or wool blankets with or without a layer of gelled water are used in the military, fire departments, steel mills, etc. More recent fire blankets are made of fire and heat resistant aramid fabrics, which are more effective than wool blankets, and will not melt, drip, burn, or support combustion in the air. New types of fire blankets have been invented: non-woven polyester impregnated with a hydrous gel (Romaine, 1986), fabric made of mineral material containing basalt or a sodocalcic glass (Calderwood et al., 2006), or chemical compound which melts and reacts endothermically (Goldberg, 2006).

For the protection of building structures, various ideas of fire blanket deployment have been documented as the U.S. patents (Wagner, 1944; Ballinger, 1973; McQuirk, 1989; Gainer, 1992; Floyd, 1997; Hitchcock, 1997; Jones and Smith, 1998; Gleich, 1999; Kilduff and Oswald, 2003; Meyer and Kessler, 2004).

1. Blankets, which are rolled around cylinders inside housings attached to various parts of a building, are deployed by rotating the cylinders typically by electric motors.