Start a fire science degree at American Military University.
By Brad Davison, alumnus, American Military University
The way an exterior fire spreads onto a structure has changed dramatically over the years, which means firefighting tactics must also change as well. Until recently, a small outside fire that ignites the exterior of a structure was slow or unable to spread into the attic or interior. Today, however, that same exterior “rubbish” fire can transition to a whole-structure fire within as little as two minutes after starting.
Firefighters and researchers attribute this significant change in the way fire reacts and spreads on the exterior of structures to changes in building codes, construction techniques, and building materials. Similar to what these professionals are seeing with interior spreads, fire dynamics are changing largely due to the use of petroleum-based products in home construction.
Changes to Building Materials and Construction Techniques
Historically, exterior walls were covered by wood, brick, or stucco. In the 1960’s, vinyl siding quickly became one of the most popular exterior wall building materials. Under the vinyl siding builders commonly installed a wood or plywood wall and vapor barrier. These wood or plywood exterior walls were slow to ignite and difficult to sustain combustion once a fire started.
Furthermore, homes did not have porous eaves, which can allow uptake ventilation (or flame) into the attic, until after building codes changes in 1942. When homes built using these materials and building techniques had exterior fires, the fire was most commonly contained to the object of origin.
Today, however, exterior siding is commonly placed atop highly flammable foam board that allows fire to easily spread into the attic. Since the start of the 21st century, the International Code Council (ICC) has increased the R-value required for exterior walls significantly.
Today, 90 percent of new homes constructed in the U.S. are mandated to have a minimum rating of R-20 thermal resistance in exterior walls. In order to achieve this stricter regulation, builders commonly use a rigid foam insulation board between the exterior siding and OSB wall. Popular interior/exterior rigid foam board insulation is made of extruded polystyrene, a derivative of styrene, which combusts quickly and is listed on the Hazardous Substance List due to its flammability and its unhealthy effects on humans.
Research Shows Exterior Fires in Modern Homes Can Spread to Attic in Two Minutes
Experiments by the Firefighter Safety Research Institute (UL FSRI) have shown the differences in fire spread by various building materials and techniques. Homes built using wood, stucco, and fiber cement siding failed to sustain burning after 10 minutes of flame exposure. However, plastic siding over extruded polystyrene transitioned to an attic fire within just two minutes of ignition.
A 100-kW fire, which is equivalent to a small grill or trash can fire, can spread up two stories of vinyl siding and foam board and into an attic within these two minutes. Flames that reached the modern porous soffits met little resistance and entered the attic without hindrance.
Comparatively, that same size exterior fire took 25 minutes to transition into an attic fire in homes that did not have the extruded polystyrene foam board. Solid, unventilated eaves resisted flame spread into the attic significantly. However, it’s important to note that these homes create an increased exposure threat to adjacent structures due to the radiant heat that is directed away from the structure of origin.
Firefighting Tactics for Exterior Fires in Modern Homes
When fighting an exterior fire, UL FSRI suggests that extinguishment should start on the exterior. Researchers have found that opening interior walls to look for void fires prior to extinguishment of the exterior fire increased ventilation and exacerbates the fire dynamics. Thorough knock-down of the exterior fire as part of firefighting tactics should then transition into an aggressive push to the interior, and the attic in particular. Finishing up, firefighters must overhaul the entire involved wall because pockets of smoldering debris are often found in the walls.
Firefighters Must Understand New Home Building Practices
As part of these advanced firefighting tactics, firefighters must also understand the evolution in building techniques so they can make informed tactical decisions. As the adage goes, firefighters should always suspect an attic fire in a balloon-frame home. It’s important for firefighters to now have a similar suspicion about an exterior fire that has neared the eaves, as there’s a very good chance the fire has also entered the attic.
UL FSRI has shown through scientific research that modern building practices and furnishings have sped fire growth by as much as 10 times. Homes with plastic siding and polystyrene exterior wall insulation have the potential to spread from an outside trashcan fire into an attic fire in the time it takes to complete a 911 call. So, it is critical that fire departments stay current as building codes and construction techniques continue to advance.
For a safe and effective operation, firefighters and commanders must base their tactical decision-making on a solid understanding of contemporary building codes and practices. Unfortunately for firefighters, legacy construction methods that once minimized fire spread have largely been replaced by materials and construction practices that might improve a home’s performance, but often worsen its fortitude during a fire.
About the Author: Brad Davison is a Firefighter/Paramedic at the Maplewood Fire Department in Minnesota. In the fall of 2017, Brad completed his Master’s degree in Public Administration, with a concentration in Emergency Management from American Military University. Brad has been a contributing author to numerous Fire and EMS publications, and enjoys teaching whenever possible. To contact the author, please email IPSauthor@apus.edu. To receive more articles like this in your inbox, please sign up for In Public Safety’s bi-monthly newsletter.