When scientists burn full-size buildings at the IBHS Research Center, we learn how buildings ignite — and how to mitigate the impact of those fire-spreading embers.
Imagine a wildfire approaching your house. Is it the direct flames you fear? The radiant heat? Or is it the ember wash you'll want to protect against? Thanks to work at the Insurance Institute for Business & Home Safety Research Center (IBHS), we don't need to just imagine the effects of a firestorm. Now we can watch and study how fires burn houses and learn how to build and maintain homes so they won't ignite.
The principal component of the IBHS Research Center is a large wind tunnel and test chamber that can hold full-size, one- and two-story structures. IBHS's general research objective is to scientifically identify actions to strengthen homes and businesses against natural disasters, including hailstorms, devastating winds and wildfire. The primary objectives of our wildfire studies are to better understand the vulnerabilities of buildings to wildfires, to develop and evaluate mitigation options, and to transfer this information to the end users in order to reduce home and building loss.
Wildfires can ignite buildings in three ways: 1) burning embers (also called firebrands), 2) direct flame contact and 3) radiant heat. Burning embers are the most important cause of home ignitions. When they land near or on a building, they can ignite vegetation or accumulated debris on the roof or in the gutter. They can also penetrate directly into the building through openings and ignite furnishings in the building or debris in the attic. Near-building ignitions will subject some portion of the building to either direct flame contact — where the flames actually touch the building — or to radiant heat.
For the initial series of wildfire studies recently completed, IBHS partnered with Savannah River National Laboratory, the U.S. Forest Service and Clemson University in a project funded by the U.S.Science and Technology Directorate. Through this collaboration, we developed the ability to subject test buildings to carefully controlled radiant heat and ember attack.
We simulate ember attack using the equipment shown in Figure 1. Five combustion chambers filled with wood dowels and bark mulch are placed below grade in a trench at the outlet of the wind tunnel. After ignition by a gas burner in the combustion chamber, a fan pushes the glowing bark and dowel embers up through vertical ducts and into the wind stream of the wind tunnel.
Radiant exposure is simulated using a radiant panel that is 50 inches wide and 63 inches tall (Figure 2). It consists of 50 infrared natural gas burner heads arranged in five rows of 10 burners each. The surface temperature of each burner is approximately 1,700°F (925°C). We adjust the amount of radiant heat exposure to the target material by moving the target closer to or further from the radiant panel. Results from calibration testing indicated that a 15 kW/m2 and 35 kW/m2 exposure is obtained at separation distances of 40 inches and 20 inches, respectively. Most of the tests during our initial studies were conducted at 35 kW/m2.
Ember testing was conducted on a building that was designed to investigate and demonstrate vulnerabilities of selected components. The building was rotated on a turntable to expose different roof coverings and other materials and design features to wind-driven embers. Radiant panel testing was conducted on individual siding, window or eave components and configurations.
Ember Testing - Selected Results
These tests demonstrated the ease with which embers can ignite vegetation debris, mulch and other combustible materials. If the debris or mulch is adjacent to or under a wall or deck, or in a gutter, direct flame contact can ignite siding and decking (Figure 3).
As long as a window screen was in place, large embers were prevented from entering the interior of the test building. The fiberglass window screen failed when subjected to direct flame contact, after which embers and flames readily entered.
Radiant Panel — Selected Results
Although other testing conducted separately on glass and curtain materials indicates that curtains located behind the annealed glass and tempered glass commonly used today will not ignite before the window breaks, we wanted to demonstrate this in a laboratory. For this test, we used a vinyl frame, dual-pane, annealed-glass window at a 35 kW/m2 exposure.
As shown in Figure 4, the curtain did ignite, but ignition occurred more than a minute after the upper section of the window fell out. The lower section stayed in place during the test. This result supports the previous testing conducted separately on individual components — the glass breaks first, and then the curtain ignites. These tests also demonstrated the effectiveness of window screens in reducing radiant heat transmitted through the window glass.
Defensible Space Is Key
The 35 kW/m2 exposure level used in the radiant exposure testing was relatively high. In a 2004 research paper, Jack Cohen, from the Fire Sciences Laboratory in Missoula, Montana, reported on large, international crown-fire-modeling experiments that took place in Canada's Northwest Territories. The results clearly showed that the heat flux at wall assemblies within 30 feet of a crown fire can reach and exceed 35 kW/m2. However, since the fire was moving so rapidly, this level was maintained for at most one minute.
Given that it can take multiple minutes for glass to break and combustible materials to ignite at this exposure level, is radiant exposure from a wildfire really a problem? If you follow recommended vegetation management practices and develop and maintain defensible space zones, this level of radiant exposure is not very likely, unless a nearby building ignites and burns. When vegetation burns, it is typically a quick process. Although it may burn intensely, it will be for a relatively short period.
As demonstrated in the ember tests, direct flame contact is a much more significant concern. Direct flame contact is likely when ember-ignited mulch and debris are close enough for the flames to actually touch the building or attachment. The intent of the defensible space requirements is to significantly reduce the opportunity for the flames from a wildfire to reach your home or business.
Steve Quarles, Ph.D., joined IBHS Research Center in 2011 as a senior scientist after 26 years with the University of California. His research and outreach focuses on wildfire protection for residential and light commercial buildings and improving the moisture-related durability and resilience of buildings following hurricane/high-wind events.
Steve occupies the South Carolina Wind and Hail Underwriting Association Hazard Resilience Chair at the Research Center.