Building ignitions during wildfires occur when a component or components of a home or building are exposed to one or more of the three basic wildfire exposures:
- Burning embers (also called firebrands)
- Direct flame contact
- Radiant heat
As part of its research effort to study and understand the vulnerabilities of buildings subjected to wildfire exposures the Insurance Institute for Business & Home Safety (IBHS) has developed the capability of simulating ember and radiant heat exposures on building components and assemblies at their Research Center in Richburg, South Carolina. The primary objective of this research is to reduce the likelihood of wildfire-caused building ignitions in communities located in wildfire-prone areas. The goal of this article is to provide a summary of findings from recent testing at the Research Center.
Burning embers are the most important cause of home ignitions. When they land near or on a building they can ignite nearby vegetation or accumulated debris on the roof or in the gutter, or enter the building through openings (an open window or vent for example) and ignite furnishings in the building or debris in the attic. Near-building ignitions will subject some portion of the building to either a direct flame contact exposure, where the flames actually touch the building, or a radiant heat exposure, the heat you feel when standing near a campfire or fireplace. The vulnerability of a building to radiant heat depends on the intensity and duration of the exposure.
Exposure to Embers (Firebrands)
In collaboration the USDA Forest Service and the Savannah River National Laboratory, IBHS developed the capability of injecting burning embers into the wind stream in the large testing chamber, effectively reproducing ember storms typically observed during wildfires. This article provides a summary of, and some of the lessons learned or confirmed from this recent ember exposure study.
Figure 1 shows the ember generating equipment discharging embers. The building that was designed and built for the ember exposure studies are shown in Figures 2 and 3. During a series of tests different components on the building investigated to evaluate their potential vulnerability. The components included different roof coverings, a roof dormer and roof valley, gable end and eave vents, gutters, windows and window screening, siding and mulch. The building was rotated on a turntable to enable expose of different roof coverings, roof designs and other building component to wind-driven embers. During the series of tests reported here, only wind speeds of 10 to 15 miles per hour were used.
Figures 4 and 5 show the results of ember-started fires in the field of the roof (i.e., away from the roof edge or roof to wall intersection). The untreated wood shakes ignited as a result of the ember exposure. These ignitions were localized and the fires, as seen here, were initially small. The fires burned through the shakes and the underlying roof sheathing, and would have entered the attic. The roof fires were extinguished early, but if left unchecked, would have spread over the roof surface and into the attic. The embers ignited the pine needle debris in the valley of the asphalt composition shingle roof (Figure 5). The shingles in this valley used a woven installation technique. Although the roof covering was damaged by the burning pine needles, and contribution of the combustible “asphalt” component in the roof covering, the Class A shingles were not threatened in terms of the fire penetrating through the covering and into the attic due to the protection offered by the fiberglass component.
Pine needles or other combustible debris can accumulate at roof-to-siding intersections. If ignited by embers, the resulting fire will impinge on the siding. As observed in Figure 5, a Class A roof covering can withstand the flame contact exposure if accumulated debris is ignited. The adjacent siding and sheathing in the dormer construction must provide similar protection, or it will become the vulnerable component. It is important to keep accumulated debris to a minimum, but if present and ignited, the vulnerability of the roof won’t be the Class A roof covering, it will be the components used to construct the dormer. In this example, fiber cement siding was used as the siding material, but a combustible under eave soffit material would also be vulnerable, particularly toward the back end of the dormer where the roof-to-soffit distance is lower.
Based on the IBHS studies to date, vents more vulnerable to the entry of embers were those whose face was perpendicular to the wind flow carrying the embers. Examples of these vents were gable end vents (Figure 7) and the under eave vents in open-eave construction. Vents in open-eave construction are located in the blocking members and are sometimes called frieze block vents. Ember entry into the attic through vents in soffited (boxed-in) eaves was minimal, and most of the ember entry observed for the boxed-in eaves resulted from embers entering through the gap between the fascia and roof sheathing (Figure 8).
Embers easily ignited both the pine needle debris that was placed in the gutters and the pine needle and bark mulch placed at the base of the exterior wall. Both vinyl and metal gutters were used in these tests. In Figure 9, the gutter on the left was vinyl and that on the right was metal. Once debris in the vinyl gutter ignited, it detached from the fascia and fell to the ground. The burning debris contributed to the fire that resulted from the ignited mulch. The metal gutter stayed in place, providing a flame contact exposure to the fascia and sheathing at the edge of roof.
When vinyl siding was used, the direct flame contact from the burning mulch and gutter debris caused the siding to deform and fall away from the wall, exposing the underlying sheathing. Flames from the ignited debris and near-building vegetation also resulted in a direct flame contact exposure to the fiber glass window screen (Figure 10), allowing both flames and embers to enter the building. Entering flames and embers can easily ignite curtains and other interior furnishings. Ignition of the near-building mulch, and the damage from the resulting flame exposure, reinforces the importance of maintaining a low-or non-combustibility zone near the home or building.
As seen in Figure 8, without effective flashing at the roof edge, this exposure could also result in additional entry of embers at the gap between the top of the fascia-to-roof sheathing.
Embers ignited the combustible mulch and vegetation located in the re-entrant corner shown in Figure 3. The wind flow in this corner resulted in rapid vertical flame spread upward to the soffited eave, impinging on the soffit material and strip vent (Figure 11). The aluminum soffit vent melted and flames entered the enclosed eave above the soffit but the fire was suppressed before any interior damage was sustained. The rapid upward spread of flames was likely exacerbated by the vortex that was created in the corner. Creating a non- or low-combustibility (e.g., irrigated lawn, use of non-woody [herbaceous] plants) in this area is critical. Use of noncombustible siding in this area may also be prudent.
Implications for Ember (Firebrand) Exposures
This series of ember experiments clearly showed the potential for embers to ignite vegetative debris that can either accumulate on the roof, in a gutter, or at the base of a wall, and combustible mulch products that can be placed adjacent to the home or business. Ignition of these combustible materials will result in a flame contact exposure to adjacent materials, including siding, windows, materials at the edge of the roof, and even nearby vegetation.
Although additional experiments are needed, results from these experiments show that gable end vents and eave vents in open eave construction are more vulnerable to the entry of embers than eave vents in soffited eave construction. The vulnerability of gable end vents suggests that foundation (crawl space) vents would also be vulnerable to ember entry. Although through-roof attic vents were not evaluated in this series of experiments, these results suggest that dormer-type trough-roof vents would also be vulnerable to the entry of wind-blown embers.
Exposure to Radiant Heat
Human skin is much more sensitive to radiant heat than most building products. Whereas skin can develop severe burns in a matter of seconds when exposed to radiant heat of 15 kW/m2, wood can withstand 20 minutes or longer to this exposure before igniting. Therefore, the radiant heat level has to be high enough, and the exposure period long enough, for combustible building products to ignite or suffer other forms of degradation, such as causing glass breakage in a window. Exposure to lower levels of radiant heat can pre-heat a material, making it easier to ignite from a direct flame contact exposure. The objective of this series of tests was to improve our understanding of the sensitivity of exterior-use construction products to radiant heat. Once siding ignites, for example, it can either enter the building through the stud cavity and/or result in vertical flame spread up the wall, impinging on a window or entering the attic by burning through materials in the eave. Once the glass in a window breaks, embers can readily enter the occupied space of the building and ignite interior furnishings.
The radiant panel was 50 inches wide and 63 inches tall (Figure 1) and consists of 50 infrared natural gas burner heads arranged in five rows of ten burners each. A 15 kilowatt per square meter (kW/m2) and 35 kw/m2 exposure was obtained at separation distances between the panel and the test assembly of 40 inches and 20 inches respectively. The tests reviewed here were conducted at the 35 kW/m2 level.
Tests were conducted for windows that used different framing materials, including wood, vinyl and aluminum, and different kinds of glass, including annealed (single and dual-pane units) and tempered (dual pane). Window “failure” can occur if the glass breaks or falls out or if the framing material ignites and the fire burns through the material into occupied space of the home or business. Testing at other research labs has indicated that the glass is the most vulnerable part of a window. Results of the IBHS testing support that finding. More importantly, IBHS’ findings support the use of dual-pane windows in that the outer pane failed first, and then the inner pane at a later time, if at all. At the 35 kW/m2 exposure, neither of the panes in dual-pane tempered glass windows broke during the 25 minute exposure.
Although other testing conducted separately on glass and curtain materials indicated that curtains located behind annealed glass and tempered glass commonly used today will not ignite before the window breaks, one objective of the IBHS tests was to demonstrate this in a laboratory. IBHS did this using a vinyl frame, dual pane annealed glass window, at the same 35 kW/m2exposure. As shown in Figure 2, the curtain will ignite, but this occurred more than a minute after the upper section of the window fell out. The lower section stayed in tack during the test. This result supports the previous testing conducted separately on individual components—the glass breaks first, and then the curtain ignites.
Another goal of the window testing was to evaluate the contribution of screens in reducing the amount of radiant heat that is transmitted into the building. Vinyl-frame, single-hung, dual-pane annealed glass windows were used for these tests, which evaluated the effects of both plastic clad glass-fiber and metal screens. The heat flux sensors were positioned behind the upper and lower sections of the windows. The screens were only positioned in front of the lower section since it was the only section that could open. Results of these tests are shown in Figure 3.
The data from the heat flux sensors behind the screens are the lower two graphed lines in Figure 4. These results showed that window screens absorb radiant heat and thereby reduce the amount that is transmitted through the glass into the occupied (living) space, in this case by about one-third. There are two other interesting observations that can be seen in this graph. First, note that the measured heat flux behind the unscreened windows was less than 12 kW/m2, indicating that the glass was effective in reflecting or absorbing radiant heat. Second, the plastic clad glass fiber screen seemed to do a slightly better job than the metal screen, indicating that the metal screen may re-radiate heat back towards the window.
The wood siding products subjected to the radiant panel exposure all ignited in times that ranged from about 4.5 minutes to 16 minutes. Such a range in ignition times is not uncommon, particularly given that the updraft created by the volatiles coming off of the wood and wood-based siding products extinguished the pilot flame located at the top of the wall sections. The time to ignition for the painted products was faster than that for the unpainted products and the time to ignition for the flat profile products (in this case the plywood T1-11 panelized siding products) was faster than that for the profiled siding product (in this case a solid wood horizontal lap siding with a bevel profile).
Two different vinyl siding products were tested, including a standard product and a “heavy” product. These products differed in their thickness, with the “heavy” product being about 0.01 inch thicker than the standard product. The response of both of the vinyl siding products to the imposed radiant exposure was similar. Neither ignited in flaming combustion but both started deforming immediately, exposing the underlying sheathing material about a minute into the test (Figure 4).
Implications for Radiant Heat Exposures
The 35 kW/m2 exposure level used in this series of experiments is relatively high. Cohen (2004) reported on a component of the large international crown fire modeling experiments that took place in the Northwest Territories, Canada between 1997 and 2000. As part of that larger study, Cohen set up wall assemblies at fixed distances from the edge of the burn plots. These assemblies were instrumented with heat flux sensors that measured the radiant heat from the fires that were intentionally set at the edge of the burn plots. The resulting crown fires moved toward the wall assemblies and the heat flux was measured and recorded as a function of time. These results clearly showed that the measured heat flux at wall assemblies within 30 feet of a crown fire could reach and exceed 35 kW/m2, however this level was maintained for at most one minute since the fire was moving so rapidly.
Since IBHS results, and others, show that it takes minutes for most combustible products to ignite and for glass in windows to break, much less ignite curtains inside the home or business, at this exposure level, is building exposure to radiant heat really a problem? If you follow recommended vegetation management practices and develop and maintain defensible space zones, this level of radiant exposure is not likely to occur. When vegetation burns, it is typically a quick process, so whereas it may burn intensely, it will be for a relatively short period. Burning vegetation presents much more of a problem when it is close enough for the flames to actually touch the building or an attachment to the building, such as a deck. 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.
A scenario that can result in an elevated radiant exposure to your home for an extended period would be the ignition of a nearby building. This could be a detached garage or outbuilding, or a neighbor’s house. Buildings are stationary—if they ignite, they burn in place (Figure 5). Figure 6 shows the side of a home located about 40 feet from another home that ignited and burned to the ground during a southern California wildfire. The “wildfire” exposure to this house was largely a radiant heat exposure from the neighbor’s home. Vegetation between the two homes was larger, well pruned pine trees which survived, and Manzanita that was located about 20 feet from the damaged home. The surviving home suffered considerable damage, but it wasn’t destroyed. This home was clad with vinyl siding that had been installed over an existing solid wood siding product. After the vinyl siding drops off, the home must rely on underlying sheathing materials for protection from radiant and direct flame contact exposures. The remnants of the remaining vinyl siding can be seen in Figure 5. The window glass was dual pane, annealed. The outer pane of the dual pane window broke and fell out but the inner pane remained in place.
Cohen, Jack D. 2004. Relating Flame Radiation to Home Ignition Using Modeling and Experimental Crown Fires. Canadian Journal of Forest Resources 34:1616-1626.