WILDFIRE RESEARCH

EMBER INTRUSION THROUGH VENTS APRIL 2014

Wildfire Research: Ember Intrusion through Vents

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There have been two series of wildfire experiments conduct-ed at the IBHS Research Center since it opened in 2010. The first was in 2011, and the second was completed in January this year, with analysis continuing. External funding was secured for both series of experiments; the most recent from a CSAA Community Safety Foundation Grant.

Research ObjectivesThe objective of the 2011 project was to demonstrate the potential for ember ignition of fine fuels, and the vulnerabil-ity of a building to the resulting flames and/or radiant heat. The objective of the 2014 project was to clarify the relative importance of vents, including style, type and location, to the entry of embers (also called firebrands). Vents are vulnerable to wildfires when embers or flames enter into an enclosed attic or crawl space, and ignite combustible materials in those areas. IBHS researchers also are evaluating the effectiveness of vent-related mitigation strategies for new and existing buildings, such as mesh screening.

Preliminary Findings

While data analysis is still underway, visual observations by IBHS researchers indicate the following:

1. Finer mesh screens at the vent openings reduces the size of ember that can enter the attic or crawl space.

2. Vents with openings that are perpendicular to the wind flow (vertical orientation) are more vulnerable to the entry of wind-blown embers than vents with openings that are parallel to the wind flow (horizontal orien-tation). Gable end vents and vents in the blocking of open eaves are examples where openings are typically perpendicular to wind flow. An example of a vent with an opening that is parallel to wind flow is one in a soffited-eave. Therefore, the preferred under-eave (inlet air) vent would be one in a soffited-eave. Ember entry through vents in open-eave blocking is shown in Figure 1.

3. Even in the worst cases of observed ember entry into the attic space, wood members (trusses and plywood) did not ignite. If embers were able to accumulate next to combustible items that are commonly stored in attics, such as old magazines, clothes and cardboard boxes, ignition would be more likely. These burning items would be able to ignite the lumber and sheathing used in the attic structural support system. Under this scenario, the house would burn from the inside, out. Minimizing the amount of combustible items stored in the attic (or crawl space) would reduce the chance of an ember ignition.

Overview

Figure 1. This photo from inside the attic of the test building shows bright streaks and dots, which are

embers that entered the attic space through vents (one of these vents circled in white) in open-eave blocking.

Wildfire Research: Ember Intrusion through Vents

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The experimental design enabled researchers to examine the relative importance of style, type, and location of vents in ember intrusion, as well as to evaluate the effectiveness of vent-related mitigation strategies for new and existing buildings. The control condition used for the experi-ments was 1/4-inch screening. The performance of 1/8-inch and 1/16-inch mesh noncombustible screening also was evaluated for gable end vents. The experimental plan included evaluating the basic types of vents commonly used in attics and crawl spaces, and included:

1. Attic inlet vents

a. Soffited-eave (strip vent)

b. Open-eave (vent in between-rafter blocking)

c. Off-ridge (through-roof ), vent located near the exterior wall

2. Attic outlet vents

a. Gable end (including the two ember- and flame-resistant vents accepted by the California Office of the State Fire Marshal)

b. Turbine

c. Ridge (including a Miami-Dade wind-driven rain-rated vent)

d. Off-ridge (including the only ember- and flame-resistant vent option of this type accepted by the California Office of the State Fire Marshal)

3. Foundation (crawl space) vents

Three wind speeds were used to evaluate the influence of wind; building orientation was varied to examine the effect of wind direction on the vulnerability of vents to ember entry.

Methodology

Wildfire Research: Ember Intrusion through Vents

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After reviewing the current condition of the ember generators used in the 2011 study, and the performance of the generators during those tests, it was decided to design a new system. Regarding performance of the generators, the batch process used in the original design resulted in limited run time during the experiments, and a relatively long interval between tests because of the re-loading procedure. At roughly two individuals per generator, personnel requirements were also a disadvantage. The new generators allowed testing to run continuously by intermittently reloading the raw material storage hoppers, and also provided a more uniform exposure to the test building and vents. Raw material loading for all generators could be accomplished by one individual. The raw material used to generate embers was dried wood chips (locally sourced) and commercially available wood dowels, mixed in an approximate 85:15 ratio. There were five generators used in the 2011 testing, and two were added to the new system designed for the current project for a total of seven (Figure 2). The two additional generators improved the ember distribution across the width of the wind tunnel.

To generate embers, a screw conveyor fed the raw material from a storage hopper to the burn chamber where it was ignited by a natural gas burner. Blown-in air entered the burn chamber from the bottom and carried the lofting burning embers upwards through the chamber and into the vertical ducting attached to the top of the burn chamber that deliv-ered the embers into the wind stream of the tunnel (Figure 3).

Ember Generator System

Figure 2. Ember generators at the IBHS Research Center.

Figure 3. The ember generation system consisted of the raw ma-terial storage hopper (left), the burn chamber (right), an auger

feed screw that delivered the raw material to the burn chamber through an in-feed chute, an air delivery system (not shown)

and vertical ducting at the outlet (top) of the burn chamber (not shown). These items were located below grade in a trench.

Figure 3. The ember generation system consisted of the raw material storage hopper (left), the burn chamber

(right), an auger feed screw that delivered the raw mate-rial to the burn chamber through an in-feed chute, an air

delivery system (not shown) and vertical ducting at the outlet and top of the burn chamber (not shown). These

items were located below grade in a trench.

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For each test, video cameras were positioned at the outlet of the ember generators and at exterior and interior vent locations. The consistency of ember production from test-to-test over the course of the experiments, and ember arrival at, and penetration through vents, is being analyzed using a post-processing procedure whereby embers are counted by using a particle tracking algorithm. Several experiments were conducted each day and therefore consistency of exposure from test-to-test is an important component of the analysis and evaluation. An example of particle tracking output is shown in Figure 4.

Research Documentation

Figure 3. The ember generation system consisted of the raw ma-terial storage hopper (left), the burn chamber (right), an auger

feed screw that delivered the raw material to the burn chamber through an in-feed chute, an air delivery system (not shown)

and vertical ducting at the outlet (top) of the burn chamber (not shown). These items were located below grade in a trench.

Figure 4. Results of the particle tracking through the 1 ft2 grids (two left-hand objects in this figure), and the counting algorithm that is being used to evaluate consistency of ember generator output. The far right-hand

object in this figure represents the final count at the end of the run of a given test. A red color indicated a larger number of embers passed through the 1 ft2 grid.

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Next StepsIBHS researchers will continue analyzing the data produced during the 2013-14 experiments to quantify the relative importance of vents to ember intrusion. Specifically, researchers anticipate issuing a final report by the end of this year, which will examine the style, size and placement of vents with regard to ember entry into the vented areas of a building.

WILDFIRE RESEARCH EMBER INTRUSION THROUGH VENTS

APRIL 2014