WUI heat transfer facade experiments

Research Continues for Heat Transfer from Structure Fires Project

November 30, 2022

New façade experiments for UL’s Fire Safety Research Institute (FSRI) Heat Transfer from Structure Fires research project were completed in October and November. Conducted outdoors at the Delaware County Emergency Services Training Center (ESTC) facility in Sharon Hill, PA, these experiments follow up similar testing by FSRI that occurred in 2021 at UL Solutions’ indoor fire lab in Northbrook, IL.

During these outdoor experiments, target façades were exposed to a post-flashover fire from a source compartment with an attached façade. The source fire was allowed to transition through flashover to simulate fire spread from the interior to the exterior of a structure. To evaluate the potential for structure-to-structure fire spread, the target façades were placed at various distances from the source based on common separation distance requirements between residential structures in high density communities. Compared to the 2021 laboratory experiments, these experiments involved longer post-flashover exposures and smaller separation distances between the structures.

In the first round of testing, 12 experiments were conducted to examine the behavior of different exterior siding materials when exposed to the post-flashover source compartment fire. Three different cladding assemblies – T1-11, fiber cement siding, and exterior insulation finishing systems (EIFS) – were tested. Heat flux gauges were installed flush with the front of the target façade to measure the incident heat flux from the source fire at numerous locations across the target. For each siding type, two experiments were conducted with the target positioned 10 ft from the source. Then, two additional experiments were performed at a separation distance of either 6 ft or 14 ft depending on the behavior of the siding during the 10 ft experiments.

WUI heat transfer facade experiment focused on siding
Exposure of a target facade with T1-11 siding to a post-flashover fire from the source compartment and facade at a 14 ft separation distance.
Remote video URL

The second round of testing examined window failure as a potential mechanism for building-to-building fire spread. During each experiment, eight 2 ft x 3 ft windows with different types of glazing were mounted in a target façade and exposed to the same source compartment fire from the first round of experiments. Four types of double pane window configurations were considered: both panes plain (i.e. annealed) glass; outer pane plain glass and inner pane tempered glass; outer pane tempered glass and inner pane plain glass; and both panes tempered glass. Heat flux gauges were installed flush with the target façade around the windows to quantify the incident heat flux at each window during the experiments. Gauges were also positioned behind the windows to measure the heat flux transmission through the window panes. Additionally, recorded video footage was used to determine the failure times of the panes due to radiative heating from the source fire.

A total of eight experiments with windows mounted in the target façade were conducted. During six of the experiments, the window pane assemblies were installed in wood frames covered by a non-combustible material. During the two additional experiments, the window pane assemblies were mounted in vinyl frames and frames constructed entirely of a non-combustible material to study the potential of early window failure due to frame failure.

WUI heat transfer experiment with windows
Target facade windows during an exposure to a post-flashover fire from the source compartment and facade. By this time in the experiment, substantial window failure as well as sustained flaming combustion at the target facade around the windows had been observed.
WUI heat transfer experiment with windows
Broken window panes shortly before suppression during one of the window failure experiments.
"There are multiple vulnerabilities in a structure that can allow a nearby fire or heat to attack that structure. These vulnerabilities provide a means for flames or embers to enter that structure if something like a window should fail. Windows and siding are things we can address by making recommendations like what type of glass or siding material can be used based on failure rate."

Joseph Willi, Research engineer

Results from these experiments will provide new insight into the thermal protection capabilities of different exterior cladding assemblies and window glazings as they pertain to radiative heating from a nearby structure fire. Findings from this project will have critical implications for establishing proper control measures and supporting code development in both urban and wildland urban interface (WUI) environments.

Heat Transfer from Structure Fires