Radiant Risks: Understanding the vulnerability of windows to wildfire

March 11, 2024

Research from the Fire Safety Research Institute explores the vulnerability of windows to wildfires and how to enhance their fire resistance

The threat of wildland-urban interface (WUI) fires is escalating globally, posing unprecedented challenges, especially in the United States, where the interface between nature and urban development is increasingly becoming a battleground.

Understanding how these fires interact with the built environment is crucial.

Windows have emerged as significant points of concern, serving as vulnerable gateways through which radiant heat, flames, and firebrands can invade, potentially sparking interior structure fires.

To delve into this, Rebekah Schrader from UL’s Fire Safety Research Institute (FSRI) and University of Maryland (UMD) discusses the Heat Transfer from Structure Fires research project, carried out by the FSRI, examining heat transfer through various window types.

Understanding WUI fire spread

The spread of WUI fires is a multifaceted phenomenon, intricately linked with the built environment’s encroachment into natural landscapes.

“Wildland fires turn into WUI fires once they reach the built environment, at which point fires can spread from structure to structure and propagate through communities,” Schrader explains.

The damage inflicted by WUI fires on homes and communities has been nothing short of devastating, with their spread primarily occurring through three avenues: direct flame contact, thermal radiation, and firebrands.

The intricacies of WUI fire spread put into perspective the critical importance of homeowners understanding the vulnerabilities inherent within their homes and surrounding property.

By fostering a defensible space around their homes and employing measures to “harden” them against fire threats, the survivability of structures when faced with a fire can be substantially enhanced.

Schrader emphasises that radiant heat transfer can be a pivotal element in the spread of WUI fires, reinforcing the need for comprehensive strategies in mitigating such devastating events.

Real world applications

In investigating the threat, Schrader’s explains the selection of windows for the study: “The types of windows selected for this study were chosen to reflect those that are commercially-available and typically installed in residences across the United States,” highlighting the practical underpinnings of her research.

This decision ensures that the findings are directly applicable to the vast majority of homes at risk from WUI fires, making the study’s implications particularly relevant for homeowners, builders, and policy makers.

The study’s design acknowledges the reality that millions of people live in areas susceptible to WUI fires, underlining the critical importance of integrating fire-resistant features into residential structures.

By focusing on commercially available window types, the research aims to offer actionable insights that can be readily adopted to enhance the fire resilience of homes, ultimately advancing strategies for safeguarding communities against the devastating effects of WUI fires.

Experimentation and findings

The methodology employed in the study involved exposing 23 cm x 23 cm windows to a radiant panel generating incident heat fluxes ranging from 10 to 50 kW/m^2.

“The radiant panel was used as a heat source to ensure that the incident heat flux to the center of the window was uniform for the duration of each test,” Schrader explains, stressing the precision of the experimental setup.

This approach allowed for a controlled study of heat transfer through the windows, simulating real-world conditions where a curtain or other combustible material may be positioned behind a window during a fire.

The findings of the study reveal a nuanced understanding of how window construction affects heat transfer.

Notably, double-pane windows demonstrated a superior ability to reduce heat transfer compared to their single-pane counterparts.

Additionally, the application of low-emissivity (low-E) coatings emerged as an effective strategy for further diminishing the heat flux passing through windows.

Interestingly, the research found no significant difference in heat flux reduction between plain glass and tempered glass windows, nor between double-pane windows filled with air versus those filled with argon.

In terms of resilience, tempered glass displayed a longer time until failure when compared to plain glass.

Additionally, double-pane windows filled with argon gas consistently outperformed those filled with air, highlighting the importance of selecting the right materials and technologies for enhancing fire resistance in window design.

The vulnerability of windows

Windows are pinpointed as particularly susceptible elements of a building’s design, acting as conduits for heat that can lead to the ignition of interior combustibles.

“Combustibles such as curtains behind a window have also been known to ignite before window failure due to heat transfer through the windows, starting fires to the interior of structures,” explains Schrader, highlighting a key pathway through which fires can breach the interior of a home.

The eventual failure of windows under the intense heat of a WUI fire exacerbates the situation by creating openings for firebrands—burning or smouldering debris—to enter the structure.

The ingress of firebrands into a building through failed windows significantly raises the likelihood of interior fires, further compromising the structure’s integrity and the safety of its occupants.

Bridging the gap in building codes

The International Wildland-Urban Interface Code and California Building Code already lay down a foundation for structure hardening strategies, essential for mitigating building damage during WUI fires.

However, Schrader points out a notable gap: “Recommendations for the best window types to use in residential structures are slim and can be supported by additional research.” This observation underscores the need for detailed, evidence-based guidance on selecting windows that can enhance a building’s resistance to fires.

The FSRI’s research is poised to provide much-needed insights into how different window types influence the heat transfer into a structure during a WUI fire.

“It is my hope that the results from this study give both fire researchers and homeowners a better understanding of how window type impacts heat transfer into a structure,” says Schrader, highlighting the dual audience that stands to benefit from her findings.

By delineating the relationship between window design and fire safety, the study aims to offer actionable recommendations that can be integrated into existing codes and guidelines.

Research implications

The FSRI’s research aims to fundamentally transform the selection and application of window technologies in buildings at risk of wildland-urban interface (WUI) fires by highlighting designs that minimize heat transfer and withstand extreme conditions, thereby reducing the risk of interior ignition and entry of firebrands.

This research has the potential to profoundly impact the fire safety industry and architectural design, offering insights that could lead to a paradigm shift in fire-resistant construction practices.

By informing both fire researchers and homeowners about the critical role of windows in fire defense, these findings are expected to enhance building resilience, pave the way for fire-adapted design principles and future innovations that safeguard against wildfires, contributing to the creation of safer and more resilient communities.

Recent developments

The latest round of experiments, part of the Heat Transfer from Structure Fires research project, meticulously examined the failure mechanisms of double-hung windows when exposed to the intense conditions of a post-flashover fire for durations of approximately 10 – 15 minutes.

A key innovation of this research was the evaluation of six different window protection methods, including both commercially available solutions and materials specifically designed for wildland/WUI fire defense.

These protections ranged from fiber cement board and plywood to advanced materials like carbon fiber heat resistant welding blankets and intumescent coatings designed for wooden structures.

The second part of the study turned its focus to the failure mechanisms of window frames made from various materials, including vinyl, fiberglass, wood, and aluminum.

This aspect of the research aimed to identify how each material responded to the heat and flames, with an eye towards understanding which frame types offer the best resistance against fire penetration and heat transfer.

Joseph Willi, a research engineer at FSRI, emphasized the critical nature of these findings, noting: “If a window fails, there is now an opening in a structure through which firebrands can enter and potentially ignite combustibles inside the structure.”

The experiments shed light on how different protection methods can mitigate the risk of window failure, highlighting the potential for internal ignition due to heat transfer through windows, even if the windows themselves do not fail.

This article was originally published in the February 2024 issue of Fire & Safety Journal Americas. To read your FREE digital copy, click here.

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