Effect of vertical cavity geometry on flame spread along insulated double-glass BIPV façades exposed to window spill fires

The integration of insulated double-glass Building-Integrated Photovoltaics (BIPV) façades are increasingly applied in modern façades, but their fire behavior under window spill plume exposure remains insufficiently understood. An experimental study was therefore carried out to examine how cavity depth factors influence flame spread in insulated BIPV façade systems exposed to window spill plume. A 1/8 scale five-story building model was constructed with systematic variations in cavity gaps, including no cavity and four different cavity depths (δ) of 2 cm, 5 cm, 8 cm, and 10 cm. Propane burners produced controlled spill flames, with corresponding window overflow heat release rates of 5.5 kW, 8.5 kW, and 11.6 kW. Flame height, spread rate, and temperature profiles were recorded. The study focuses on how changes in cavity depth influence vertical flame propagation. At higher heat release rates, the spread rate decreases non-linearly with δ, while at lower rates a threshold exists below which flames cannot sustain spread. A temperature analysis confirms that cavity confinement amplifies convective and radiative feedback, accelerating flame growth. Importantly, the findings are interpreted in light of window spill plume and flame spread theories, providing a theoretical basis for understanding cavity-driven façade fire dynamics and supporting fire-safe design of BIPV façade systems. © 2026 Elsevier Ltd.