ABSTRACT The risk of external wall flame spread has become a global concern following incidents where combustible façade materials contributed to fire propagation. The 2017 Grenfell Tower fire highlighted the risks of combustible cladding and insulation. A common fire control method is the installation of cavity barriers within wall systems. However, research on the performance of improperly installed Open‐State Cavity Barriers (OSCBs) is limited. This intermediate‐scale experimental study investigates the impact of installation defects identified through site inspections and surveys. Six common defects were evaluated using intermediate‐scale fire tests, including gaps between barriers, obstructions, excessive cavity depths, and improper corner installations. The results revealed that gaps exceeding 30 mm between cavity barriers allowed vertical flame spread, with gaps over 45 mm significantly increasing temperature and flame penetration into upper compartments. Gaps of 15 mm did not significantly contribute to flame spread. Blockages of up to 50 mm in front of barriers reduced temperature rise and flame spread. A 50% increase in cavity depth delayed cavity closure by 30% compared to installations following manufacturer specifications. Improper corner installations, where cavity closure was inadequate, led to higher temperatures in upper compartments and increased flame spread risk. The key finding demonstrates that open or mis‐installed cavity barriers notably compromise façade fire safety through accelerated flame propagation compared to properly closed barriers. These findings highlight the critical role of proper OSCB installation in limiting external flame spread and emphasize the need for robust regulations, improved installation quality control, and further research into cavity barrier performance.
Cherukat et al. (2026) studied this question.