Evaluation of exterior wall cavity fires using an intermediate scale test method

Cavities form an integral part of many external wall systems (EWSs). Numerous external wall fires worldwide have been primarily due to combustible exterior cladding. However, the Grenfell Tower and Knowsley Heights fire incidents (in the UK) are examples where wall cavity materials have impacted fire spread. Wall cavity materials are typically regulated by small-scale fire test methods which do not necessarily represent the actual fire conditions that can exist within wall cavities. This experimental study proposes an intermediate-scale test (IST) protocol to examine cavity wall fire behaviour. This protocol is a modified version of the FM Global Cavity Fire Test method (within the FM 4411-2020 series). The study examines a broad range of cavity materials including sarking, polyester insulation, phenolic foam, PIR, and EPS. A low-intensity (6–8 kW) and high-intensity (~80 kW) ignition sources were used to represent two types of cavity fire scenarios. These two fire sizes were shown to differentiate reaction to fire behaviour between these materials and explore the “tipping point” in resulting fire behaviour (which may lie between these two intensities). This proposed cavity fire test protocol provides a suitable “elevated fire risk assessment tool” for combustible cavity materials.