Layer-Specific Thermal Degradation and Fire Response of a CLT Sandwich Wall Assembly Under Medium-Scale Radiant Exposure

The fire response of cross-laminated timber (CLT) sandwich wall assemblies depends on interactions among the structural core, lining, cavities and secondary timber members. This study examines the thermal roles of these layers under medium-scale radiant exposure. The thermogravimetric mass change, mass-loss rate and heat-flow response were obtained for CLT, Fermacell® gypsum–fiber board and timber stud specimens by simultaneous thermal analysis. One complete, artifact-free record per material group was selected for mechanistic comparison; the data were not used to characterize material variability statistically. The degradation intervals were compared qualitatively with temperatures, visual observations and post-test damage recorded during a 90 min exposure of the wall assembly at 20 kW/m2. Fermacell® retained 77.8% of its initial mass; CLT and stud specimens retained 22.5% and 20.8%, respectively. The largest mass loss of both wood-based materials occurred at 280–430 °C, with mass-loss-rate peaks at 372.1 °C for the CLT and 359.4 °C for the stud. The lining initially delayed heat transfer. After board cracking, cavity heating was followed by the degradation and glowing of the timber stud. Layer-specific thermal analysis supports the mechanistic interpretation of the tested assembly, but it neither predicts event timing nor replaces standardized fire resistance testing or classification.