Sandwich panels were tested according to the ISO 13784-1 standard by the Fire Insurers Laboratories of Korea and the Korea Institute of Construction Technology in Korea, the Royal Institute of Technology in Sweden, and the Commonwealth Scientific and Industrial Research Organization in Australia. The tested sandwich panels consisted of steel sheets, expanded polystyrene, and glass wool and were supplied by the Fire Insurers Laboratories of Korea. The heat release rate results were collated and compared, and the variation in the data distribution was analyzed statistically. The precision of the maximum heat release rate (HRR) and that of the THR determined according to ISO 13784-1 was +/- 10.16 kW and +/- 37.00 MJ, respectively, suggesting suitable consistency of HRR and THR values among the participating institutions. The inter-laboratory comparison for the assessment of the same product according to the ISO 13784-1 standard was found to be feasible despite variations in the laboratory facilities, specifically in terms of the extraction hoods and calorimeter HRR capacity.