Summary Nowadays, buildings contain more and more synthetic insulation materials in order to meet the increasing energy‐performance demands. These synthetic insulation materials have a different response to fire. In this study, the mass loss and flammability limits of different sandwich panels and their cores (polyurethane (PUR), polyisocyanurate (PIR) and stone wool) are studied separately by using a specially designed furnace. Expanded polystyrene and extruded polystyrene are tested on their cores only. The research has shown that the actual mass loss of synthetic and stone wool‐based cores is comparable up to 300 °C. From 300 °C onwards, the mass loss of PUR panels is significant higher. The mass losses up to 350 °C are 7%, 29% and 83% for stone wool, PIR and PUR respectively, for the influenced area. Furthermore, delamination can be observed at exposure to temperatures above 250 °C for the synthetic and 350 °C for the mineral wool panels. Delamination occurs due to the degradation of the resin between core and metal panels and the gasification of the (PUR) core. The lower flammability limits have been established experimentally at 9.2% m/m (PUR) and 3.1% m/m (PS). For PUR, an upper limit of 74% was found. For PIR and mineral wool, no flammability limits could be established. Copyright © 2017 John Wiley & Sons, Ltd.