Summary The experimental study presented herein investigates the influence of the substrate thermal conditions on the behaviour of thin intumescent coatings. Steel plates coated with a commercially available solvent‐based thin intumescent coating were exposed to a constant incident radiant heat flux of 50 kW/m2 in accordance with the heat‐transfer rate inducing system (H‐TRIS) test method. The influence of different substrate thermal conditions was investigated using sample holders capable of controlling the thermal boundary conditions at the unexposed surface of tested steel plates and comparing them to coated timber samples. Experimental results evidence that the substrate thermal conditions govern the swelling of intumescent coatings, thus their effectiveness in protecting load‐bearing structural elements. The substrate temperature controls the swelling of intumescent coatings because it defines the temperature experienced by the reacting virgin coating located close to the coating‐substrate interface. The physical and thermal properties of the substrate control the capacity of the system to concentrate/dissipate heat in proximity of the coating‐substrate interface. In this way, the substrate thermal conditions govern the temperature evolution of the reacting intumescent coating, consequently the swelling process. Accordingly, high swelling rates were recorded for highly insulating conditions (timber substrate), while low swelling rates for poorly insulating conditions (water‐cooled heat sink).