Composite slabs often use steel decks, which act as a formwork as well as bottom reinforcement to the slab. When such slabs are exposed to fire, the steel deck debonds and separates from the concrete and the gap generated due to this separation provides a layer of insulation. To study the insulation effect generated by this gap, an interface element between the steel deck and the concrete slab is modeled. Two different analysis methods namely, sequentially coupled iterative approach and fully coupled heat transfer and stress analysis are employed to predict the gap between the steel deck and concrete slab. Both of these approaches simulate radiation and conduction through the layer of air at the interface. The results obtained from both these methods are validated by comparing them with the experimental results available in the literature on this topic. The work presented in this paper demonstrates that considering the insulation effect of the gap between the steel deck and concrete slab is critical for an accurate prediction of temperatures in the concrete slab and this can significantly change the predicted fire resistance rating of the slab. In addition, a detailed parametric study suggests that the distance between the locations of embedment and the shape of the deck profile have a significant effect on the thermal insulation, whereas the thickness of the deck or the slab have little influence on the rate of heat flow through the interface. Finally, an empirical equation to predict the concrete temperatures based on such parameters is proposed. © 2019 Elsevier Ltd