Flammability of intumescent coatings at the materials-scale is “expected” to have an impact on their fire resistance offered to the steel member. However, to what extent and at what stage during the exposure to an imposed fire curve in a furnace are not thoroughly established. The role of different functionality of the flame-retardant additives in this process is also not clear despite a thorough understanding of their thermal decomposition mechanisms. In an effort to better understand some of these aspects, in this work, model systems based on different flame-retardant additives in epoxy are chosen to cover various strategies (like intumescent, radical quenching, and endothermic behavior) that are typically employed to reduce the flammability of a coating system. The flame-retardant additives chosen are ammonium polyphosphate, tetrabromobisphenol-A, 9,10-dihydro-9-oxa-10-phosphaphenanthrene oxide, and aluminum trihydroxide. The effects of flame-retardant additives on the curing, thermal decomposition, and flammability of epoxy resin are investigated. Reactive flame-retardant additives, in particular, are found to reduce the enthalpy associated with curing of epoxy by more than 45%. However, as the amount of hardener incorporated in these formulations is not controlled despite the presence of reactive flame-retardant additives, this had a negative effect on the thermal decomposition behavior of the epoxy/flame-retardant formulations, and consequently on the calculated decomposition activation energies. Heating rate seems to be a dominating parameter influencing the flammability of the formulations. Peak heat release rate values, irrespective of the formulation, increased by more than 1382%–1970% by changing the heating rate in a pyrolysis flow combustion calorimeter from 6°C/min to 100°C/min. As the flammable volatiles released per unit time are higher at high heating rates, ignition and subsequent flaming of the coating occurred in the furnace tests, where a cellulosic fire curve was simulated. This has significantly influenced the fire resistance times.