A 1-D transient mathematical model was studied for a composite system consisting of a substrate (steel) and an intumescent coating exposed to radiative heating. By applying a simple heat conduction equation for the first material, a general model proposes the coating consisting of three components, which degrade according to independent finite rate reactions. Mass and heat transfer take place across a variable volume medium, including a simplified mechanism for bubble dynamics and material swelling. A parametric analysis clarifies the role of several parameters on the fire retardancy performance of the coating. Good agreement is obtained between predictions and measurements of the substrate temperature profiles.