An intumescent is a substance that swells because of heat exposure, thus increasing in volume and decreasing in density. Intumescent materials used in fire protection products will increase their volume significantly under the influence of heat (typically at around 200°C).

The swelling when exposed to heat will  typically forms a multi-cellular, carbonaceous, or ceramic layer. This layer acts as a thermal barrier, effectively protecting the substrate against a rapid increase in temperature to maintain its structural integrity.