As intumescent paint heats up and burns, it expands and forms a carbon foam that does a poor job of transferring heat. The specific heat of graphite (pure carbon) is very high, meaning it takes a lot of energy to increase its temperature, while the higher volume makes for a thicker barrier.
Degree of Expansion
According to Fire Retardants Inc, a coating of paint 6 mm thick expands to about an inch of black foam in the presence of fire, increasing the volume of the barrier between the wall material and the fire.
Intumescent paint, upon expanding, also prevents fire from penetrating the substrate material by leeching oxygen from the material. A lower-oxygen environment cannot combust as readily, further increasing the durability of the coated medium.
Intumescent paint lowers total damage, and often the substrate material is able to be salvaged. The expanded carbon foam is simply scraped off and the surface can be repainted.
In many cases, the coated material will still be structurally sound.
While specific instructions may vary by brand, most intumescent paints can be rolled or brushed on like regular household paints, while others strongly recommend spray-on application. Consult the directions on each product and remember to coat all areas, as a single exposed region can compromise the fire-retardant nature of the entire substrate.