An intumescent coating is a material that swells when exposed to the heat produced by a fire. The coating has special chemicals suspended within a binder. As the binder is exposed to elevated temperatures it softens allowing the chemicals to react. This reaction forms a thick solid foam-like char that can be over 50 times the original film thickness. This char burns very slowly and provides thermal protection to the underlying substrate by insulating it from heat transfer.

There are two generic types of intumescent coatings: thin film and thick film:

1. Thin film intumescent coatings – the most common coatings – are comparatively easy to apply, and often have a good aesthetic finish. They are waterborne or solventbased and are primarily for internal use, although some are suitable for exterior applications. A sealer coat is often required to protect the intumescent coating from extended weathering during the construction phase unless the coating system has been tested and proven for durability and performance under such conditions.
2. Thick film intumescent coatings – often (but not exclusively) epoxy-based coatings – are typically used in exterior applications, severe service environments, or when a longer period of fire resistance is required. In most instances, certified thick or thin film intumescent coatings are used in New Zealand. Certified coatings are formally assessed for the fire protection of structural steel by an accredited laboratory in accordance with either AS 1530.4 or NZS/BS 476 part 21, the standards cited in the NZ Building Code Acceptable Solutions and Verification Methods.

An intumescent coating solution will not always be suitable, especially where there is insufficient space available for the coating to fully expand. Examples may also include partitioning along the line of a structural steel frame. In such cases alternative measures should be considered (for example, boarding systems, flexible blanket systems, concrete encasement).