How it works
At normal temperatures, an intumescent coating looks like a thick paint film. When heated, chemical reactions in the film release gases and form a carbon-rich foam that swells to many times its original thickness. This char has low thermal conductivity and shields the steel from the fire. It does not stop the steel from heating; it delays the moment the steel reaches the temperature at which its strength becomes insufficient.
Main types
| Type | Fire scenario | Typical setting |
|---|---|---|
| Water-borne thin film | Cellulosic (building) fires | Interior, controlled environments |
| Solvent-borne thin film | Cellulosic fires | Interior and some protected exterior exposures |
| Epoxy intumescent | Cellulosic and hydrocarbon fires | Industrial plants, exterior, harsh environments |
Cellulosic fire resistance is tested in Canada under CAN/ULC-S101, with UL 263 and ASTM E119 as the American counterparts. Hydrocarbon pool fires are covered by UL 1709. The fire scenario and rating come from the building code analysis or the process risk assessment, not from the coating supplier.
How the thickness is set
The required dry film thickness is not a single value per product. It is read from the tested and listed design for the specific product, according to:
- the required fire-resistance rating;
- the steel section's mass-to-heated-perimeter ratio (W/D or Hp/A, depending on the method);
- the type of member: beam, column, brace;
- the exposure and the critical steel temperature used in the design.
The schedule of thicknesses by member is a project document and must be followed exactly.
A system, not a single coat
An intumescent coating is normally applied over a compatible primer, sometimes a zinc-rich primer, and may need a topcoat for exterior or humid exposure. The intumescent layer itself is not the corrosion protection; the corrosivity category of the site determines the full system. Primer and topcoat must be approved for use with the intumescent product, since an incompatible coat can impair char expansion.
Inspection
Thickness is checked member by member against the schedule, following SSPC-PA 2 and, for thin-film products, the AWCI Technical Manual 12-B. Inspectors also check primer compatibility, adhesion, cure and damage repairs. See our intumescent coatings service.