What is intumescent paint? And the types of intumescent strips, explained.
If it is your building, the intumescent spec is what keeps the steel frame standing for its rated period. Get it right and the frame holds. Specify it from a colour chart and you cannot demonstrate the required rated protection. That difference is what a third-party-certified installer is there to get right.
Intumescent paint is a reactive coating that swells into an insulating char when fire heats it, protecting the structural steel underneath.
Scope is agreed up front, with tight adherence to it once work starts.
Intumescent meaning: start with what the word does
Intumescent paint is a reactive fire-protection coating applied to structural steel. It looks like ordinary paint at room temperature. Once a fire heats it past roughly 200–250°C it reacts, swelling to many times its original thickness and forming a carbonaceous char that insulates the steel beneath it.
What does intumescent mean?
Intumescent describes a material that swells when heated. The word does the whole job: an intumescent product sits inert on a surface until a fire raises it past its activation temperature, then expands to many times its original volume. That single behaviour is what separates a reactive coating from a passive one. A board or a spray protects steel by being thick and insulating from the day it goes on. An intumescent coating protects steel by being thin until the moment it is needed. BS EN 13381-8, the British and European test standard for these products, is titled Applied reactive protection to steel members, and reactive is the operative word.
Is intumescent paint the same as an intumescent coating?
They are the same product under two names. Intumescent paint is what most people search for and what most site teams call it, because it arrives in tins and goes on with a spray gun or a roller. Intumescent coating is the term used in specifications, test evidence and standards. The distinction matters on a project for one reason. A decorative paint is chosen for how it looks, while a coating is chosen against a tested thickness for a named steel section and a named fire resistance period. Skyline lists this work as Intumescent Coatings for that reason. The words on the drawing should match the words on the test certificate.
How does intumescent coating protect structural steel?
Steel does not burn, and that is exactly the problem. It softens. As an unprotected section climbs through a fire it loses strength until it can no longer carry its load, and the frame moves. An intumescent coating buys back the time in between. The heat of the fire triggers a chemical reaction in the coating rather than in the steel, and the coating sacrifices itself to build an insulating char over the metal. The steel still heats. It heats slowly. That delay is the product: the frame stands for the period the building was designed around.
- It stays inert until it is needed SteelConstruction.info puts the reaction temperature at typically about 200–250°C. Below that it behaves like normal paint, and takes a decorative topcoat.
- It expands about fifty times The published expansion ratio for thin film coatings is about 50:1. A 1mm coating becomes roughly 50mm of char.
- The char is the insulation The swollen carbonaceous layer has very low thermal conductivity, so it slows the rate at which heat reaches the steel section behind it.
Inert film
The coating is applied to a specified dry film thickness and left. It can be over-coated and looks like a painted finish.
Activation, 200–250°C
An acid catalyst attacks the carbon source while a spumific agent releases non-flammable gases. The softened resin traps them and the film foams.
Expanded char
The char holds the steel below its limiting temperature for the rated period. SteelConstruction.info gives 620°C for thin film intumescents on hot-rolled beams carrying concrete slabs. For sprays and boards it gives 550°C.
What is intumescent paint made of?
A thin film intumescent basecoat is built from four ingredients that only make sense together. SteelConstruction.info lists them as a catalyst that produces a mineral acid (phosphoric acid), a carbonific such as starch that forms the carbonaceous char, a binder or resin that softens at a predetermined temperature, and a spumific agent that releases non-flammable gases including carbon dioxide, ammonia and water vapour. In a fire the acid attacks the carbon source, the resin softens at the right moment, and the gases blow the softening mixture into a thick foam that then sets as char. Timing is the whole trick. Each ingredient has to act in sequence, which is why a tested product cannot be substituted for a similar-looking one.
What is the difference between thin film and thick film coatings?
Both swell, at very different rates, for very different fires. Thin film products are solvent or water-based and are the normal choice inside buildings, where the fire that matters is a cellulosic one burning timber, paper and furnishings. Thick film products are epoxy-based, go on far heavier, and are built for external and severe environments and for hydrocarbon fires, which climb much faster. The number that separates them is the expansion ratio. Choosing between them is a specification decision taken against the building's fire strategy, not a preference.
Thin film
Solvent or water-based, applied to a specified dry film thickness. The usual choice for steel inside a building. It can take a decorative finish.
- Expansion ratio
- ~50:1
- Typical periods
- 30 · 60 · 90 min
- Fire type
- Cellulosic
Thick film
Epoxy-based and much heavier. Specified for external or aggressive environments. It is also the choice for hydrocarbon fires, which climb far faster than a building fire.
- Expansion ratio
- ~5:1
- Typical use
- External · process
- Fire type
- Hydrocarbon
What decides how thick an intumescent coating must be?
Three inputs decide it, and none of them is the size of the tin. The first is the section factor of the steel member, which describes how quickly that particular section heats up. The second is the limiting temperature the design allows that member to reach. The third is the fire resistance period the building has to achieve. Feed those into the manufacturer's tested data and a required dry film thickness comes out for every member on the frame. Change the section and the thickness changes with it. This is why an intumescent schedule is drawn up member by member, and why applied thickness is measured and recorded rather than assumed.
Section factor
The surface area of the member per unit length divided by its volume per unit length, written Am/V. A slender section has a high section factor and heats faster. It needs more protection than a stocky one.
17–300+m⁻¹ rangeLimiting temperature
The temperature the design lets the steel reach before it counts as failed. For thin film intumescents on hot-rolled beams carrying concrete slabs, SteelConstruction.info gives 620°C. Sprays and boards work to 550°C.
620°CThin film, beamsFire resistance period
How long the element must keep performing. Thin film intumescents mainly cover 30, 60 and 90 minutes. Newer products reach 120.
30–120minutesHow is intumescent coating tested?
The evidence comes from BS EN 13381-8, published in 2010 and titled Test methods for determining the contribution to the fire resistance of structural members. Applied reactive protection to steel members. It covers beams and columns, across a range of coating thicknesses, section factors and design temperatures, and it heats the specimens to the standard time and temperature curve set out in EN 1363-1. Two things get measured. Whether the protection stays attached to the steel, and how well it holds the temperature down. The resulting data feeds the fire design calculations in EN 1993-1-2 and EN 1994-1-2. A coating without that evidence behind it is a colour, not a fire protection system. Skyline's free guide, The Golden Thread Explained, covers what that same documentation discipline looks like as a building's own golden thread record.
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BM TRADA Q-Mark register. Scope is agreed before any work begins.
What are intumescent strips?
An intumescent strip is a narrow seal fitted around a protected opening that expands once a fire starts, closing the gap it guards. It is the same chemistry as the coating on a steel beam, doing a different job on a different element: the coating protects the member it covers, while the strip seals a joint that would otherwise let smoke and flame straight through. Strip materials, typically sodium silicate or graphite, vary between manufacturers and expand at different rates. Those material differences are why the types of intumescent strips an assembly needs are set by its own test evidence, not chosen by look.
How intumescent strips are fitted follows the tested assembly's evidence, not site judgement: placement and sizing are set by that evidence, and the gap a strip protects is governed by it too. A strip is only ever tested as part of a complete tested assembly, so every component stands or falls together. A strip retro-fitted to an assembly it was never tested with proves nothing.
Strips get a section here for one reason. Most people searching for intumescent products are trying to tell the two apart. Skyline's own services: Compartmentation Surveys, Firestopping installation, Fire Barriers & Cavity Barriers, Penetration & Linear Gap Sealing, and Intumescent Coatings. For structural steel, our Structural Fire Protection & Intumescent Coatings work is described on the structural fire protection page.
Scope of this guideThe questions people actually ask
What does intumescent mean?
It describes a material that swells when heated. In fire protection it means a product that stays inert at room temperature, then expands into an insulating carbonaceous char once a fire takes it past its activation temperature.
Is intumescent paint the same as an intumescent coating?
Yes. Intumescent paint is the everyday name. Intumescent coating is the term used in specifications, test reports and standards such as BS EN 13381-8, and it is the name Skyline uses for the service.
At what temperature does intumescent paint activate?
SteelConstruction.info puts the reaction at temperatures typically of about 200–250°C. Below that the coating behaves like an ordinary decorative finish and can be over-coated.
How much does intumescent paint expand?
For thin film products the published ratio is about 50:1, so a 1mm coating expands to roughly 50mm of char. Thick film epoxy coatings expand much less, at about 5:1.
How long can intumescent coatings protect steel?
Thin film coatings are mainly used where 30, 60 and 90 minutes are required, and SteelConstruction.info notes products developed in recent years that provide 120 minutes. The period for any given building comes from its fire strategy.
What types of intumescent strips are there?
They differ mainly by base material — commonly sodium silicate or graphite — with different profiles and expansion rates between manufacturers. Which of the types of intumescent strips a given door, frame or duct needs is set by that assembly's own fire test evidence, not picked by eye.
What is an intumescent strip?
An intumescent strip is a narrow seal fitted around a protected opening that expands once a fire starts, closing the gap it guards. It is the same chemistry as the coating on a steel beam, doing a different job on a different element.
How to fit intumescent strips?
How intumescent strips are fitted follows the tested assembly's evidence, not site judgement: placement and sizing are set by that evidence, and the gap a strip protects is governed by it too. A strip retro-fitted to an assembly it was never tested with proves nothing.
From the definition to the work itself
Structural Fire Protection & Intumescent
Firestopping & Compartmentation
Firestopping Surveys
Intumescent coatings for structural steel
Sources for the figures on this page
- SteelConstruction.info, Fire protecting structural steelwork. Reaction temperature, coating composition, expansion ratios, limiting temperatures and fire resistance periods.
- SteelConstruction.info, Calculating section factors. Definition of section factor and the range of values.
- BSI, BS EN 13381-8:2010, Test methods for determining the contribution to the fire resistance of structural members. Applied reactive protection to steel members. Scope, test basis and the link to EN 1363-1, EN 1993-1-2 and EN 1994-1-2.